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Chapter 11: Acids, bases and the pH value
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Chapter 13: Reactions of acids with metals
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Chapter overview
2 weeks
The central challenge of this chapter is to establish that acid-base reactions are exchange reactions. A fragment of the acid is exchanged with a fragment of the base and a salt and water are the resulting products of the reaction. The type of salt that forms depends on the identities of the acid and the base that were combined during the reaction.
Once learners understand this, they have taken an important step to understanding acid-base chemistry. We will spend some time developing a frame for explaining this at the start of the chapter, to which we will return frequently.
In light of the fact that learners have yet to learn about cations and anions, we have considered it pedagogically justifiable to make the following simplifications to currently accepted acid-base theory, in order to bring the concept of exchange across to the learners:
Acids can be thought of as contributing H (instead of H+); and
Bases can be thought of as contributing O or OH (instead of O2- and OH-).
Water (H2O) is a combination of 2 H and 1 O, or alternatively 1 H and 1 OH.
We are well aware that writing H + OH → H2O has no meaning in science and for this reason we have avoided this usage in the text. But we do consider the use of simplified symbols (H instead of H+ and so forth) to have an advantage over their scientifically correct (but potentially confusing) counterparts in this context.
There is also a danger that misconceptions and sloppy usage of symbols may result further down the line, when simplifying in this way. However, we feel these risks are counterbalanced by the greater likelihood of learners understanding the concept of exchange if the symbols they work with are not cluttered with additional information - like the charges on the ions - that have no meaning for them yet.
Other skills that will be reinforced in this chapter are:
A word of caution: Acid-base reactions are neutralisation reactions. However, this does not mean that the mixture of an acid with a base will be a neutral solution and you should avoid language that reinforces this notion. Even if equivalent quantities (stoichiometric quantities) of the acid and base are mixed - which would imply that both have been neutralised - the resulting solution will only be neutral (i.e. pH = 7) under very special circumstances. The reason is that not all salts are 'neutral substances'; in fact most salts have acid-base properties of their own. The chemistry required for learners to understand this is beyond them at this stage and will only be dealt with in Physical Sciences in Grade 12. Our suggestion is that you simply refrain from calling salts 'neutral substances'. If questions arise around the issue you could point out that the salts they will encounter in this chapter may be neutral substances, but that this is not true of all salts.
Take note that although there is no section specifically named 'Applications' as indicated in CAPS, this content has rather been dealt with under other sections where it is more appropriate.
6.1 Neutralisation and pH (1.5 hours)
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Tasks |
Skills |
Recommendation |
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Investigation: The reaction between vinegar and baking soda |
Hypothesising, measuring, preparing, observing, comparing, recording, plotting graphs |
CAPS suggested |
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Activity: CO2 bubbled through water |
Observing, measuring, comparing |
Optional |
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Activity: What is acid rain? |
Observing, reading, researching, interpreting, analysing, summarising |
CAPS suggested |
6.2 The general reaction of an acid with a metal oxide (1.5 hours)
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Tasks |
Skills |
Recommendation |
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Investigation: The reaction between magnesium oxide and hydrochloric acid |
Hypothesising, preparing, observing, measuring, recording, plotting graphs |
CAPS suggested |
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Activity: Writing the chemical equation |
Writing and balancing chemical equations |
Optional (Suggested) |
6.3 The general reaction of an acid with a metal hydroxide (1.5 hours)
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Tasks |
Skills |
Recommendation |
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Investigation: The reaction between sodium hydroxide and hydrochloric acid |
Hypothesising, preparing, measuring, observing, measuring, recording, plotting graphs |
CAPS suggested |
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Activity: Writing the chemical equation |
Writing and balancing chemical equations |
Optional (Suggested) |
6.4 The general reaction of an acid with a metal carbonate (1.5 hours)
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Tasks |
Skills |
Recommendation |
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Investigation: The reaction between calcium carbonate (chalk) and hydrochloric acid |
Hypothesising, preparing, comparing, measuring, recording, plotting graphs |
CAPS suggested |
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Activity: Writing the chemical equation |
Writing and balancing chemical equations |
Optional (Suggested) |
In the previous chapter we learnt about a new concept, namely pH. If we want to know whether something is an acid or a base, we can measure its pH:
Another useful thing we learnt in the previous chapter is that we can use universal indicator to measure the pH of a solution. Universal indicator has different colours at different pH values. Below is a colour chart showing the range of colours for universal indicator and the pH values they correspond to. You will need it for all the activities of this chapter, because we are going to do lots of pH measurements!
