Exercises: Chemical equations and balancing

When lighting a gas ring on a cooker, methane ($\text{CH}_4$) reacts with oxygen in the air and turns into carbon dioxide and water vapour. The diagram shows the particles of each substance:

Identify the reactants and products of this reaction.

Complete the sentence with the missing words: 'In a chemical equation, the substances that are transformed are called $\dots$ and are written to the $\dots$ of the arrow; the new substances formed are called $\dots$ and are written to the $\dots$.'

Read equation $\text{N}_2 + 3\ \text{H}_2 \rightarrow 2\ \text{NH}_3$ carefully and explain in your own words what it tells us: how many molecules of each substance react and how many molecules are formed.

What does the subscript $2$ in $\text{H}_2$ mean? And the coefficient $2$ in front of $2\ \text{H}_2\text{O}$? Explain in your own words the difference between a subscript and a coefficient.

Nadia says: 'When paper burns, matter disappears because the ash weighs much less than the original paper.' Is she right? Reason your answer considering all the substances involved and formed, including gases.

Why must chemical equations be balanced before using them? Name the law on which balancing is based and explain it in a simple sentence about what happens to atoms in a reaction.

Count the hydrogen and oxygen atoms on each side of equation $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$, represented with particles in the figure, and state, without changing it yet, whether it is balanced or not. Justify your answer.

Balance the equation for the formation of water by inspection: $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$. Write the coefficients needed for each substance.

The natural gas in many homes is methane. Balance its combustion by inspection: $\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$. State the coefficients in the order in which the substances appear.

Ammonia is manufactured industrially from nitrogen and hydrogen and then used to make fertilisers. Balance the equation: $\text{N}_2 + \text{H}_2 \rightarrow \text{NH}_3$.

The magnesium in sparklers and fireworks burns with a very intense white light, forming magnesium oxide. Balance the equation: $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$.

When iron rusts completely, iron(III) oxide is formed. Balance the equation: $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$. Then check by counting the atoms of each element on both sides.

How many atoms of each element are there in total in $3\ \text{CO}_2$? And in $2\ \text{H}_2\text{O}$? The figure shows these molecules:

Explain how atoms are counted when a formula has a coefficient in front of it.

In a closed container, $12\ \text{g}$ of carbon are completely burned with $32\ \text{g}$ of oxygen, forming carbon dioxide. The diagram represents the closed container before and after the reaction:

What mass of carbon dioxide is obtained? Which law allows you to state this without needing to weigh it?

The balanced equation for the combustion of methane is $\text{CH}_4 + 2\ \text{O}_2 \rightarrow \text{CO}_2 + 2\ \text{H}_2\text{O}$. If $5$ methane molecules are burned:

1. a) How many oxygen molecules are consumed?
2. b) How many carbon dioxide molecules and how many water molecules are formed?

To 'balance' the equation $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$, a student changes the product and writes $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}_2$. Explain why this trick is not allowed: what substance $\text{H}_2\text{O}_2$ actually is and what changing a subscript in a formula means.

The butane in gas cylinders, $\text{C}_4\text{H}_{10}$, burns with oxygen in the air, forming carbon dioxide and water.

1. a) Balance the equation: $\text{C}_4\text{H}_{10} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$.
2. b) Verify, by counting the carbon, hydrogen and oxygen atoms on both sides, that your balanced equation satisfies the conservation of atoms.

In a closed container, $10\ \text{g}$ of a substance A react completely with $5\ \text{g}$ of a substance B to produce $12\ \text{g}$ of a substance C together with a certain mass of a substance D. The diagram summarises the known masses:

1. a) What mass of D has been formed?
2. b) Justify your calculation by stating the law you applied.

We place a lit candle on a precision balance in two situations:

1. a) In the open air: the reading on the balance decreases as the candle is consumed.
2. b) Inside an airtight glass bell jar: the reading barely changes.

The figure shows the two set-ups:

Explain both results using the law of conservation of mass, and reason whether matter really 'disappears' in either case.

Hydrogen peroxide from the first-aid kit, $\text{H}_2\text{O}_2$, gradually decomposes into water and oxygen.

1. a) Balance the equation: $\text{H}_2\text{O}_2 \rightarrow \text{H}_2\text{O} + \text{O}_2$.
2. b) The bottle of hydrogen peroxide states that it should be kept tightly closed and in a cool, dark place. Relate these instructions to the reaction above and explain what would happen if they were not followed.

The thin layer that protects aircraft aluminium parts from corrosion is formed when the metal reacts with oxygen in the air.

1. a) Balance the equation: $\text{Al} + \text{O}_2 \rightarrow \text{Al}_2\text{O}_3$.
2. b) Explain why, when balancing by inspection, it is advisable to start with an element that appears in only one substance on each side of the arrow.

The balanced equation for the formation of water is $2\ \text{H}_2 + \text{O}_2 \rightarrow 2\ \text{H}_2\text{O}$. Someone suggests dividing all coefficients by two and writing $\text{H}_2 + \frac{1}{2}\ \text{O}_2 \rightarrow \text{H}_2\text{O}$.

1. a) Would conservation of atoms still hold?
2. b) What is the problem with referring to 'half a molecule' of oxygen?
3. c) Justify why, by convention, the coefficients of a balanced equation are the simplest possible integers.

Explain, using the particle model, exactly what happens to atoms in a chemical reaction: what happens to the atoms of the reactants, why their total number does not change, and how a chemical reaction differs from simply mixing substances. Illustrate your explanation with the reaction of hydrogen with oxygen to form water, represented using particles in the figure:

Some rockets use hydrogen as fuel, which burns with oxygen to form water vapour.

1. a) Write the equation for the reaction and balance it.
2. b) If $100$ hydrogen molecules are burned in a ground test, how many oxygen molecules are consumed and how many water molecules are formed?
3. c) How many hydrogen atoms are there in total before the reaction? And after? Relate your answer to the law of conservation.