4CH1/2CR — January 2021 (Edexcel IGCSE Chemistry): Past Paper

Substances can exist as solids, liquids or gases.

Give the change of state that occurs when a substance melts.

Complete the word equation for the sublimation of iodine. iodine (s) → iodine ( )

The circle in the diagram represents a particle. Complete the diagram to show the arrangement of particles in a gas.

The table lists some statements about particles. Place ticks (✓ ) in boxes to show which two statements are correct for water particles. Statement Tick the particles only vibrate the particles do not move the particles have no gaps between them the particles move randomly the particles have more energy than in ice the particles have a regular arrangement

This question is about elements in Group 7 and their compounds. The table gives information about some of these elements

Predict the colour of fluorine at room temperature.

How many of the elements in the table are liquids at room temperature (20 °C)?

The element astatine is below iodine in Group 7. Predict the formula of a molecule of astatine.

Sea water contains bromide ions. Bromine can be obtained by bubbling chlorine through a sample of sea water. The ionic equation for the reaction is Cl₂ (g) + 2Br⁻ (aq) → 2Cl⁻ (aq) + Br₂ (aq)

Explain which species acts as an oxidising agent in this reaction.

The reaction occurs because chlorine is more reactive than bromine. Bromine is below chlorine in Group 7. Explain the decrease in reactivity from chlorine to bromine.

Elements in Group 7 react with elements in Group 1 to form ionic compounds. Which pair of ions both have the electronic configuration 2.8.8?

Explain why metals conduct electricity but covalent compounds do not conduct electricity.

Hydrogen chloride, HCl, is a covalent substance. When hydrogen chloride is added to water, a solution of dilute hydrochloric acid is formed. This solution does conduct electricity. Name the type of particle in the solution of the dilute hydrochloric acid that allows it to conduct electricity.

The teacher uses this apparatus to investigate the electrolysis of a solution of dilute hydrochloric acid. The ammeter measures the current. variable power supply ammeter gas syringe Y gas syringe X positive electrode negative electrode solution of dilute hydrochloric acid The teacher wants to find out if there is a relationship between current and volume of gas collected at each electrode. She adjusts the power supply until the current is 0.1 amp. After 5 minutes she records the volume of gas collected in syringe X and syringe Y. The teacher repeats the experiment several times, using a different current each time. The table gives the teacher’s results for syringe Y. Current in amp 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Volume of gas in cm³ 8 15 22 25 37 44 52 60

Plot the results for syringe Y.

Draw a circle around the anomalous result.

Draw a line of best fit. Volume of gas in syringe Y in cm³ 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Current in amp

Explain a possible cause of the anomalous result, other than misreading the apparatus.

Deduce the relationship between current and volume of gas collected in syringe Y.

The ionic half‑equation for the reaction that produces the gas in syringe X is 2Cl⁻ → Cl₂ + 2e– The ionic half‑equation for the reaction that produces the gas in syringe Y is 2H⁺ + 2e– → H₂

Suggest how these ionic half‑equations show that the volume of chlorine collected in syringe X should be the same as the volume of hydrogen collected in syringe Y.

Suggest why the volume of chlorine collected in syringe X is always less than the volume of hydrogen collected in syringe Y.

This question is about alcohols, carboxylic acids and esters.

The table gives information about some alcohols. Alcohol Structural formula Relative formula mass methanol CH₃OH 32 ethanol C₂H₅OH C₄H₉OH 74 Complete the table by giving the missing information.

Ethanol can be oxidised to ethanoic acid by heating with potassium dichromate(VI) and another reagent.

Name the other reagent.

State the colour change that occurs during this reaction. from to

Alcohols react with carboxylic acids to form esters.

Name the ester that forms when ethanol reacts with ethanoic acid.

Complete the equation for the reaction between methanol and ethanoic acid. CH₃OH + → + H₂O

Hydrogen peroxide solution decomposes slowly at room temperature to form water and oxygen. The equation for the reaction is 2H₂O₂ → 2H₂O + O₂

A catalyst increases the rate of this reaction. State one other property of a catalyst.