Can you remember how we used the universal indicator paper in the previous chapter? Here are some suggestions for the investigations of this chapter:
Later, if you want to write down a note or an observation, you can do so directly on the paper and copy it to your workbook afterwards.
Instead of dipping the paper in the solutions you are testing, use a glass rod or drinking straw to transfer a drop of the test solution to the indicator paper.
For some of the investigations in this chapter, you will be using droppers or syringes to measure out quantities. Tell learners that they may not use droppers or syringes to squirt water at other learners! There are many reasons why this is not a good idea. The most important reason is that the dropper or syringe may contain acid, that could end up in someone's eye where it could cause permanent damage or even blindness. So, squirting each other with the droppers or syringes is not allowed.
What do you think would happen if we mixed an acid and a base?
Get learners to discuss this in class or in small groups. Allow them to speculate and guide them to recall their Grade 7 learning: An acid will lose its potency when it is mixed with a base and vice versa. So the acid will be weakened by the base and the base will be weakened by the acid. 'Weaken', however, is a term best avoided, because 'weak' and 'strong' have very specific meanings when speaking about acids and bases. In a sense their acid-base properties will be destroyed, because they will be converted to products that won't be acids or bases. (Often the salt that results from the reaction between an acid and a base will have acid-base properties of its own, but we will not be discussing that now.)
We are going to do an investigation to find out. We are going to mix vinegar with baking soda. But first, a little revision: is vinegar an acid or a base? If you are not sure, imagine putting a drop of vinegar on your tongue. What would it taste like?
It would taste sour, therefore it is an acid.
Is baking soda an acid or a base? If you are not sure, turn back to the previous chapter and look at the activity 'The pH scale'.
Baking soda is a base.
Quantities for this investigation are as follows: Every 1 g of baking soda will require approximately 15 ml of vinegar for complete neutralisation. We recommend that you measure out 1 teaspoon of baking soda and approximately 50 ml vinegar for each group.
The purpose of this experiment is to investigate how the pH changes when vinegar is added to baking soda.
INVESTIGATIVE QUESTION(S):
A possible answer is: What will happen to the pH of the solution if we add vinegar to baking soda?
OVERVIEW OF THE INVESTIGATION:
In the range pH > 7
The pH will decrease.
HYPOTHESIS:
When we add vinegar to baking soda, the pH of the mixture will decrease.
MATERIALS:
METHOD:
RESULTS:
Present your results in a neat table. Use appropriate headings for your table. 'Number of teaspoons of vinegar added' and 'Colour of the universal indicator paper' and 'pH of the test solution' are suggested headings for your columns.
Draw a line graph to illustrate your results. What will be on the x-axis and what will be on the y-axis? Give your graph a heading.
Learners must draw a graph with the 'number of teaspoons of vinegar added' on the x-axis (independent variable) and the pH of the solution on the y-axis (dependent variable).
CONCLUSIONS:
What conclusions can be made from the results of your investigation? Here you can rewrite your hypothesis, but change it to reflect your findings if they are different from what you predicted earlier.
Were you able to confirm or reject your hypothesis?
In this investigation, you probably noticed that the pH of the mixture dropped every time you added more vinegar to the baking soda! Why did this happen?
When an acid and a base are mixed (in the right amounts), they will neutralise each other. That means that, together, they will change into something that is neither an acid nor a base. So, the acid will lose its 'acidity' and the base will lose its 'basicity'.
What have we learnt so far? We have learnt that acids and bases neutralise each other:
If we add a base to an acid, the pH of the resulting solution will increase, because the acid will lose some of its potency.
What are the products of an acid-base reaction? Can we predict what they will be?
In order to understand how an acid-base reaction works, we have to take a quick detour and say something about exchange reactions. Acid-base reactions are exchange reactions.
In the reaction below, two substances AB and CD are undergoing an exchange reaction:
AB + CD → AD + CB
Can you see that A and C have exchanged partners so that A is now combined with D, while C combined with B?
What does this have to do with acids and bases? Well, acids and bases undergo exchange reactions too. Here are some examples. See if you can figure out which parts have exchanged with which.
