Chapter 6 and 7

Question 1

What is an energy level diagram?

Answer 1

A diagram to show the energy released when the bonds are broken in the reactant and remade in the products.

It shows the overall change in energy for that reaction

The overall energy change for a reaction is the difference between the energy need to break bonds and the energy need to form bonds

Question 2

What are exothermic reactions?

Answer 2

Reactions that give out heat to their surroundings (you will see an increase in temperature)

Thermal energy is transferred from the chemicals to the surroundings

Question 3

What are endothermic reactions?

Answer 3

Reactions that take in heat from their surroundings (you will see a decrease in temperature)

Thermal energy is transferred from the surroundings to the chemicals

Question 4

What is electrolysis?

Answer 4

The use of electricity to split ionic compounds into their ions.

Question 5

What is an electrolyte?

Answer 5

An aqueous or molton solution of ions

Question 6

What is oxidation?

Answer 6

The gain of oxygen or loss of electrons

Question 7

What is reduction?

Answer 7

The loss of oxygen or gain of electrons

Question 8

Electrolysis requires what to conduct electricity?

Answer 8

Charged particles that are free to move

Question 9

Why is cryolite added to the electrolysis process of Aluminium oxide?

Answer 9

To lower the temperature at which aluminium oxide melts

Question 10

Why is the electrolysis of aluminium oxide so expensive?

Answer 10

It requires a large amount of electricity to produce the temperatures needed to melt the aluminium oxide

Question 11

What is the +ve electrode called?

Answer 11

Anode – it attracts -ve ions (anions)

Question 12

What is the –ve electrode called?

Answer 12

Cathode – it attracts +ve ions (cations)

Question 13

What happens if two +ve ions are present in the electrolyte?

Answer 13

In aqueous solutions, the least reactive +ve ions will go to the cathode

Question 14

Which -ve ion will be attracted to the anode in aqueous electrolytes?

Answer 14

In aqueous solutions oxygen (-ve ion) will be attracted to the anode unless a halogen ion is present

Question 15

What happens at the anode?

Answer 15

The -ve ions lose electrons to turn back into atoms. We can show this using half ionic equations

Question 16

What happens at the cathode?

Answer 16

The +ve ions gain electrons to turn back into atoms. We can show this using half ionic equations

Question 17

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

Answer 17

2O^- ————–> O₂ + 2e^-

Question 18

What is the half equation for the formation of aluminium at the cathode?

Answer 18

Al³⁺ + 3e ^- ——————> Al

Question 19

What is mean bond energy?

Answer 19

The energy needed to break a particular chemical bond

Question 20

What is activation Energy?

Answer 20

The minimum energy needed to start a reaction

Question 21

How is activation energy shown on an energy level diagram?

Answer 21

Activation energy is shown as an arrow from the reaction line to the top of the curve

Question 22

How would you show a change in activation energy when a catalyst is used?

Answer 22

Question 23

What is the test for chlorine?

Answer 23

Bleaches wet litmus paper

Question 24

At the electrode, Mg​²⁺ ion is converted back into an atom. Has it been oxidised or reduced? Explain your answer.

Answer 24

Mg²⁺ has been reduced as it has gained two electrons to become an atom of Mg.

Mg²⁺ + 2e^- ————-> Mg

Question 25

What does OIL RIG mean in terms of oxidation and reduction?

Answer 25

Oxidation

is

loss

(of electrons)

Reduction

is

gain

(of electrons)

Question 26

Give two examples of an exothermic reaction.

Answer 26

Combustion, neutralisation, some oxidation reactions such as rusting, respiration, heat packs

Question 27

Give two examples of an endothermic reaction.

Answer 27

Thermal decomposition, photosynthesis, cold packs

Question 28

How is a current formed in an electrochemical cell?

(higher only)

Answer 28

In a fuel cell, the electrodes are two different metals. The potential difference (voltage) between the metals cause electrons to flow

Electrons flow from the most to least reactive metal producing an electrical current

Question 29

What is a battery?

(higher only)

Answer 29

A battery is formed from two or more cells

Question 30

Why is the reactivity series important in electrochemical cells?

(higher only)

Answer 30

The position of the metals in the reactivity determines the voltage. The greater the difference in the position of the two metals used as electrodes the higher the voltage

Question 31

Explain why a current won’t flow if the electrodes are made of the same metal?

(higher only)

Answer 31

For a current to flow there must be a difference in the metals position in the reactivity series. This can only happen if the metals are different.

Question 32

What is voltage?

(higher only)

Answer 32

Voltage is a measure of the potential difference in electrical energy between two parts of a circuit i.e. the different metal electrodes in the electrochemical cell.

The bigger the difference in energy, the bigger the voltage.

In Chemistry, we are using the position of the metal in the reactivity series to reflect its electrical energy.

Note: Current is the rate at which charge is flowing

Question 33

What is a happening in an electrochemical cell?

(higher only)

Answer 33

Chemical energy is being converted into electrical energy.

Non-rechargeable batteries have a limited amount of chemicals that are eventually used up.

Question 34

How is a fuel cell different from a non- rechargeable battery?

(higher only)

Answer 34

Non-rechargeable batteries have a limited supply of chemicals that are used up

Fuel cells have a continuous supply of chemicals most often oxygen (from the air) and a fuel (hydrogen)

Question 35

What is the law of conservation of energy?

Answer 35

Energy can not be created or destroyed it can only be transferred from one form to another

Question 36

What two factors can change the voltage of an electrochemical cell?

(higher only)

Answer 36

Choice of metals used for the electrodes

Choice of the electrolyte (type or concentration)

Question 37

How would you calculate bond energy?

(higher only)

Answer 37

To calculate bond energy

Add together the bond energies for all the bonds in the reactants – this is the ‘energy in’.

Add together the bond energies for all the bonds in the products – this is the ‘energy out’.

Calculate the energy change = energy in – energy out.

Question 38

Why are bond energies important?

(higher only)

Answer 38

In chemical reactions energy is transferred as bonds in the reactants are broken and new bonds form in the products

Bond breaking is endothermic and bond forming is exothermic. We can work out the overall energy change of a reaction by looking at the bond energies

The overall energy change for a reaction is the difference between the energy need to break bonds and the energy need to form bonds