A student has samples of three solids, X, Y and Z. The student uses this apparatus to find out which solids act as catalysts in the decomposition of hydrogen peroxide solution. hydrogen peroxide solution 0 20 40 60 80 100 oxygen Describe the method that the student should use to find out which solids act as catalysts.

The decomposition of hydrogen peroxide solution is exothermic. On the diagram, draw and label the reaction profiles for the reaction • without a catalyst • with a catalyst 2H₂O₂ Energy 2H₂O + O₂ Progress of the reaction

The equation for the reaction can be shown using displayed formulae. 2H O O H → 2H O H + O O ΔH = –204 kJ The table gives the bond energies for two of the bonds. Bond Bond energy in kJ/mol H O 463 O O 146

Use this information to calculate the total amount of energy needed to break all the bonds in two moles of H₂O₂ energy needed = kJ

Use this information to calculate the total amount of energy released when all the bonds in two moles of H₂O are formed. energy released = kJ

Use the value of ΔH and your answers for (i) and (ii) to calculate the bond energy, in kJ/mol, for the O O bond. bond energy = kJ/mol

A student does a titration using dilute sulfuric acid to find the concentration of a solution of potassium hydroxide. The student adds 25.0 cm³ of the potassium hydroxide solution to a conical flask. He then adds a few drops of methyl orange indicator. The student does the titration four times.

Name the piece of apparatus the student should use to add the potassium hydroxide solution.

What is the colour of methyl orange in an alkaline solution? .

The table shows the student’s results. titration 1 2 3 4 volume of acid added in cm³ 20.65 20.60 20.90 20.55 concordant results Concordant results are those within 0.20 cm³ of each other.

Place ticks (✓ ) in the table to show which results are concordant.

Use the concordant results to calculate the mean (average) volume of acid added. mean volume = cm³

This table shows the student’s results for another titration. volume of potassium hydroxide solution used in cm³ 25.0 concentration of potassium hydroxide solution in mol/dm³ 0.0370 mean volume of sulfuric acid added in cm³ 21.20 The equation for the reaction is 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

Calculate the amount, in moles, of KOH in 25.0 cm³ of the potassium hydroxide solution. amount of KOH = mol

Calculate the amount, in moles, of H₂SO₄ in 21.20 cm³ of sulfuric acid. amount of H₂SO₄ = mol

Calculate the concentration, in mol/dm³, of the sulfuric acid. concentration of sulfuric acid = mol/dm³

A sample of a gaseous hydrocarbon, X, has a volume of 600 cm³ at room temperature and pressure (rtp).

Calculate the amount, in moles, of hydrocarbon X in the sample. [molar volume of a gas = 24 000 cm³ at rtp] amount of hydrocarbon X = mol

The mass of the sample of hydrocarbon X is 1.45 g. Show that the relative molecular mass (Mr) of X is 58 Mr =

Hydrocarbon X is an alkane. Show that the molecular formula of X is C₄H₁₀

Give the displayed formula of the branched‑chain isomer of hydrocarbon X.

This question is about ammonia gas, NH₃

Ammonia can be prepared in a laboratory from the reaction between ammonium chloride, NH₄Cl, and sodium hydroxide. The other products of the reaction are sodium chloride and water.

Give a chemical equation for this reaction.

Give a test for ammonia gas.

In industry, ammonia is produced from nitrogen and hydrogen. The equation for this reaction is N₂(g) + 3H₂(g) ⇌ 2NH₃(g) In a sealed container, the reaction can reach a position of dynamic equilibrium. Explain the meaning of the term dynamic equilibrium.

The graph shows the percentage yield of ammonia at equilibrium for different temperatures and pressures. 80 350 °C 450 °C Percentage yield of ammonia at 40 equilibrium 550 °C 0 100 200 300 400 Pressure in atmospheres Using the graph, explain if the forward reaction is exothermic or endothermic.