Half equations at electrodes

A scientist reacts lead(II) nitrate solution with sodium bromide solution.

This is the equation for the reaction.

Pb(NO₃)₂(aq) + 2NaBr(aq) PbBr₂(s) + 2NaNO₃(aq)

Describe how the scientist could obtain a pure dry sample of lead(II) bromide (PbBr₂) from the reaction mixture.

The scientist reacts an excess of lead(II) nitrate solution with 25 cm³ of sodium bromide solution of concentration 2.0 mol/dm³.

Show that the amount of sodium bromide used is 0.050 mol.

Show that the maximum theoretical mass of lead(II) bromide is approximately 9 g.

[for PbBr₂ M_r = 367]

The scientist electrolyses molten lead(II) bromide using this apparatus.

Explain why lead(II) bromide needs to be molten rather than solid for electrolysis to occur.

The electrolyte is at a temperature of 400C.

Explain a suitable material for the electrodes.

Give the half‑equation that occurs at the negative electrode.

Bromine forms at the positive electrode.

This is the half‑equation for the reaction at the positive electrode.

State what is observed at the positive electrode.

State why the half‑equation represents an oxidation reaction.

This question is about electrolysis.

This apparatus is used to collect the gases produced when an electric current passes through dilute sulfuric acid.

inert electrode inert electrode

Name a suitable material for the inert electrodes.

Give a test to show that the gas produced at the positive electrode is oxygen.

Give a test to show that the gas produced at the negative electrode is hydrogen.

Give an ionic half-equation for the formation of hydrogen at the negative electrode.

The oxygen gas in the tube has a volume of 17.8 cm³ at rtp.

Calculate the mass, in grams, of oxygen gas in the tube.

Give your answer to 3 significant figures.

[for O₂ at rtp, molar volume = 24 000 cm³]

This apparatus is used to electrolyse a concentrated solution of sodium chloride.

Give a reason why sodium chloride solution conducts electricity.

The positive ions in the solution are H⁺ and Na⁺.

Give a reason why hydrogen forms at the negative electrode rather than sodium.

A sample of the solution is taken from near the negative electrode. The sample is tested with universal indicator.

Explain the final colour of the universal indicator.

Sodium chloride is an ionic compound.

Explain why sodium chloride conducts electricity when it is molten or in solution, but not when it is solid.

A solution of sodium chloride can be electrolysed using this apparatus.

If the solution is dilute, a significant amount of oxygen collects at P.

Complete the ionic half-equation for this reaction.

If the solution is concentrated, chlorine is the main product that collects at P.

This is the ionic half-equation for the reaction.

State why this is an oxidation reaction.

The gas that collects at Q is hydrogen.

Give a test for hydrogen.

Explain how hydrogen gas forms at the negative electrode.

Hydrogen reacts with oxygen to form water.

The equation shows the covalent bonds in the molecules.

The table gives the bond energies.

Use the equation and the values in the table to calculate the enthalpy change, Δ H, for the reaction.

Include a sign in your answer.

Complete the diagram to show the energy levels of the reactants and products, and the enthalpy change, Δ H.

When copper(II) sulfate solution is electrolysed, copper forms at the negative electrode.

A student uses this apparatus to investigate the electrolysis of copper(II) sulfate solution.

Describe how the student could test a sample of copper(II) sulfate solution to show that it contains copper(II) ions.

Describe how copper metal forms at the negative electrode.

State the appearance of the copper that forms on the negative electrode.

Oxygen forms at the positive electrode.

Give a test for oxygen.

Complete the half-equation for the formation of oxygen at the positive electrode.

2H₂O _........ +_........ +_........

State why the formation of oxygen at the positive electrode is an oxidation reaction.

This question is about metals.

Explain why metals are malleable.

Explain why molten sodium chloride conducts electricity.

Explain how sodium metal forms at the negative electrode.

Write an ionic half-equation for the formation of chlorine gas at the positive electrode.

Give a reason why sodium metal does not form in the electrolysis of an aqueous solution of sodium chloride.

Copper can be produced by reacting an oxide of copper with methane.

The diagram shows the apparatus used.

The oxide of copper is heated until the reaction is complete.

The table shows the results.