Example 1
HCl + NaOH → NaCl + HOH
In the above equation HOH should actually be written: H2O. The reaction equation becomes:
HCl + NaOH → NaCl + H2O
or, in words:
hydrochloric acid + sodium hydroxide →sodium chloride + water
In this example, the following happened:
Example 2
2 HCl + MgO → MgCl2 + HOH
In the above equation HOH should actually be written: H2O. The reaction equation becomes:
2 HCl + MgO → MgCl2 + H 2O
or, in words:
hydrochloric acid + magnesium oxide →magnesium chloride + water
In this example, the following happened:
Acid-base reactions always produce water and a salt. In both of the examples above the general equation was:
acid + base → salt + water
There is one class of acid-base reactions that produces an additional product, but we will learn more about that later.
In Grade 11 you will learn that the mechanisms of these reactions are actually slightly more complex than this, but for now, understanding it at this level is good enough.
When we investigated acids and bases in the previous chapter, we considered only household acids like lemon juice and vinegar. There are a few laboratory acids that you should know the names and formulae of and they have been listed in the following table:
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Name of the acid |
Formula of the acid |
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hydrochloric acid |
HCl |
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nitric acid |
HNO3 |
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sulfuric acid |
H2SO4 |
These acids are very corrosive, even when they have been diluted with water and should always be handled with great care.
What happens if you put a burger in concentrated hydrochloric acid? (video)
In the next sections will discuss the classes of substances that are typically acids or bases. Two important things to remember are the following:
First, we will look at the non-metal oxides.
Can you name a few non-metal oxides? Write down their formulae. If you are not sure you can take a peek at the Periodic Table and pick a few non-metals from the right-hand side of the table. Add oxygen and you have a non-metal oxide!
CO2 and SO2
How do we know that non-metal oxides form acidic solutions? Experiments have shown this.
You may not know this, but when CO2 gas is bubbled through water some of it dissolves in the water to form carbonic acid. Here is the reaction equation:
CO2 + H2O → H2CO3
To see this happen, try the following quick activity.
A variation of this activity is if you have liquid universal indicator, you can add it to the tap water at the start to show the pH is 7 (it should be green). Then as you blow into the water, the universal indicator will change colour. Make sure you use a clear glass so learners can observe the colour change as it becomes more yellow. This links to the next activity on acid rain and how it forms.
MATERIALS:
INSTRUCTIONS:
We could measure the pH of the water with universal indicator paper.
Now exhale into the water using a straw. Your breath contains CO2 and some of this will dissolve in the water if you carry on doing this for a few minutes.
The pH will be below 7.
Carbonic acid is added to soft drinks to make it fizzy. The carbonic acid decomposes and forms carbon dioxide (CO2)
The pH of the solution is now below 7 because it contains carbonic acid (H2CO3). Carbonic acid is not a very strong acid, but still acidic enough to have a pH lower than 7.
When sulfur dioxide (a gas) is bubbled through water it dissolves in the water to form an acid called sulfurous acid:
SO2 + H2O → H2SO3
These are two of the reactions that produce a phenomenon called acid rain. SO2 and CO2 are released as waste products from factories and power stations. For example, burning wood and fossil fuels releases carbon dioxide and sulfur dioxide into the atmosphere. These gases then dissolve in water droplets in the atmosphere to form acids, in a similar way that the CO2 in your breath dissolved in the water in the last activity to produce an acidic solution. When it rains, these acids are present in the raindrops that fall back to earth. Sulfurous acid (H2SO3) is strong enough to damage plant life and to acidify water sources.
For the next activity, you have to do some research on acid rain.
Volcanoes also release non-metal oxides into the air (mainly SO2) that can contribute to acid rain.
INSTRUCTIONS:
QUESTIONS:
They are sulfur dioxide (SO2), carbon dioxide (CO2) and nitrogen dioxide (NO2).
The main sources of these gases which contribute to acid rain are from human activity, such as electricity generation in fossil fuel power plants (especially coal), factories emitting smoke and the exhaust fumes from motor vehicles. Acid rain can also occur due to natural phenomena, such as volcanoes which emit sulfur dioxide into the atmosphere. Some processes in the ocean and in wetlands also produce the gases which form acids.
