IGCSE Chemistry 0620 Paper 42, May/June 2025

percentage of nitrogen in clean, dry air

typical operating temperature of the Contact process in °C

number of metals in Period 3 of the Periodic Table

number of halogens which are gases at r.t.p.

typical temperature for the fermentation of aqueous glucose in °C

number of unbranched structural isomers of ${\mathrm{C}}_4{\mathrm{H}}_8$ which decolourise aqueous bromine

number of covalent bonds in one molecule of ethanol

number of carbon atoms in one molecule of propyl butanoate.

State what is meant by a covalent bond.

Chlorine(I) oxide, ${\mathrm{C}\mathrm{l}}_2\mathrm{O}$, is a simple molecule with covalent bonds. State what is meant by (I) in the name chlorine(I) oxide.

Complete the dot-and-cross diagram in Fig. 2.1 of a molecule of chlorine(I) oxide. Show outer electrons only.

Explain, in terms of structure and bonding, why ${\mathrm{C}\mathrm{l}}_2\mathrm{O}$ boils at a low temperature and does not thermally decompose into its constituent elements, ${\mathrm{C}\mathrm{l}}_2$ and ${\mathrm{O}}_2$.

Give two reasons why liquid ${\mathrm{C}\mathrm{l}}_2\mathrm{O}$ is a poor conductor of electricity.

Carbon and silicon(IV) oxide both exist as giant covalent structures. Name a giant covalent structure of carbon which conducts electricity.

Identify the particles responsible for the conduction of electricity in this covalent structure of carbon.

Silicon(IV) oxide contains silicon atoms, $\mathrm{S}\mathrm{i}$, and oxygen atoms, $\mathrm{O}$. Fig. 2.2 shows part of the giant covalent structure of silicon(IV) oxide. Complete the diagram in Fig. 2.2 by adding the symbol for each of the 9 atoms shown.

Complete the definition of electrolysis by filling in the missing words. The decomposition of ........................................ compounds when ........................................ or ........................................ by the passage of an electric current.

Electrolysis of aqueous copper(II) sulfate is carried out using the apparatus shown in Fig. 3.1. The electrodes are made of platinum. State whether the mass of the cathode increases, decreases or remains the same when platinum electrodes are used.

Describe the change in appearance, if any, of the electrolyte when platinum electrodes are used.

Describe what is seen at the anode when platinum electrodes are used.

Write the ionic half-equation for the reaction at the anode.

The electrolysis is repeated using copper electrodes. State whether the mass of the cathode increases, decreases or remains the same when copper electrodes are used.

Describe the change in appearance, if any, of the electrolyte when copper electrodes are used.

Aluminium is extracted from its purified ore by electrolysis. Name this ore of aluminium.

Name the substance added to this process to reduce the operating temperature.

Explain why the anodes need to be continually replaced during this process.

The main ore of chromium is chromite. Chromite contains ${\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_4$. ${\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_4$ reacts with carbon. Complete the equation for this reaction. $${\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_4+4\mathrm{C}\to \mathrm{F}\mathrm{e}+2\mathrm{C}\mathrm{r}+\text{..............................}$$

Suggest one disadvantage of extracting chromium by reacting chromite with carbon.

Chromium can be mixed with nickel and other elements to form stainless steel. Name the type of substance formed when a metal is mixed with other elements.

$({\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7$ is a compound containing chromium. The negative ion in $({\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7$ is ${\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7^{2-}$. State the sum of the oxidation numbers in the ${\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7^{2-}$ ion.

The oxidation number of each $\mathrm{O}$ in ${\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7^{2-}$ ions is $-2$. Determine the oxidation number of each $\mathrm{C}\mathrm{r}$ in ${\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7^{2-}$ ions. Show your working.

