C2- electrolysis Flashcards Preview

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Flashcards in C2- electrolysis Deck (21)
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1
Q

What is electrolysis?

A

Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.

2
Q

What needs to happen for electrolysis to work?

A

For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or when melted.

3
Q

What happens during electrolysis?

A

Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.
Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.

4
Q

What is the electrolyte?

A

A substance which in a solution will conduct an electric current.

5
Q

How do you use electrolysis to electroplate objects?

A

The negative electrode should be the object that is to be electroplated
The positive electrode should be the metal that you want to coat the object with
The electrolyte should be a solution of the coating metal, such as its metal nitrate or sulfate

6
Q

How would you electroplate with silver?

A

The object to be plated, such as a metal spoon, is connected to the negative terminal of the power supply. A piece of silver is connected to the positive terminal. The electrolyte is silver nitrate solution.

7
Q

How would you electroplate with copper?

A

The object to be plated, such as a metal pan, is connected to the negative terminal of the power supply. A piece of copper is connected to the positive terminal. The electrolyte is coppersulfate solution.

8
Q

How would you purify copper in copper manufacture?

A

In this case, both electrodes are made from copper. The negative electrode gradually gets coated with pure copper as the positive electrode gradually disappears.

9
Q

Why is aluminum so expensive?

A

largely because of the amount of electricity used up in the extraction process.

10
Q

What is the name of aluminum ore?

A

bauxite.

11
Q

How is aluminum extracted before electrolysis?

A

The bauxite is purified to yield a white powder - aluminium oxide - from which aluminium can be extracted. The extraction is done by electrolysis. But first the aluminium oxide must be melted so that electricity can pass through it. Aluminium oxide has a very high melting point (over 2000°C) so it would be expensive to melt it. Instead, it is dissolved in moltencryolite - an aluminium compound with a lower melting point than aluminium oxide. The use of cryolite reduces some of the energy costs involved in extracting aluminium.

12
Q

What happens during electrolysis in the extraction of aluminum?

A

Aluminium metal forms at the negative electrode and sinks to the bottom of the tank, where it is tapped off. Oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away. As a result, the positive electrodes have to be replaced frequently. This adds to the cost of the process.

13
Q

What do you get at the negative electrode in electrolysis?

A

positively charged ions gain electrons. This is reduction, and you say that the ions have been reduced.Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series: The metal will be produced if it is less reactive than hydrogen
Hydrogen will be produced if the metal is more reactive than hydrogen

14
Q

What do you get at the positive electrode in electrolysis?

A

At the positive electrode, negatively charged ions lose electrons. This is oxidation, and you say that the ions have been oxidised.

15
Q

What happens if an electric current is passed through a concentrated sodium chloride solution

A

hydrogen gas forms at the negative electrode and chlorine gas forms at the positive electrode. A solution of sodium hydroxide forms.

You might have expected sodium metal to be deposited at the negative electrode. But sodium is too reactive for this to happen, so hydrogen is given off instead.

16
Q

What happens during the electrolysis of sodium chloride solution?

A

Hydrogen ions H+ (from the water) are discharged at the negative electrode as hydrogen gas, H2
Chloride ions Cl– are discharged at the positive electrode as chlorine gas, Cl2
Sodium ions Na+ and hydroxide ions OH– (from the water) stay behind - they form sodium hydroxide solution, NaOH

17
Q

What are the uses of the three products of the electrolysis of concentrated sodium chloride solution

A

Hydrogen is used as a fuel and for making ammonia
Chlorine is used to kill bacteria in water, and to make bleach and plastics
Sodium hydroxide is used to make soap and bleach

18
Q

How do you write a half-equation?

A

Write the formulae of the reactant and product.
Adjust the number of ions, if needed.
Count the number of charges. Add enough electrons so that both sides have the same total number of charges.

19
Q

What are electrons shown as in half-equations?

A

Electrons are shown as e- in half-equations

20
Q

How are half-equations balanced?

A

A half-equation is balanced by adding, or taking away, a number of electrons equal to the total number of charges on the ions in the equation.

21
Q

What does a half-equation show you?

A

A half-equation shows you what happens at one of the electrodes during electrolysis

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