SO2 + H2O → H2SO3
CO2 + H2O → H2CO3
Sulphurous acid and carbonic acid.
The impacts include:
Acids are corrosive and so they can corrode surfaces over time.
Learners need to justify their answers. They may say that it helps the local environment as the gases are carried further away and therefore do not pollute the town or city that the factory is in or near. But this does not do anything to minimize the acid rain that could potentially form as the same amount of gases are still emitted; they are just carried further away. The acid rain therefore can still form and fall on the vegetation and areas outside of the towns and cities.
There are several solutions to minimizing the formation of acid rain. For example, coal-powered stations can use filters and other processes in their smoke towers to remove sulfur gases before the smoke is released into the atmosphere. Countries can take bigger steps by signing treaties to reduce their sulfur and other greenhouse gas emissions. The move towards using renewable energy sources will also help to reduce the reliance on coal and other fossil fuels, thereby reducing the emission of acid-producing gases into the atmosphere.
We have now learnt about non-metal oxides, but what about metal oxides? What kind of solutions do they form in water? We will find out more about them and other metal compounds in the next section.
Do you remember learning about some of the metal oxides in Chapter 3? We already learnt these rules to write the formulae of metal oxides.
Metal oxides from group 1 on the Periodic Table will have the formula M2O, where M represents any metal.
Can you write two examples? Look at the Periodic Table at the front of the book, pick any two metals from group 1 and write their formulae using this rule.
Any two of the following: Li2O, Na2O, K2O, Rb2O, Cs2O
Metal oxides from group 2 will have the formula MO.
Can you write 2 examples?
Any two of the following: BeO, MgO, CaO, SeO, BaO
What do you think the pH will be of a solution of a metal oxide in water?
The pH will be above 7.
The next class of compounds that form basic solutions in water are the metal hydroxides.
A metal hydroxide forms when a metal reacts with water. A metal hydroxide has the general formula MOH or M(OH)2. In the formula, M represents a metal atom, O represents an oxygen atom and H represents a hydrogen atom.
To know whether the MOH or M(OH)2 will be the correct formula, here are two simple rules for you to remember:
Metal hydroxides from group 1 on the Periodic Table will have the formula MOH.
Can you write two examples? Look at the Periodic Table at the front of the book, pick any two metals from group 1 and write their formulae using this rule.
Any two of the following: LiOH, NaOH, KOH, RbOH, CsOH.
Metal hydroxides from group 2 will have the formula M(OH) 2.
Can you write two examples?
Any two of the following: Be(OH)2, Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2.
What do you think the pH will be of a solution of a metal hydroxide in water?
The pH will be above 7.
The final class of compounds that forms basic solutions in water is the metal carbonates. Baking soda is a special kind of carbonate, called a bicarbonate (or hydrogen carbonate). You may remember that it was one of the bases we tested with universal indicator earlier.
A metal carbonate has the general formula MCO3 or M2CO3. In the formula, M represents a metal atom, C represents a carbon atom and O represents an oxygen atom.
To know whether the MCO3 or M2CO3 will be the correct formula, there are two simple rules to remember:
Metal carbonates from group 1 on the Periodic Table will have the formula M2CO3.
Can you write two examples?
Any two of the following: Li2CO3, Na2CO3, K2CO3,Rb2CO3, Cs2CO3.
Metal hydroxides from group 2 will have the formula MCO3.
Can you write two examples?
Any two of the following: BeCO3, MgCO3, CaCO3, SrCO3, BaCO3.
What do you think the pH will be of a solution of a metal carbonate in water?
The pH will be above 7
In the next sections we will be investigating real reactions!
In the previous section we learnt about two classes of oxides, namely metal oxides and non-metal oxides. Here is what we know about them so far:
Here is the same summary, in table form, with some examples added:
|
Metal oxides |
Non-metal oxides |
|
metal + oxygen → metal oxide |
non-metal + oxygen → non-metal oxide |
|
basic |
acidic |
|
pH > 7 |
pH |
|
Examples: Li2O, Na2O, MgO, CaO |
Examples: CO2, SO2, NO2, P2O5 |
In this section, we are going to learn about the reactions between metal oxides and acids.
This investigation requires magnesium oxide from the reaction when magnesium ribbon burns in oxygen. If you have set some aside from the earlier activity 'The reaction of magnesium with oxygen' (Chapter 3), learners can use it for this investigation. If you did not, you can easily repeat that demonstration to produce more white magnesium oxide powder for this next investigation. This investigation is also suitable to scale up as a demonstration.
