Topic 7 - Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

Where the reaction transfers energy to the surroundings. This causes a rise in temperature.

Question 2

What is an endothermic reaction?

Answer 2

Where energy is taken in from the surroundings during a reaction. This causes a fall in temperature.

Question 3

What is the most common example of an endothermic reaction?

Answer 3

The combustion of hydrocarbons.

Question 4

What is an example of an endothermic reaction?

Answer 4

Thermal decomposition.

Question 5

In a reaction profile diagram, the peak of the graph going from reactants to products is what?

Answer 5

The activation energy - the amount of energy required to start the reaction.

Question 6

What would an exothermic reaction profile diagram look like?

Answer 6

The reactants have a higher energy level than the products since energy is released.

Question 7

What would an endothermic reaction profile diagram look like?

Answer 7

The products would have a higher energy level than the reactants, as energy is taken in from the surroundings during the reaction.

Question 8

Is breaking bonds an endothermic or exothermic process? Why?

Answer 8

Energy has to be supplied to break chemical bonds, therefore it is an endothermic process. Then new chemical bonds are formed to make the products.

Question 9

Is making bonds an endothermic or exothermic process? Why?

Answer 9

When making new bonds, energy is transferred to the surroundings, therefore it is an exothermic process.

Question 10

What is bond energy? What are the units for it?

Answer 10

Bond energy is the energy needed to break the bond between two atoms. This is measured in kJ/mol. You can use bond energies to work out the energy change for many chemical reactions.

Question 11

How do you calculate the overall energy change during the reaction?

Answer 11

How much energy is needed to break bonds in the reactants - how much energy is released making new bonds in the products.

Question 12

What is the short equation for energy change during a reaction?

Answer 12

Break - Make.

Question 13

What is the rule for discharging ions at the cathode?

Answer 13

The ion discharged will be the least reactive out of hydrogen and another ion.

Question 14

What is the rule for discharging ions at the anode?

Answer 14

The least complex out of hydroxide and another ion is discharged.

Question 15

What are spectator ions?

Answer 15

In a displacement reaction, they are not changed.

Example Zn + CuSO4 –> ZnSO4 + Cu.
SO4 (sulfate) has not changed and is a spectator ion.

Question 16

What is an ionic equation?

Answer 16

A displacement reaction without spectator ions.

Question 17

What can a redox reaction be represented as?

Answer 17

Two half equations of the ions or atoms discharged or charged.

Question 18

What can you do with the difference in reactivity in a displacement reaction?

Answer 18

You can make electrical cells and batteries.

Question 19

How do you use reactivity to make a cell?

Answer 19

If you join the metals together by a wire and dip them into an electrolyte, electrons will flow through the wire.

Question 20

What is the rule for voltage produced in a battery?

Answer 20

The greater the difference in reactivity between two metals used, the higher the voltage produced.

Question 21

What were the first mass-produced batteries?

Answer 21

Primary cells, they cannot be recharged.

Question 22

How are batteries recharged?

Answer 22

The battery is connected to a power supply that reverses the reactions that occur at each electrode when recharging, regenerating the original reactants.

Question 23

Why are hydrogen powered vehicles useful?

Answer 23

Using hydrogen as a fuel reduces human impact on climate change as the reaction doesn’t produce carbon dioxide.

Question 24

What are the problems with using hydrogen as a fuel?

Answer 24

Supplying the hydrogen uses electrolysis, which requires energy from non-renewable fossil fuels, so it still produces CO2 indirectly.

Question 25

What is a more efficient use of energy from oxidising hydrogen?

Answer 25

Fuel cells

Question 26

How do fuel cells work?

Answer 26

They are fed with hydrogen and oxygen to produce water. Most of the energy released in the reaction is transferred to electrical energy which can be used to run a vehicle but a constant supply of hydrogen is needed.

Question 27

In a fuel cell, what happens at the cathode?

Answer 27

Hydrogen gas is supplied that diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to an external circuit.

Question 28

In a fuel cell, what happens at the anode?

Answer 28

Oxygen gas is supplied which diffuse through the graphite to form hydroxide ions, accepting electrons from the external circuit.

Question 29

In a fuel cell, what is the half equation at the cathode?

Answer 29

2H2 (g) + 4OH- (aq) –> 4H2O (l) + 4e-

Question 30

In a fuel cell, what is the half equation at the anode?

Answer 30

O2 (g) + 2H2O (l) + 4e- –> 4OH- (aq)

Question 31

What is the reaction equation in a fuel cell?

Answer 31

2H2 (g) + O2 (g) –> 2H2O (l)

Question 32

What waste product is produced in a fuel cell?

Answer 32

Water, so they provide an alternative to fossil fuels which produce greenhouse gases.

Question 33

What are the advantages of hydrogen fuel cells?

Answer 33

-They don’t need to be electrically charged
-No pollutants are produced
-Can be a range of sizes for different uses.

Question 34

What are the disadvantages of hydrogen fuel cells?

Answer 34

-Hydrogen is highly flammable
-Hydrogen is sometimes produced for the cell by non-renewable sources
-Hydrogen is difficult to store.