When $({\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7$ is heated the following reaction occurs. $$({\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7\to {\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_3+4{\mathrm{H}}_2\mathrm{O}+{\mathrm{N}}_2$$ Calculate the volume of nitrogen gas produced at r.t.p., in ${\mathrm{c}\mathrm{m}}^3$, when $1.26\mathrm{g}$ of $({\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7$ is heated using the following steps. The $M_r$ of $({\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7$ is 252. • Calculate the number of moles of $({\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7$ used. .............................. mol • Determine the number of moles of ${\mathrm{N}}_2$ formed. .............................. mol • Calculate the volume of ${\mathrm{N}}_2$ formed at r.t.p. in ${\mathrm{c}\mathrm{m}}^3$. .............................. ${\mathrm{c}\mathrm{m}}^3$

State the group number of the alkali metals.

Identify the alkali metal which: has the highest melting point

Identify the alkali metal which: has the highest density

Identify the alkali metal which: has the lowest reactivity

Identify the alkali metal which: burns with a lilac flame

Identify the alkali metal which: is found in fertilisers to improve plant growth.

Sodium sulfide, ${\mathrm{N}\mathrm{a}}_2\mathrm{S}$, is an ionic compound. Complete Fig. 5.1 to show the electronic configurations of the ions in sodium sulfide. Show the charges on the ions.

Rubidium has only two naturally occurring atoms, ${}^{85}\mathrm{R}\mathrm{b}$ and ${}^{87}\mathrm{R}\mathrm{b}$. State the term given to these naturally occurring atoms of rubidium.

Complete Table 5.1 to show the number of protons, neutrons and electrons in the atom and ion of rubidium shown. Table 5.1

The relative atomic mass of rubidium to one decimal place is 85.5. Determine the relative abundance of ${}^{85}\mathrm{R}\mathrm{b}$ in rubidium. Express your answer as a percentage.

State the term for a family of similar compounds which contain the same functional group.

Determine the difference in relative molecular mass between ${\mathrm{C}}_2{\mathrm{H}}_4$ and the next member in this family of similar compounds.

Write the symbol equation for the complete combustion of ${\mathrm{C}}_2{\mathrm{H}}_4$.

${\mathrm{C}}_2{\mathrm{H}}_4$ reacts with steam to form ethanol. $${\mathrm{C}}_2{\mathrm{H}}_4(\mathrm{g})+{\mathrm{H}}_2\mathrm{O}(\mathrm{g})\rightleftharpoons {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}(\mathrm{g})\mathrm{\Delta }H=-45\mathrm{k}\mathrm{J}\mathrm{/}\mathrm{m}\mathrm{o}\mathrm{l}$$ The process happens in a closed system and the reaction reaches an equilibrium. The conditions for this process are $300{}^{\circ }\mathrm{C}$ and $60\mathrm{a}\mathrm{t}\mathrm{m}$ pressure. ${\mathrm{H}}_3{\mathrm{P}\mathrm{O}}_4$ is used as a catalyst. Complete Table 6.1 to show the effect, if any, on the concentration of ${\mathrm{C}}_2{\mathrm{H}}_4(\mathrm{g})$ when the following changes to the conditions are applied. Only use the words increases, decreases or no change. Table 6.1

Explain, in terms of collision theory, why the rate of the forward reaction increases if the temperature increases.

Compound B has the displayed formula shown in Fig. 6.1. Deduce the molecular formula of compound B.

State why compound B is unsaturated.

Draw the structure of one repeat unit of the polymer formed when compound B undergoes addition polymerisation.

Explain why $1\mathrm{m}\mathrm{o}\mathrm{l}$ of compound B reacts with $2\mathrm{m}\mathrm{o}\mathrm{l}$ of sodium hydroxide, $\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}$.

Calculate the volume, in ${\mathrm{c}\mathrm{m}}^3$, of $0.250{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{/}\mathrm{d}\mathrm{m}}^3$ $\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}$ that reacts with $0.100\mathrm{m}\mathrm{o}\mathrm{l}$ of compound B.