Exercises: 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)

In cement kilns, limestone is transformed according to $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$.

1. a) Classify the reaction according to its structure and justify your answer.
2. b) Name the two products, knowing that $\text{CaO}$ is quicklime used in construction.

When an iron nail is placed in a blue copper(II) sulfate solution, the nail becomes coated with a reddish deposit and the blue colour of the solution fades. The reaction is $\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}$. The situation is shown in the figure.

1. a) Classify the reaction according to its structure.
2. b) State which element replaces (displaces) which and explain which substance forms the reddish deposit.

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.

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?

In a blast furnace, iron is obtained according to $\text{Fe}_2\text{O}_3 + 3\ \text{CO} \rightarrow 2\ \text{Fe} + 3\ \text{CO}_2$. Recall the classical definition: the substance that gains oxygen is oxidised and the one that loses oxygen is reduced.

1. a) Which substance is reduced? Justify your answer by comparing its formula before and after.
2. b) Which substance is oxidised?
3. c) The reducing agent is the substance that causes the reduction of another. Which acts as the reducing agent here?

Classify each reaction according to its structure (synthesis, decomposition, displacement or double displacement), reasoning your answer based on the number and type of substances involved:

1. a) $2\ \text{SO}_2 + \text{O}_2 \rightarrow 2\ \text{SO}_3$ (key stage in the manufacture of sulfuric acid)
2. b) $2\ \text{KClO}_3 \rightarrow 2\ \text{KCl} + 3\ \text{O}_2$ (preparation of oxygen in the laboratory)
3. c) $\text{Cl}_2 + 2\ \text{KBr} \rightarrow 2\ \text{KCl} + \text{Br}_2$ (extraction of bromine from brines)
4. d) $\text{BaCl}_2 + \text{Na}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\ \text{NaCl}$ (sulfate analysis in water quality control)

In masonry, quicklime (calcium oxide) is "slaked" by adding water to it, obtaining slaked lime (calcium hydroxide), releasing a lot of heat.

1. a) Write the equation for the reaction from the formulas of the reactants and product and balance it.
2. b) Classify it according to its structure and justify your answer.
3. c) Explain, in view of the heat given off, why the vessel in which the lime is slaked becomes so hot.

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 demonstration known as the "silver tree", a copper wire immersed in a silver nitrate solution gradually becomes covered with grey silver crystals, while the solution turns bluish due to the copper(II) nitrate formed. The setup is shown in the figure.

1. a) Write the equation (reactants: copper and silver nitrate; products: copper(II) nitrate, $\text{Cu(NO}_3\text{)}_2$, and silver) and balance it.
2. b) Classify it according to its structure and state which metal displaces which.

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.

Milk of magnesia, an antacid sold in pharmacies, contains magnesium hydroxide, which neutralises excess hydrochloric acid in the stomach: $\text{Mg(OH)}_2 + \text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O}$ (unbalanced equation).

1. a) Balance the equation.
2. b) Classify it as a neutralisation and, at the same time, according to its structure, as a double displacement, explaining both labels.
3. c) Identify the salt formed.

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.

Compare these two formation reactions of sodium chloride:

1. a) $2\ \text{Na} + \text{Cl}_2 \rightarrow 2\ \text{NaCl}$ (synthesis from the elements)
2. b) $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$ (neutralisation)

Assign the oxidation numbers of sodium and chlorine on both sides of each equation and decide which of the two is a redox process (with electron transfer) and which is not, justifying your answer.

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 aluminium 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 sulfur dioxide emitted by old thermal power plants is transformed in the atmosphere according to $2\ \text{SO}_2 + \text{O}_2 \rightarrow 2\ \text{SO}_3$, a step prior to the formation of acid rain.

1. a) Classify the reaction by its structure (synthesis, decomposition, substitution or double substitution).

1. b) Assign the oxidation numbers of sulfur ($+4$ in $\text{SO}_2$, $+6$ in $\text{SO}_3$) and of oxygen ($0$ in $\text{O}_2$, $-2$ when combined) and show that it is also a redox process, indicating what is oxidized and what is reduced.

1. c) Conclude whether the same reaction can belong to two types depending on the criterion used, and give another example, different from this one, of a reaction that is both of a structural type (synthesis, decomposition, substitution…) and a redox process. Indicate in it which species is oxidized and which is reduced.

The corrosion of steel on ships can be summarised as $4\ \text{Fe} + 3\ \text{O}_2 \rightarrow 2\ \text{Fe}_2\text{O}_3$. To protect the hulls, zinc blocks called sacrificial anodes are attached to them, which wear away instead of the iron. The arrangement of the blocks is shown in the figure.

1. a) Classify the corrosion reaction by its structure and, by assigning oxidation numbers ($0$ in the elements, $+3$ and $-2$ in the oxide), check that it is also redox: what is oxidised?
2. b) Zinc is oxidised more easily than iron (it gives up its electrons first). Reasoning with this fact, explain why the zinc blocks prevent the hull from rusting.
3. c) Explain why these blocks must be replaced periodically and what would happen if they were not replaced.

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).

A student states that the neutralisation $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$ is a redox reaction "because particles are exchanged in it".

1. a) Assign the oxidation numbers of all elements (H: $+1$; Cl: $-1$; Na: $+1$; O: $-2$) in each substance on both sides.
2. b) Check if any change when moving from reactants to products.
3. c) Evaluate the student's statement: explain the difference between the exchange of ions in a double displacement and the transfer of electrons in a redox process.

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.

Photosynthesis can be summarised as $6\ \text{CO}_2 + 6\ \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\ \text{O}_2$.

1. a) Try to classify it by its structure (synthesis, decomposition, displacement or double displacement) and comment on why it does not fit neatly into any of the four types.
2. b) Assign the oxidation numbers of carbon ($+4$ in $\text{CO}_2$; average value $0$ in glucose) and oxygen ($-2$ in water; $0$ in $\text{O}_2$) and identify which element is oxidised and which is reduced.
3. c) Conclude which classification criterion (structure or electron transfer) is more general for processes such as photosynthesis or cellular respiration, which is its reverse reaction.

Sodium chloride is to be obtained in the laboratory via three different routes. Write and balance an equation for each type and classify it:

1. a) A synthesis from the elements sodium and chlorine.
2. b) A neutralisation between a suitable acid and base.
3. c) A double displacement between sodium carbonate and calcium chloride, in which calcium carbonate precipitates.
4. d) Common salt is obtained industrially by evaporating seawater in salt pans: explain why this industrial process is not a chemical reaction, even though it produces the same substance.