Stoichiometry in industry, the environment and society

Cement factories heat limestone in large kilns: $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$.

1. a) Identify the reactant and products of the reaction.
2. b) Explain why a cement plant emits carbon dioxide even if the kiln were heated using only renewable electricity: where does this gas come from?

In a blast furnace, iron ore reacts with carbon monoxide: $\text{Fe}_2\text{O}_3 + 3\ \text{CO} \rightarrow 2\ \text{Fe} + 3\ \text{CO}_2$.

1. a) Identify the reactants and products.
2. b) Explain in a sentence what carbon monoxide does to iron oxide: which substance gains its oxygen?

A thermal power plant burns coal, which to simplify is carbon: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$. Calculate what mass of carbon dioxide is emitted for every $12\ \text{kg}$ of carbon burned (atomic masses: $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$).

In a test, an industrial boiler burns $120\ \text{kg}$ of coal, which can be considered pure carbon: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$. Calculate, by converting through moles or using proportions, the mass of carbon dioxide emitted (atomic masses: $\text{C} = 12\ \text{u}$; $\text{O} = 16\ \text{u}$).

The production manager of a chemical plant says that their reactor operates "with a yield of $90\ \%$".

1. a) Explain in your own words what this figure means, thinking about the difference between what the balanced equation predicts and what actually comes out of the reactor.
2. b) Reason why in practice the yield of an industrial process never reaches exactly $100\ \%$.

The ammonia manufactured in large plants, $\text{N}_2 + 3\ \text{H}_2 \rightarrow 2\ \text{NH}_3$, is the raw material for fertilisers.

1. a) How many moles of ammonia are obtained, at most, from $1\ \text{mol}$ of nitrogen?
2. b) Explain why this reaction is said to "fix" nitrogen from the air and why it is so important for agriculture.

So-called green hydrogen is obtained by the electrolysis of water: $2\ \text{H}_2\text{O} \rightarrow 2\ \text{H}_2 + \text{O}_2$.

1. a) What mass of hydrogen is obtained by decomposing $2\ \text{mol}$ of water? (molar mass: $\text{H}_2 = 2\ \text{g/mol}$).
2. b) Explain what condition the electricity used must satisfy for this hydrogen to be called "green".

Some coals contain sulphur as an impurity, which reacts when burned: $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$.

1. a) Calculate what mass of sulphur dioxide is formed when burning $32\ \text{g}$ of sulphur (molar masses: $\text{S} = 32\ \text{g/mol}$; $\text{SO}_2 = 64\ \text{g/mol}$).
2. b) Name the environmental problem associated with emissions of $\text{SO}_2$.

To clean the flue gases from a thermal power station, the smoke is passed through a bed of limestone, which traps the sulfur dioxide: $\text{SO}_2 + \text{CaCO}_3 \rightarrow \text{CaSO}_3 + \text{CO}_2$. The power station emits $128\ \text{kg}$ of $\text{SO}_2$ per day. Calculate, using moles or proportions, the mass of calcium carbonate needed daily to trap all of it (molar masses: $\text{SO}_2 = 64\ \text{g/mol}$; $\text{CaCO}_3 = 100\ \text{g/mol}$).

In photosynthesis, plants manufacture glucose and release oxygen: $6\ \text{CO}_2 + 6\ \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\ \text{O}_2$. The proportion in the equation is: for every $264\ \text{g}$ of $\text{CO}_2$ absorbed, $192\ \text{g}$ of $\text{O}_2$ are released. It is estimated that the trees in an urban park fix $1\,320\ \text{kg}$ of $\text{CO}_2$ per year, as shown in the figure.

Calculate:

1. a) The mass of oxygen they return to the air in one year.
2. b) Comment on why urban parks are considered the "lungs" of a city.