The purpose of this investigation is to:
determine whether the reaction between an aqueous solution of magnesium oxide and hydrochloric acid is a neutralisation reaction.
INVESTIGATIVE QUESTION(S):
What are the questions you hope to answer with this investigation? Write them in the space below. There are a few words to start you off.
When magnesium oxide is dissolved in water, will the resulting solution be acidic, basic or neutral?
When a solution of magnesium oxide is treated with hydrochloric acid, will the pH of the mixture increase, decrease, or stay the same?
OVERVIEW OF THE INVESTIGATION:
In the range pH > 7
The pH will decrease
HYPOTHESIS:
What are your predictions? Your hypothesis should be a prediction of the finding(s) of the investigation. You should write it in the form of a possible answer to the investigative question(s). Here are a few words to start you off:
When magnesium oxide is dissolved in water, the resulting solution will be basic (have a pH > 7).
When a solution of magnesium oxide is treated with hydrochloric acid, the pH of the mixture will decrease.
MATERIALS:
Notes for the investigation:
Learners must not dilute hydrochloric acid themselves as it reacts strongly with water. Make sure to add the acid slowly to the water and NOT the other way around.
If you have universal indicator solution, this will work very nicely as you can observe the colour changes as you add the drops.
If you decide to give the learners droppers to measure out the HCl, you will have to enforce very strict rules for handling the droppers. Learners find the temptation to squirt water at each other very difficult to resist and they must be made aware of the hazards of accidentally squirting acid at another learner.
METHOD:
Use the glass rod (or plastic straw) to stir the solution until most the magnesium oxide has dissolved. We will be calling this the test solution from now on.
RESULTS:
Learner-dependent answer
Draw a graph of your results. Here are some hints to help you decide which variable to put on which axis:
Number of drops of HCl
pH
The number of drops of HCl should be on the x-axis of the graph and pH should be on the y-axis. There should be a general trend downwards (since acid is added to a base, we can expect the pH to drop), but it should not be linear. This experiment is a very rudimentary 'titration' and an example of a titration curve from this experiment is given here:
It is therefore not expected that learners' curves will be linear, but rather that there will be a gradual decline in pH at first, followed by a rapid drop when all the base has been neutralised. After this the curve levels out again.
CONCLUSIONS:
What conclusions can be made from the results of your investigation? Rewrite your hypothesis, but change it to reflect your findings if they are different from what you predicted earlier.
Were you able to confirm or reject your hypotheses?
Now that we have investigated a reaction between a metal oxide (MgO) and an acid (HCl), we can write an equation for the reaction. We will begin by writing a general equation and end with one that matches the reaction that we have just investigated.
A video showing the reaction of copper(II)oxide with hydrochloric acid
Can you remember learning that an acid-base reaction is an exchange reaction? We learnt that:
Whatever is left of the the acid and the base after making a H2O molecule, combines to form a salt.
The general word equation for the reaction between an acid and a base is:
acid + base → salt + water
Since the base in our reaction is a metal oxide we can write:
acid + metal oxide → salt + water
This is the general word equation for the reaction between an acid and a metal oxide. The type of salt that forms will depend on the specific acid and metal oxide which were used in the reaction.
Now we are going to learn how to write equations for our actual reaction.
The following steps will guide you:
HCl
Magnesium oxide (MgO)
Now,let's try to predict the products of the reaction. We know that water will be one of the products.
Mg
Write what remains of the acid (HCl) after we have taken away the H (to make water). (Remember we need two H to make one H2O).
2 Cl (we used 2 HCl)
MgCl2
Now, let's put it all together, first the reactants, then the products:
2 HCl + MgO → MgCl2 + H2O
Let's check quickly if the reaction is balanced.
2 H atoms left and 2 H atoms right. The H's are balanced.
2 Cl atoms left and 2 Cl atoms right. The Cl's are balanced.
1 O atoms left and 1 O atoms right. The O's are balanced.
Since the numbers of each type of atom is the same on either side of the equation, we can confirm that it is balanced.
Finally, let's use the chemical equation to write a word equation for the reaction:
hydrochloric acid + magnesium oxide → magnesium chloride + water
In the next section we are going to look at the reactions between acids and metal hydroxides.
