Electrolytic Processes Flashcards

All 30 cards in this deck

What is an electrolyte?

An ionic compound that is molten or dissolved in water, so its ions are free to move.

What is electrolysis?

The process in which electrical energy from a direct current (d.c.) supply decomposes an electrolyte.

Why does a solid ionic compound not conduct electricity?

Its ions are held in fixed positions and are not free to move.

In an electrolysis cell, what are the names and charges of the two electrodes?

Anode = positive electrode; cathode = negative electrode.

During electrolysis, where do the cations and the anions move to?

e.g. molten lead bromide

Cations to the negative cathode, anions to the positive anode.

e.g. $\mathrm{Pb^{2+}}$ to the cathode, $\mathrm{Br^-}$ to the anode.

True or false? Calcium carbonate mixed with water behaves as an electrolyte.

False. Calcium carbonate is insoluble, so its ions are not free to move in the water.

What are the steps to predict the products of electrolysis of a molten binary ionic compound?

e.g. molten zinc chloride

1. Identify the ions: $\mathrm{Zn^{2+}}$ and $\mathrm{Cl^-}$
2. Cation goes to the cathode: zinc at the cathode
3. Anion goes to the anode: chlorine at the anode

What are the products of the electrolysis of molten lead bromide?

Lead at the cathode and bromine at the anode.

What is the half equation at the cathode in the electrolysis of molten lead bromide?

$\mathrm{Pb^{2+} + 2e^- \rightarrow Pb}$

What is the half equation at the anode in the electrolysis of molten lead bromide?

$\mathrm{2Br^- \rightarrow Br_2 + 2e^-}$

Why must an ionic compound such as lead bromide be melted before it can be electrolysed?

Melting frees the ions to move, so the compound can conduct and be decomposed.

True or false? Electrolysis of molten potassium bromide produces hydrogen at the cathode.

False. Potassium is produced at the cathode; there is no water, so there are no hydrogen ions.

What is the rule for the product at the cathode when an aqueous solution is electrolysed with inert electrodes?

Hydrogen is produced, unless the metal is less reactive than hydrogen, in which case the metal is produced.

e.g. copper from copper chloride solution.

What is the rule for the product at the anode when an aqueous solution is electrolysed with inert electrodes?

The halogen is produced if a halide ion is present; otherwise oxygen is produced (from hydroxide ions).

What are the products of the electrolysis of sodium chloride solution with inert electrodes?

Hydrogen at the cathode and chlorine at the anode.

What are the products of the electrolysis of copper chloride solution with inert electrodes?

Copper at the cathode and chlorine at the anode.

What are the products of the electrolysis of water acidified with sulfuric acid, and in what volume ratio?

Hydrogen at the cathode and oxygen at the anode, in a volume ratio of 2 hydrogen : 1 oxygen.

True or false? Electrolysis of sodium sulfate solution produces sodium at the cathode.

False. Hydrogen is produced, because sodium is more reactive than hydrogen.

What is oxidation in terms of electrons?

Loss of electrons.

What is reduction in terms of electrons?

Gain of electrons.

In electrolysis, which electrode is the site of reduction and which of oxidation?

Reduction at the cathode (cations gain electrons); oxidation at the anode (anions lose electrons).

What is the half equation for the formation of hydrogen gas from hydrogen ions at the cathode?

$\mathrm{2H^+ + 2e^- \rightarrow H_2}$

What is the half equation for the formation of oxygen from hydroxide ions at the anode?

$\mathrm{4OH^- \rightarrow O_2 + 2H_2O + 4e^-}$

True or false? In the half equation $\mathrm{2Cl^- \rightarrow Cl_2 + 2e^-}$ the chloride ions are oxidised.

True. They lose electrons, and this happens at the anode.

True or false? When copper sulfate solution is electrolysed with copper electrodes, the anode loses mass and the cathode gains mass.

True. Copper atoms dissolve from the anode as ions and copper is deposited on the cathode.

What are the half equations at the anode and cathode when copper sulfate solution is electrolysed with copper electrodes?

Anode: $\mathrm{Cu \rightarrow Cu^{2+} + 2e^-}$
Cathode: $\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$

Why does the blue colour of copper sulfate solution stay the same when it is electrolysed with copper electrodes?

$\mathrm{Cu^{2+}}$ ions enter the solution at the anode at the same rate as they leave at the cathode, so the concentration is unchanged.

What are the steps to measure the mass change of an electrode in the electrolysis of copper sulfate solution?

e.g. a copper cathode electrolysed for 10 minutes

1. Clean with emery paper to remove surface oxide/grease, dry and weigh
2. Electrolyse at a set current for 10 minutes
3. Rinse, dry and reweigh: mass change = final mass − initial mass

How is impure copper purified by electrolysis?

Impure copper is the anode and pure copper the cathode in copper sulfate solution; copper dissolves from the anode and is deposited as pure copper on the cathode.

How does the mass of copper deposited on the cathode depend on the current and the time?

It is directly proportional to both.

e.g. three times the current for the same time deposits three times the mass.