Use the results to show that this oxide has the empirical formula CuO

[for Cu, A_r = 63.5 for O, A_r = 16]

Iron metal was discovered several thousand years ago. Iron is produced industrially from iron(III) oxide by extraction using carbon.

Sodium is a metal in Group 1 of the Periodic Table. Sodium was discovered by Sir Humphrey Davy in 1807. It is produced industrially by the electrolysis of sodium chloride.

Explain, using the reactivity series, why carbon can be used to extract iron from iron(III) oxide.

Suggest why sodium was not discovered until 1807.

The diagram shows a simplified version of the Downs cell, which can be used to produce sodium industrially. The electrolyte is molten sodium chloride.

Explain why the molten electrolyte conducts electricity.

What is the formula of the cation in the electrolyte?

During the production of sodium, small explosions can be heard.

Explain why using sodium chloride that is not completely dry could cause these small explosions.

Give ionic half-equations for the reactions at the anode and cathode.

Explain why the reactivity of metals in Group 1 increases from lithium to potassium.

This question is about magnesium and magnesium compounds.

Magnesium burns in oxygen to form magnesium oxide.

State two observations that would be seen during the reaction.

The diagram shows the electron configurations of a magnesium atom and an oxygen atom.

Describe the changes in the electronic configurations when magnesium reacts with oxygen to form the ionic compound magnesium oxide, MgO

Magnesium can be produced by the electrolysis of molten magnesium chloride.

State why magnesium cannot be produced by heating magnesium oxide with carbon.

Explain the different ways that magnesium and magnesium chloride conduct electricity.

magnesium

magnesium chloride

During the electrolysis of molten magnesium chloride, magnesium is formed at the negative electrode.

The ionic half‑equation for the reaction is

State why this is a reduction reaction.

Write an ionic half‑equation for the formation of chlorine at the positive electrode.

The diagram represents the structure of copper metal.

Explain three properties of copper that make it a suitable metal to use in electrical wiring.

The diagram shows the electrolysis of copper(II) sulfate solution, using graphite electrodes.

Copper forms at the negative electrode and oxygen forms at the positive electrode.

Give the formula of the copper ion and the formula of the sulfate ion in copper(II) sulfate.

copper ion

sulfate ion

State what would be seen at the positive electrode.

Give a test for oxygen.

Give an ionic half‑equation for the formation of oxygen at the positive electrode.

Suggest why the copper(II) sulfate solution contains some OH^– ions.

This question is about the electrolysis of copper(II) sulfate solution.

The diagram shows the apparatus used for the electrolysis.

positive electrode negative electrode

copper(II) sulfate solution

A student records the total increase in mass of the negative electrode every minute for 8 minutes.

The table shows the results.

Plot the student’s results.

Draw a curve of best fit.

Explain the shape of the graph.

The product at the positive electrode is oxygen gas.

The student repeats the electrolysis using different apparatus.

Describe how the student should collect a sample of pure oxygen at the positive electrode.

Give an ionic half-equation for the formation of oxygen.

The wire used to connect the power supply to the electrodes is made of copper metal.

The diagram shows the arrangement of the ions in a metal.

Metals that are malleable can also be stretched to form long, thin wires.

Suggest why metals can be stretched to form wires.

Explain why metals conduct electricity.

A teacher prepares the insoluble salt lead(II) bromide (PbBr₂) by mixing solutions of lead(II) nitrate and sodium bromide.

Describe what the teacher should do next to obtain a pure, dry sample of lead(II) bromide.

The teacher then sets up a circuit in a fume cupboard using the pure, dry sample of lead(II) bromide.

Explain why the lamp does not light when the lead(II) bromide is solid.

The teacher heats the lead(II) bromide.

When the lead(II) bromide is molten, the lamp lights and bromine forms at the positive electrode.

State what observation would be made at the positive electrode.

Explain how bromide ions in the molten lead(II) bromide become bromine molecules at the positive electrode.

Write an ionic half-equation for the reaction that occurs at the negative electrode.

Include state symbols in your equation.

This question is about ionic compounds.

State the formula of the cation and the anion in magnesium sulfate.

The diagram shows the electronic configuration of a potassium atom and an oxygen atom.

Potassium oxide (K₂O) is an ionic compound.

Draw the electronic configuration of a potassium ion and an oxide ion.

