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Chemical misconceptions 33
Word equations
Target level
These materials are designed for use with 11–14 year old students who have been taught to use word equations. The materials will also be useful for 14–16 year olds students who need to revisit this topic.
Topic
Using word equations to represent chemical reactions.
Rationale
Many students find it difficult to write word equations, which require an appreciation of the nature of chemical change (including conservation of matter), and familiarity with chemical names and the patterns of common reaction types. These materials provide probes for exploring whether students can complete word equations, and a set of practice exercises. These ideas are discussed in Chapter 9 of the Teachers’ notes. In the pilot teachers judged the materials ‘excellent’, ‘very useful’ and ‘helpful for revision’. Some teachers found the responses of some of their pupils to be ‘disappointing’ (or even ‘shocking’). The probes were thought to provide an interesting ‘look into [students’] minds’ and to lead to useful classroom discussion. Although teachers found it useful that students were asked to give reasons for their answers, some of the students did not like having to try to explain their reasons. (Some teachers may wish to ask students to just complete the equations in the probes, and to leave the spaces for making notes when going through the answers.) Students were reported to find the materials helpful and easy to follow, and were considered to have greater understanding afterwards.
Instructions
These materials may either be used with students who should have mastered word equations as a pre-test (to identify students needing practice), a remedial exercise, and a post-test; or as end-of-topic review material with students meeting word equations for the first time. The materials include: Completing word equations (1) A probe to explore whether students can complete word equations. Word equations A study exercise including an information sheet providing practice in completing word equations for five common types of reaction. This exercise could be set as a private study (homework) task. Completing word equations (2) A probe to explore whether students can complete word equations.
34 Word equations
Resources
Student worksheets
- Completing word equations (1)
- Word equations – Information sheet
- The reaction between acids and alkalis
- The reaction between acids and metals
- The reaction between acids and metal carbonates
- Displacement reactions
- Synthesis reactions
- Completing word equations (2)
Feedback
Teachers’ answer sheets are included for each of the probes, and the study activity.
36 Word equations
Word equations – the reaction between acids and metal carbonates
1. Copper chloride, Copper sulfate Zinc chloride, Zinc nitrate Iron nitrate, Iron sulfate 2. Magnesium chloride
Word equations – Displacement reactions
1. Iron 2. Zinc sulfate 3. Magnesium, copper 4. Copper sulfate 5. Zinc chloride, iron 6. Iron sulfate 7. Zinc, copper nitrate 8. Copper 9. Iron nitrate, copper It may be worth pointing out that the order of the reactants or products in the equation is irrelevant.
Word equations – synthesis reactions
1. Sulfur Fluoride 2. Magnesium sulfide, Magnesium chloride Iron fluoride, Iron chloride Zinc oxide, Zinc sulfide, Zinc fluoride Copper oxide, Copper sulfide, Copper chloride 3. Sodium iodide Nickel sulfide Calcium
Completing word equations (2)
Teachers will have their own ideas about what is an acceptable explanation for students’ completions, depending upon the age of the students and the depth to which ideas about reactions have been studied. Suggested explanations are provided.
1. sulfuric acid + sodium hydroxide → sodium sulfate + water In this type of reaction a salt is formed from an acid and an alkali. The reactants include elements which must also be present in the products. As the reacting alkali is sodium hydroxide, then the product will be a sodium salt. As the reacting acid is sulfuric acid, then the salt produced will be a sulfate. 2. iron + copper chloride → iron chloride + copper In this type of reaction a more reactive metal displaces a less reactive metal from its salt. The reactants include elements which must also be present in the products. As the displaced metal is copper, then the reacting salt must have been a copper salt. As the product was a chloride, then the reacting salt will also be a chloride.
Chemical misconceptions 37
3. hydrochloric acid + magnesium carbonate → magnesium chloride + water + carbon dioxide In this type of reaction an acid reacts with a carbonate to give a salt, water and carbon dioxide. The reactants include elements which must also be present in the products. As the product is a chloride, then the reacting acid must be hydrochloric acid. 4. zinc + oxygen → zinc oxide In this type of reaction a binary compound is prepared from two elements. The reactants include elements which must also be present in the products. 5. calcium + nitric acid → calcium nitrate + hydrogen In this type of reaction an acid reacts with a metal to produce a salt and hydrogen. The reactants include elements which must also be present in the products. As the reacting acid is nitric acid, then the salt produced will be a nitrate. As the reacting metal is calcium, then the product must be a calcium salt.
Word equations –
information sheet
Word equations are a shorthand used to describe chemical reactions. Although there are many millions of possible chemical reactions you are not expected to know about them all. It is useful to remember that many reactions are of similar types.
