C5: Energy changes

Question 1

What is an exothermic reaction?

Answer 1

One which transfers energy to the thermal energy store of the surroundings.

Question 2

Give 3 examples of exothermic reactions.

Answer 2

Combustion, neutralisation and many oxidation reactions.

Question 3

Give 2 everyday uses of exothermic reactions.

Answer 3

Self-heating cans and hand warmers.

Question 4

What is an endothermic reaction?

Answer 4

One which takes energy from, and decreases the thermal energy store of, the surroundings.

Question 5

Give 2 examples of endothermic reactions.

Answer 5

Thermal decomposition, and the reaction between citric acid and sodium hydrogencarbonate.

Question 6

Give one use of an endothermic reaction.

Answer 6

Sports injury packs.

Question 7

Describe how you could test the effect of concentration of HCl on the energy released in a neutralisation between HCl and NaOH.

Answer 7

1) Place equal volumes of HCl and NaOH (of equal concentrations) in beakers.
2) Put these in a water bath at a set temperature and wait for them to reach this.
3) To minimise energy lost to the surroundings, line a beaker with cotton wool and put a polystyrene cup inside it.
4) Add the HCl and NaOH to the cup and place a lid on it, recording the starting temperature w/ a thermometer.
5) Record the temperature every 20 seconds until the temperature stops increasing.
6) Repeat steps 1-5 with different concentrations of HCl, and repeat each stage 3 times to find an average.

Question 8

Is the process of breaking bonds exothermic or endothermic, and why?

Answer 8

Endothermic, because it takes energy to overcome the forces holding together the bonds.

Question 9

Is the process of forming bonds exothermic or endothermic, and why?

Answer 9

Exothermic, because particles become more stable when they form compounds.

Question 10

Why do exothermic reactions release energy to the surroundings?

Answer 10

The energy released by forming bonds is greater than the energy used to break them.

Question 11

Why do endothermic reactions take energy from the surroundings?

Answer 11

The energy used to break bonds is greater than the energy released by forming them.

Question 12

What is the activation energy of a reaction?

Answer 12

The minimum amount of energy that particles must have to react.

Question 13

What are reaction profiles?

Answer 13

Diagrams that show the changes in the relative energies of the reactants and products of a reaction. The progress of the reaction is plotted against energy.

Question 14

Draw a reaction profile for an exothermic reaction.

Answer 14

Question 15

Draw a reaction profile for an endothermic reaction.

Answer 15

Question 16

Using the bond energies given below, calculate the energy change for the reaction between hydrogen and chlorine forming hydrochloric acid.

• H–H = +436kJ/mol
• Cl–Cl = +242kJ/mol
• H–Cl = +431kJ/mol

Answer 16

1. Symbol equation: H₂ + Cl₂ →2HCl
2. Find energy needed to break original bonds: 436 + 242 = 678kJ/mol
3. Find energy released by making new bonds: 2 x 431 = 862kJ/mol
4. Find overall energy change: 678 - 862 = -184kJ/mol

Question 17

What is a cell?

Answer 17

A device which contains chemicals which react to produce electricity.

Question 18

How are simple electrochemical cells made and how do they produce electricity?

Answer 18

Two different metal electrodes are connected in contact with an electrolyte (a salt or acid solution).

The electrodes react with the electrolyte, and the electrodes gain different charges (as they react differently).

A wire is connected between the electrodes, so electricity flows through it.

Question 19

Name 2 factors which determine the voltage produced by a cell.

Answer 19

The type of electrodes and electrolyte.

Question 20

What are batteries?

Answer 20

Two or more cells connected together in series to provide a greater voltage.

Question 21

Why do non-rechargable cells/batteries get used up?

Answer 21

The chemical reactions stop when one of the reactants has been used up. Alkaline batteries are non-rechargeable.

Question 22

Why can rechargeable cells be recharged?

Answer 22

The chemical reactions are reversed when an external electrical current is supplied.

Question 23

What are fuel cells?

Answer 23

Cells which are supplied by an external source of fuel (e.g. hydrogen) and oxygen/air.

The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.

Question 24

What does the reaction in a hydrogen-oxygen fuel cell involve?

Answer 24

The oxidation of hydrogen to produce water. This is a redox reaction: hydrogen + oxygen → water.

Question 25

Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries.

Evaluate the use of hydrogen fuel cells in comparison with rechargeable cells and batteries.

Answer 25

1. Batteries are more polluting to dispose of than fuel cells, because they’re made from highly toxic metal compounds.
2. There is a limit to how many times batteries can be recharged before they need replacing.
3. Batteries are more expensive than fuel cells.
4. Batteries store less energy than fuel cells.

Question 26

How do hydrogen-oxygen fuel cells work?

Answer 26

1. Hydrogen goes to the anode (negative) and is oxidised (loses electrons) to form H⁺ ions.
2. The H⁺ ions move to the cathode.
3. At the cathode (positive), oxygen is reduced (gains electrons) to form oxide ions, which react with the H⁺ ions to form water.

*The electrodes are named the opposite way to in electrolysis.

Question 27

Write the half equation for the reaction at the anode in a hydrogen fuel cell.

Answer 27

1. Hydrogen goes to the anode and is oxidised.
2. H₂ → 2H⁺ + 2e^-

Question 28

Write the half equation for the reaction at the cathode in a hydrogen fuel cell.

Answer 28

1. Oxygen goes to the cathode and is reduced, then reacts with H⁺ ions to form water.
2. O₂ + 4H⁺ + 4e^- →2H₂O