Show the charge on each ion.

A sample of solid potassium oxide is added to water.

A reaction occurs and a colourless solution forms.

When a few drops of phenolphthalein indicator are added to the solution it turns pink.

Identify the ion responsible for the colour change.

Give a chemical equation for the reaction between potassium oxide and water.

Explain why ionic compounds conduct electricity when molten or in aqueous solution, but not when in the solid state.

The diagram shows the apparatus a teacher uses to demonstrate the electrolysis of a concentrated aqueous solution of sodium chloride.

During the electrolysis two gases, X and Y, are formed. One of the gases produces a squeaky pop when tested with a lighted splint.

Use ionic half‑equations to identify X and Y.

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

Hydrogen chloride, HC l, 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.

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.

Plot the results for syringe Y.

Draw a circle around the anomalous result.

Draw a line of best fit.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 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

The ionic half‑equation for the reaction that produces the gas in syringe Y is

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 the metal aluminium.

Aluminium is malleable and conducts electricity.

The diagram shows the arrangement of the ions in aluminium metal.

Explain why aluminium is malleable.

Explain why aluminium conducts electricity.

Aluminium cannot be extracted by heating a mixture of carbon and aluminium oxide.

Give a reason why heating a mixture of aluminium oxide and carbon does not produce aluminium.

Aluminium is extracted industrially by the electrolysis of molten aluminium oxide Al₂O₃ at a temperature of about 950 °C.

Aluminium metal forms at the negative electrode and oxygen gas forms at the positive electrode. The positive and negative electrodes are made of graphite.

The diagram shows the apparatus used.

Explain how aluminium metal forms at the negative electrode.

Write an ionic half-equation for the formation of oxygen gas at the positive electrode.

Suggest why carbon dioxide gas is also produced at the positive electrode.

Aluminium reacts with iron(III) oxide. The reaction is exothermic.

The equation for the reaction is

State how the equation shows that iron(III) oxide is reduced.

Draw an energy level diagram for the reaction between aluminium and iron(III) oxide.

This question is about the metal aluminium.

Draw a labelled diagram to represent the structure and bonding in a metal.

Explain why a metal conducts electricity.

Aluminium is used to make cans for drinks.

Give two properties of aluminium that make it suitable for this use.

Aluminium is extracted from aluminium oxide (Al₂O₃) by electrolysis.

The electrolyte is aluminium oxide dissolved in molten cryolite.

State why aluminium cannot be extracted by heating aluminium oxide with carbon.

Aluminium is produced at the negative electrode.

The ionic half-equation for the reaction is

State why this is a reduction reaction.

Complete the ionic half-equation for the reaction at the positive electrode.

This question is about the metal, lead.

Explain why metals, such as lead, are malleable.

A teacher uses this apparatus in a fume cupboard to demonstrate the electrolysis of lead(II) bromide.

The lead(II) bromide is heated until it melts.

When the lead(II) bromide melts, the lamp lights.

One of the products of this electrolysis is lead.

State why solid lead(II) bromide does not conduct electricity.

Bromine is formed by the oxidation of bromide ions at the positive electrode.

Complete the ionic half-equation for the oxidation of bromide ions.

Explain why lead metal forms at the negative electrode.

The teacher stops heating the mixture and allows it to solidify.

Suggest why the lamp stays alight.

This question is about the halogens and their compounds.

The table gives the colour and physical state at room temperature of the halogens.

Complete the table by predicting the colour of astatine and the physical state of fluorine at room temperature.

Chlorine gas is bubbled into a colourless solution of potassium bromide.

Explain why the solution turns orange.

Potassium bromide is an ionic compound.

Draw diagrams to show the outer electrons in a potassium ion and in a bromide ion.

Include the charges on the ions.

A student sets up a circuit to test the electrical conductivity of water, solid sodium chloride and aqueous sodium chloride.

The table shows the student’s results.

Explain these results, with reference to the structure and bonding of the substances.

A concentrated aqueous solution of sodium chloride is electrolysed using graphite electrodes.

Chlorine is formed at the positive electrode (anode).

Give an ionic half‑equation for the formation of chlorine at the positive electrode.

State why this ionic half‑equation represents an oxidation reaction.

Which substance is formed at the negative electrode (cathode)?