1. Metal + acid
One type of reaction is that between a metal and an acid. When a metal reacts with an acid the reaction produces a salt, and hydrogen gas is released. The salt produced depends upon the metal and the acid. If magnesium reacts with hydrochloric acid, then the salt produced is magnesium chloride. metal + acid → salt + hydrogen
2. Metal carbonate + acid
Metal carbonates also react with acid, to give a salt. When a carbonate reacts with acid the gas carbon dioxide is given off. The salt produced depends upon which acid, and which metal carbonate react. If zinc carbonate reacts with sulfuric acid, then the salt produced is zinc sulfate. metal carbonate + acid → salt + carbon dioxide + water
3. Acid + alkali
When an alkali and an acid react the product is a salt solution. The particular salt produced depends upon which acid and which alkali reacted. When nitric acid reacts with potassium hydroxide the salt produced is potassium nitrate. acid + alkali → salt + water
4. Metal + salt solution
When a reactive metal is placed in the solution of a salt of a less reactive metal, a ‘displacement’ reaction occurs. The more reactive metal is said to displace the less reactive metal from solution. For example zinc is added to copper nitrate solution the copper is displaced and the solution will contain zinc nitrate.
5. Element + element
When two elements react together to form a compound the compound is given a name to show which elements reacted. So if calcium reacts with chlorine the compound is called calcium chloride.
These examples show you the patterns that are found in five important types of reaction. If you look for patterns you should find it easier to work out how to complete word equations. In this worksheet you will find some examples to help you practice thinking about word equations.
Word equations – information sheet – page 1 of 1 H P
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Word equations – the reaction
between acids and alkalis
When an acid reacts with an alkali, a salt and water is produced:
acid + alkali → salt + water
for example
hydrochloric acid + sodium hydroxide → sodium chloride + water
The salt that is produced depends upon which acid and which alkali react. The following table provides a summary of the name of the salt produced by different reactions between acids and alkalis.
1. Complete the table (Hint – look for the patterns)
Name of alkali Name of acid Sodium hydroxide Potassium hydroxide Hydrochloric acid Sodium chloride
Nitric acid Potassium nitrate
Sulfuric acid Sodium sulfate Potassium sulfate
Salts produced when acids react with alkalis
2. Complete the following word equation
(acid + alkali → salt + water)
nitric acid + _______________________________ → potassium nitrate + water
H P Word equations – the reaction between acids and alkalis – page 1 of 1
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Word equations – the reaction
between acids and metal carbonates
When an acid reacts with a metal carbonate, a salt, water and carbon dioxide are produced:
Acid + metal carbonate → salt + water + carbon dioxide
for example
sulfuric acid + zinc carbonate → zinc sulfate + water + carbon dioxide
The salt that is produced depends upon which acid and which metal react. The following table provides a summary of the name of the salt produced by different reactions between acids and metal carbonates.
1. Complete the table (Hint – look for the patterns)
Name of metal carbonate Name of acid Hydrochloric acid Nitric acid Sulfuric acid Copper carbonate Copper nitrate
Zinc carbonate Zinc sulfate
Iron carbonate Iron chloride
2. Complete the following word equation
(acid + metal carbonate → salt + water + carbon dioxide )
hydrochloric acid + magnesium carbonate → ________________________________ + water + carbon dioxide
H P Word equations – the reaction between acids and metal carbonates – page 1 of 1
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Word equations –
displacement reactions
When a reactive metal is added to a solution containing the salt of a less reactive metal, a reaction occurs.
See if you can complete the following word equations (Hint – look for the patterns).
eg magnesium + iron chloride → magnesium chloride + iron
1. magnesium + iron nitrate → magnesium nitrate + __________________________ 2. magnesium + _______________________________ → magnesium sulfate + zinc 3. _________________ + copper sulfate → magnesium sulfate + ________________ 4. zinc + ____________________________ → zinc sulfate + copper 5. zinc + iron chloride → ______________________ + ________________________ 6. zinc + ___________________________ → zinc sulfate + iron 7. ______________________ + ______________________ → zinc nitrate + copper 8. iron + copper sulfate → iron sulfate + __________________________ 9. iron + copper nitrate → ______________________ + ______________________
Word equations – displacement reactions – page 1 of 1 H P
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Completing word equations (2)
Word equations are used to describe chemical reactions. Look at the word equations below. In each case complete the word equation by adding the name of the missing substance. (Explain your answers.)
1. sulfuric acid + sodium hydroxide → ______________________________________ + water
I think this is the answer because
2. iron + ______________________________________ → iron chloride + copper
I think this is the answer because
3. ___________________________ acid + magnesium carbonate → magnesium chloride + water + carbon dioxide
I think this is the answer because
4. zinc + oxygen → ____________________________________
I think this is the answer because
5. calcium + nitric acid → _____________________________________ + hydrogen
I think this is the answer because
Completing word equations (2) – page 1 of 1 H P
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46 Word equations
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