Classification of Chemical Reactions

The basic reaction in the fertiliser industry is the synthesis of ammonia: $\text{N}_2 + 3\ \text{H}_2 \rightarrow 2\ \text{NH}_3$. The diagram represents it using the particle model:

1. a) Which type does it belong to according to its structure: synthesis, decomposition, displacement or double displacement?
2. b) Justify your answer based on the number of reactants and products in the equation.

Classify each of these reactions as synthesis, decomposition, displacement or double displacement, justifying your choice in each case:

1. a) $2\ \text{H}_2 + \text{O}_2 \rightarrow 2\ \text{H}_2\text{O}$ (hydrogen engine)
2. b) $2\ \text{H}_2\text{O} \rightarrow 2\ \text{H}_2 + \text{O}_2$ (electrolysis of water on the International Space Station to obtain breathable oxygen)
3. c) $\text{Zn} + 2\ \text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$ (zinc with acid in the laboratory)
4. d) $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$ (neutralisation)

Hydrogen peroxide in first-aid kits ($\text{H}_2\text{O}_2$) decomposes slowly into water and oxygen, which is why old bottles lose their disinfectant effectiveness.

1. a) Write and balance the decomposition equation: $\text{H}_2\text{O}_2 \rightarrow \text{H}_2\text{O} + \text{O}_2$.
2. b) Classify the reaction according to its structure and justify your answer.
3. c) The packaging states "store in a cool, dark place": reason which phenomenon they want to slow down with this advice.

In the laboratory, when mixing separate colourless solutions of silver nitrate and sodium chloride, a white solid appears and settles at the bottom of the tube: $\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3$. The tube is shown in the figure.

1. a) Explain what a precipitate is and identify which one it is in this reaction.
2. b) Reason why this reaction is classified as double displacement (or exchange).

Hydrochloric acid and sodium hydroxide react according to $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$.

1. a) Identify which reactant is the acid and which is the base.
2. b) Name the two types of substances that are always formed in a neutralisation reaction.
3. c) State which structural type (synthesis, decomposition, displacement or double displacement) neutralisation belongs to.

When mixing colourless solutions of lead(II) nitrate and potassium iodide in a beaker, a striking yellow precipitate of lead(II) iodide appears; potassium nitrate remains dissolved in the solution. Observe the diagram:

1. a) Write and balance the reaction equation.
2. b) Classify it according to its structure and justify why it is called a precipitation reaction.

Flares and old photographic flashbulbs take advantage of the fact that magnesium burns with a very intense white light: $2\ \text{Mg} + \text{O}_2 \rightarrow 2\ \text{MgO}$. According to the classical definition, an element is oxidised when it gains oxygen.

1. a) Which substance is oxidised in this reaction? Justify your answer.
2. b) The substance that supplies the oxygen is called the oxidising agent. What is the oxidising agent here?

The combustion of natural gas is written as $\text{CH}_4 + 2\ \text{O}_2 \rightarrow \text{CO}_2 + 2\ \text{H}_2\text{O}$. Use oxidation numbers: in $\text{CH}_4$ carbon acts with $-4$, in $\text{CO}_2$ with $+4$; oxygen is $0$ in $\text{O}_2$ and $-2$ in the products.

1. a) State which element increases its oxidation number (is oxidised) and which decreases it (is reduced).
2. b) The one that gives up electrons is the reducing agent and the one that accepts them is the oxidising agent: identify them.
3. c) Conclude whether all combustion is, fundamentally, an oxidation-reduction reaction.

To weld railway tracks in situ, the thermite reaction is used: $\text{Fe}_2\text{O}_3 + 2\ \text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\ \text{Fe}$, so exothermic that the iron is obtained molten. The welding setup is shown in the figure.

1. a) Classify it by its structure (synthesis, decomposition, displacement or double displacement) and justify your answer.
2. b) Assign the oxidation numbers of iron and aluminum on both sides ($+3$ in the oxides, $0$ in the metals) and identify the oxidising agent and the reducing agent.
3. c) Relate the exothermic nature of the reaction to its usefulness for this welding and explain why it does not matter that the other product is an oxide.

The reactivity series of some metals (from most to least reactive) is: $\text{K} > \text{Ca} > \text{Na} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Pb} > \text{H} > \text{Cu} > \text{Ag}$. A metal displaces from its compounds those to its right in the series, and only those situated to the left of hydrogen react with acids releasing $\text{H}_2$.

Predict, justifying using the series, whether a reaction will occur in each case and write the balanced equation where applicable:

1. a) Copper with hydrochloric acid.
2. b) Zinc with hydrochloric acid (products: zinc chloride and hydrogen).
3. c) Silver with copper(II) sulfate solution.
4. d) Zinc with copper(II) sulfate solution (products: zinc sulfate and copper).

The well-known "effervescent volcano" experiment (and effervescent antacids) mixes a carbonate with an acid. With sodium carbonate and hydrochloric acid, the overall reaction is $\text{Na}_2\text{CO}_3 + 2\ \text{HCl} \rightarrow 2\ \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2$.

1. a) Classify the overall reaction by its structure.
2. b) In reality, the exchange first produces carbonic acid, $\text{H}_2\text{CO}_3$, an unstable compound that decomposes into water and carbon dioxide: write and balance this second equation and classify it.
3. c) Conclude that a transformation observed in the laboratory can be the linking of two reactions of different types, and relate the bubbling to the gas formed.