Electrolysis Flashcards

All 34 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 and it can conduct electricity.

True or false? A solid ionic compound conducts electricity.

False. In the solid the ions are in fixed positions and cannot move, so it only conducts when molten or dissolved.

What are the names of the positive and negative electrodes in electrolysis?

Positive electrode = anode; negative electrode = cathode.

In electrolysis, which electrode does each type of ion move to, and why?

Positive ions move to the negative electrode (cathode) and negative ions move to the positive electrode (anode), because opposite charges attract.

What is electrolysis?

Passing an electric current through an electrolyte so that ions move to the electrodes and are discharged, producing elements.

True or false? Electrons travel through the electrolyte to carry the charge.

False. Ions move through the electrolyte; electrons only travel through the wires and electrodes.

What are the steps to predict the products of electrolysis of a molten binary ionic compound?
e.g. molten lead bromide

1. The metal ion is positive, so the metal forms at the cathode (negative electrode): lead.
2. The non-metal ion is negative, so the non-metal forms at the anode (positive electrode): bromine.

What is meant by an inert electrode?

An electrode that does not react with the electrolyte or the products, e.g. graphite (carbon) or platinum.

Why must the two products be kept apart during the electrolysis of molten sodium chloride?

So that the products do not react together and reform sodium chloride.

True or false? When molten zinc chloride is electrolysed, zinc is produced at the positive electrode.

False. The metal is produced at the negative electrode (cathode); chlorine forms at the positive electrode.

When is electrolysis used to extract a metal instead of reduction with carbon?

When the metal is too reactive to be extracted by reduction with carbon (more reactive than carbon), or if the metal reacts with carbon.

Why is extracting a metal by electrolysis expensive?

Large amounts of energy are needed to melt the compound and to produce the electric current.

What is the electrolyte used in the manufacture of aluminium?

A molten mixture of aluminium oxide and cryolite.

Why is cryolite mixed with the aluminium oxide in aluminium extraction?

It lowers the melting point of the mixture, so less energy (and cost) is needed to melt it.

Why must the carbon positive electrode be continually replaced in aluminium extraction?

The oxygen produced at the anode reacts with the hot carbon to form carbon dioxide, so the electrode wears away.

What is produced at each electrode when molten aluminium oxide is electrolysed?

Aluminium at the negative electrode (cathode) and oxygen at the positive electrode (anode).

What are the steps to predict the products of electrolysis of an aqueous solution of one ionic compound?
e.g. sodium chloride solution

1. Cathode: hydrogen if the metal is more reactive than hydrogen, otherwise the metal — sodium is more reactive, so hydrogen.
2. Anode: the halogen if halide ions are present, otherwise oxygen — chloride ions are present, so chlorine.

Which two ions from water are present in every aqueous solution being electrolysed?

Hydrogen ions ($\mathrm{H^+}$) and hydroxide ions ($\mathrm{OH^-}$), made when water molecules break down.

What is seen at each electrode when copper sulfate solution is electrolysed with inert electrodes?

A pink-brown coating of copper on the negative electrode and bubbles of oxygen gas at the positive electrode.

Why does the blue colour of copper sulfate solution fade during electrolysis?

The copper ions are discharged (removed from the solution), so the solution becomes less concentrated.

True or false? When potassium iodide solution is electrolysed, potassium is produced at the negative electrode.

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

Name the alkaline solution left behind when sodium chloride solution is electrolysed.

Sodium hydroxide ($\mathrm{NaOH}$).

Which electrodes are used in the electrolysis required practical, and why?

Inert graphite (carbon) electrodes, so they do not react with the electrolyte or the products.

How are hydrogen and oxygen collected at the electrodes identified?

Hydrogen: a lit splint gives a squeaky pop. Oxygen: a glowing splint relights.

How is chlorine collected at the positive electrode identified?

Damp litmus paper is bleached white.

What is a hypothesis in a practical investigation?

A testable idea or prediction based on scientific knowledge that the experiment is designed to test.

How can the volumes of the two gases produced be compared more accurately than by using test tubes?

Collect them in measuring cylinders, inverted burettes or gas syringes, because these have a scale for measuring volume.

At the cathode, what happens to the positive ions and what type of reaction is this?

They gain electrons, so the reaction is reduction.

At the anode, what happens to the negative ions and what type of reaction is this?

They lose electrons, so the reaction is oxidation.

What are the steps to write the half equation for a metal formed at the cathode?
e.g. $\mathrm{Al^{3+}}$ ions in molten aluminium oxide

1. Write the metal ion on the left and the metal atom on the right: $\mathrm{Al^{3+}} \rightarrow \mathrm{Al}$
2. Add electrons to the left, one for each positive charge: $\mathrm{Al^{3+}} + 3\mathrm{e^-} \rightarrow \mathrm{Al}$

What are the steps to write the half equation for a halogen formed at the anode?
e.g. bromide ions, $\mathrm{Br^-}$

1. Two halide ions give one diatomic molecule: $2\mathrm{Br^-} \rightarrow \mathrm{Br_2}$
2. Add electrons to the right, one for each negative charge: $2\mathrm{Br^-} \rightarrow \mathrm{Br_2} + 2\mathrm{e^-}$

What is the half equation for oxygen being produced from hydroxide ions at the anode?

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

True or false? In a half equation for the cathode reaction, the electrons are written on the right-hand side.

False. Electrons are gained at the cathode, so they are written on the left, e.g. $\mathrm{Na^+} + \mathrm{e^-} \rightarrow \mathrm{Na}$

True or false? When sodium sulfate solution is electrolysed using inert electrodes, hydrogen is produced at the negative electrode.

True. Sodium is more reactive than hydrogen, so hydrogen is discharged instead of the metal.