We will start this section with an investigation to illustrate the reaction between an acid and a metal hydroxide.
Notes for the investigation:
The purpose of this investigation is to:
INVESTIGATIVE QUESTION(S):
Here are some ideas:
OVERVIEW OF THE INVESTIGATION :
In the range pH > 7
The pH will decrease
HYPOTHESIS:
Some ideas:
MATERIALS:
Prepare 0.1 M NaOH solution by dissolving approximately 4 g of NaOH pellets in 1 liter of cold tap water. Wear safety goggles and gloves since there is a chance the sodium hydroxide solution could splash up.
Instructions for preparation are given with the previous investigation: The reaction between magnesium oxide and hydrochloric acid
METHOD:
Use the syringe to transfer 2 ml of the sodium hydroxide solution into the test tube or small glass beaker. We will be calling this the test solution from now on.
Learner dependent answer. Should be around 2 ml.
NaCl forms in this reaction and the idea is for learners to let it dry out in the window sill and examine it later. It is probably not a good idea to let them taste it, as there is a possibility that not all of the acid or base has been neutralised.
RESULTS:
learner-dependent answer
Draw a graph of your results.
Volume of HCl
pH
The volume of HCl added should be on the x-axis of the graph and pH should be on the y-axis. There should be a general trend downwards (since acid is added to a base, we can expect the pH to drop), but it should not be linear. See comments and graph provided with the previous investigation.
CONCLUSIONS:
What conclusions can be made from the results of your investigation? Here you can rewrite your hypothesis, but change it to reflect your findings if they are different from what you predicted earlier.
Were you able to confirm or reject your hypothesis?
Now that we have investigated a reaction between a metal hydroxide (NaOH) and an acid (HCl), we can write an equation for the reaction. We will begin by writing a general equation and end with one that matches the reaction that we have just investigated.
A video illustrating the reaction of magnesium hydroxide with hydrochloric acid in the presence of universal indicator
You learnt that an acid-base reaction can be represented by the following general word equation:
acid + base → salt + water
The base in our reaction was a metal hydroxide, so the general equation becomes:
acid + metal hydroxide → salt + water
This is the general equation for the reaction between an acid and a metal hydroxide. The type of salt that forms will depend on the specific acid and metal hydroxide which were used in the reaction.
Now we are going to learn how to write equations for our actual reaction.
The following steps will guide you:
HCl
Sodium hydroxide (NaOH)
Now,let's try to predict the products of the reaction. We know that water will be one of the products.
Na
Write what remains of the acid after we have taken away the H to make water. Remember we need two H to make one H2O, but NaOH has already contributed one O and one H. Now put the two fragments together. Place the metal from the base first and the non-metal from the acid. One Na and one Cl makes...
NaCl
Now, let's put it all together, in the following order: Acid + metal hydroxide → salt + water
HCl + NaOH → NaCl + H2O
Let's check quickly if the reaction is balanced.
2 H atoms on the left and 2 H atoms on the right. The H's are balanced.
1 Cl atoms on the left and 1 Cl atoms on the right. The Cl's are balanced.
1 O atoms on the left and 1 O atoms on the right. The O's are balanced.
Once you have performed this reaction and you are left with a neutral solution, you decide you want to recover the sodium chloride (table salt). How will you do this?
You need to evaporate the water so that the salt crystallizes, either by leaving it in a sunny spot or boiling the solution.
Finally, let's use the chemical equation to write a word equation for the reaction:
hydrochloric acid + sodium hydroxide → sodium chloride + water
In the next section we are going to look at the reactions between acids and metal carbonates.
In this section we will investigate the reaction between an acid and a metal carbonate.
Grind up a few pieces of white chalk for this experiment. The calcium carbonate will not actually dissolve well in water, but it should be possible to determine that the solution is basic, from the tiny amount of calcium carbonate that will dissolve when the chalk dust is suspended in water.
Learners will need their colour charts and results tables before they start.
The purpose of this investigation is to:
INVESTIGATIVE QUESTIONS:
Some ideas:
OVERVIEW OF THE INVESTIGATION :
We will measure the pH of a suspension of calcium carbonate (CaCO3) with universal indicator paper to confirm whether it is acidic or basic. Within what range do you expect the pH of the calcium carbonate to fall?
In the range pH > 7
The pH will decrease
HYPOTHESIS:
Some ideas:
MATERIALS:
METHOD:
Place approximately 2 ml of the calcium carbonate suspension into the test tube or small glass beaker. We will be calling this the test solution from now on.
Perform the experiment in a conical flask, as follows:
2 into another conical flask containing lime water (see diagram below). The CO2 gas should be poured out. Shake the conical flask containing the lime water and CO2 to facilitate mixing. Allow the learners to make their observations.2 is denser than air and will remain in the conical flask for a few minutes before diffusing into the air. It is during this time that you should pour it over into the lime water. Be careful not to let any of the test solution flow over into the clear lime water. Only the CO
Alternatively, you could use a setup like the one shown in the diagram below:
These experiments can also be done using combo plates.
Carbon dioxide, CO2
RESULTS:
Draw a graph of your results.
Volume of HCl
pH
CONCLUSIONS:
What conclusions can be made from the results of your investigation? Here you can rewrite your hypothesis, but change it to reflect your findings if they are different from what you predicted earlier.
Were you able to confirm or reject your hypothesis?
Now that we have investigated a reaction between a metal carbonate (CaCO3) and an acid (HCl), we can write an equation for the reaction. We will begin by writing a general equation and end with one that matches the reaction that we have just investigated.
A video showing the reaction between metal carbonates and acids
The general equation for the reaction between an acid and a base is as follows:
acid + base → salt + water
If we replace 'base' with 'metal carbonate', we get:
acid + metal carbonate → salt + water
But wait, there was a third product in our reaction! Can you remember what it was? (Hint: Bubbles formed, so it was a gas.)
CO2
We need to make it clear that CO2 was a product of the reaction, so the correct general word equation would be:
acid + metal carbonate → salt + water + carbon dioxide
The type of salt that forms will depend on the specific acid and metal carbonate which were used in the reaction.
Now we are going to learn how to write equations for our actual reaction.
The following steps will guide you:
HCl
Calcium carbonate (CaCO3)
Write what remains of the base after we have taken away the CO3 to make CO2 and leave one O to make water.
Ca
Write what remains of the acid after we have taken away the H to make water. Remember we need two H to make one H2O and CaCO3 has only contributed one O.
2 HCl are needed, so 2 Cl will remain.
CaCl2
Now, let's put it all together, first the reactants, then the products:
2 HCl + CaCO3→ CaCl2 + H2O + CO2
Let's check quickly if the reaction is balanced.
2 H left and 2 H right. The H's are balanced.
2 Cl left and 2 Cl right. The Cl's are balanced.
3 O left and 3 O right. The O's are balanced.
1 C left and 1 C right. The C's are balanced.
Finally, let's use the chemical equation to write a word equation for the reaction:
hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
Calcium carbonate is found in many places outside of the laboratory. It is found in different types of rocks around the world, for example limestone, chalk and marble.
Calcium carbonate is also the main part of shells of various marine organisms, snails, pearls, oysters and bird eggshells. It is also found in the exoskeletons of crustaceans (such as crabs, prawns and lobsters).
Dark green leafy vegetables such as broccoli, kale and cabbage are a dietary source of calcium carbonate, providing the body with calcium. You can also take calcium carbonate in the form of tablet supplements.
Calcium carbonate also has many applications. In industry, the main application is in construction as it is used in various building materials and in cement. Calcium carbonate is used in many adhesives, paints and in ceramics. It is also used in swimming pools to adjust the pH. When do you think it would be added? If the pool was too acidic and you wanted to make it more basic, or if the pool was too basic and you wanted to make it more acidic?
CaCO3 forms a basic solution in water so it is used if the pH is too low (too acidic) and you want to make the pool water more basic.
Calcium carbonate is also used in agriculture in the form of lime powder. Agricultural lime is made by grinding up limestone or chalk. It is added to the soil if the soil is too acidic to increase the pH. It also provides plants with a source of calcium.
In this chapter we have investigated a number of reactions of acids with bases. We have learnt to write word equations for these reactions and practised converting between word and balanced chemical equations.
Some common laboratory acides are sulfuric acid (H2SO4), nitric acid (HNO3) and hydrochloric acid (HCl).
acid + metal oxide → salt + water
acid + metal hydroxide → salt + water
acid + metal carbonate → salt + water + carbon dioxide
Concept map
Complete the concept map by filling in the blank spaces..
This is the completed concept map.
Fill in the missing words in these sentences. Write the word on the line below. [10 marks]
To know if something is an acid or a base, we measure its _______.
pH
The name of the laboratory acid with the formula H2SO4, is _______.
sulfuric acid
The formula of the laboratory acid named hydrochloric acid, is _______.
HCl
When a metal oxide reacts with an ______, a salt and water will be formed.
acid
When a metal hydroxide reacts with an acid, a salt and _____ will be formed.
water
When a metal carbonate reacts with an acid, a salt, water and _______ will be formed.
carbon dioxide
Metal oxides, metal hydroxides and metal carbonates all dissolve in water, forming ______ solutions. This means the solutions will have pH values ____ than 7.
basic; greater
The reaction of an acid with a base is called a _______ reaction.
neutralisation
Non-metal oxides tend to form ______ solutions when they dissolve in water.
acidic
Write a short paragraph (3 or more sentences) to explain what you understand each of the following terms to mean, in your own words. [2 x 3 = 6 marks]
Learner's paragraph should contain at least the following ideas:
If they are mixed in the right amounts, they will neutraliseeach other.
Learner's paragraph should contain at least the following ideas:
For each of the following reactions, complete the tables by providing the missing equations.
The reaction between hydrochloric acid and magnesium oxide [4 marks]
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Word equation |
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Chemical equation |
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General equation |
acid + metal oxide → salt + water |
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Word equation |
hydrochloric acid + magnesium oxide → magnesium chloride + water |
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Chemical equation |
2 HCl + MgO → MgCl2 + H2O |
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General equation |
acid + metal oxide → salt + water |
The reaction between hydrochloric acid and sodium hydroxide [6 marks]
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Word equation |
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Chemical equation |
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General equation |
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Word equation |
hydrochloric acid + sodium hydroxide → sodium chloride + water |
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Chemical equation |
HCl + NaOH → NaCl + H2O |
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General equation |
acid + metal hydroxide → salt + water |
The reaction between hydrochloric acid and calcium carbonate [4 marks]
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Word equation |
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Chemical equation |
2 HCl + CaCO3 → CaCl2 + H2O + CO2 |
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General equation |
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Word equation |
hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide |
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Chemical equation |
2 HCl + CaCO3 → CaCl2 + H2O + CO2 |
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General equation |
acid + metal carbonate → salt + water + carbon dioxide |
The reaction between hydrochloric acid and magnesium hydroxide [4 marks]
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Word equation |
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Chemical equation |
2 HCl + Mg(OH)2 → MgCl2 + 2 H2O |
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General equation |
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Word equation |
hydrochloric acid + magnesium hydroxide → magnesium chloride + water |
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Chemical equation |
2 HCl + Mg(OH)2 → MgCl2 + 2 H2O |
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General equation |
acid + metal hydroxide → salt + water |
The reaction between hydrochloric acid and calcium oxide [4 marks]
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Word equation |
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Chemical equation |
2 HCl + CaO → CaCl2 + H2O |
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General equation |
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Word equation |
hydrochloric acid + calcium oxide → calcium chloride + water |
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Chemical equation |
2 HCl + CaO → CaCl2 + H2O |
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General equation |
acid + metal oxide → salt + water |
The reaction between hydrochloric acid and potassium hydroxide [6 marks]
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Word equation |
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Chemical equation |
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General equation |
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Word equation |
hydrochloric acid + potassium hydroxide → potassium chloride + water |
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Chemical equation |
HCl + KOH → KCl + H2O |
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General equation |
acid + metal hydroxide → salt + water |
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Word equation |
hydrochloric acid + sodium carbonate → sodium chloride + water + carbon dioxide |
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Chemical equation |
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General equation |
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Word equation |
hydrochloric acid + sodium carbonate → sodium chloride + water + carbon dioxide |
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Chemical equation |
2 HCl + Na2CO3 → 2 NaCl + H2O + CO2 |
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General equation |
acid + metal carbonate → salt + water + carbon dioxide |
Total [48 marks]
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Previous
Chapter 11: Acids, bases and the pH value
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Table of Contents |
Next
Chapter 13: Reactions of acids with metals
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