5. Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

One which transfers energy to the surroundings, usually by heating, shown by a rise in temperature.

Question 2

What are examples of exothermic reactions?

Answer 2

Combustion, neutralisation, most oxidation reactions.

Question 3

What are everyday uses of exothermic reactions?

Answer 3

Handwarmers (oxidation of iron in air with salt solution catalyst). Self heating cans of coffee.

Question 4

What is an endothermic reaction?

Answer 4

One which takes in energy from the surroundings, shown by a fall in temperature.

Question 5

Are endo or exo thermic reactions more common?

Answer 5

Exothermic.

Question 6

What are examples of endothermic reactions?

Answer 6

Reaction between citric acid and sodium hydrogencarbonate. Thermal decomposition.

Question 7

What is an everyday use of endothemic reactions?

Answer 7

Sports injury packs.

Question 8

Describe a practical where you can measure energy released in a chemical reaction.

Answer 8

Taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction.

Question 9

How can you reduce the energy lost to the surroundings in this practical?

Answer 9

Putting the polystrene cup in a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation.

Question 10

How could you test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid and sodium hydroxide?

Answer 10

1. Put 25cm³ of 0.25 mol/dm³ of hydrochloric acid and sodium hydroxide into seperate beakers.
2. Place the beakers in a water bath set to 25*C until they are both at the same temperature.
3. Add the HCl followed by the NaOH to a polystrene cup with a lid.
4. Take the temperature of the mixture every 30 seconds, and record the highest temperature.
5. Repeat steps 1-4 using 0.5 mol/dm³ and then 1 mol/dm³ of hydrochloric acid.

Question 11

What do reaction profile show?

Answer 11

The relative energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction.

Question 12

What is activation energy?

Answer 12

The minimum amount of energy the reactants need to collide with each other and react.

Question 13

What would the activation profile for an enothermic reaction look like?

Answer 13

The products would be at a higher energy than the reactants.

Question 14

Is bond breaking endo or exothermic?

Answer 14

Endothermic as energy must be supplied.

Question 15

Is bond forming endo or exothermic?

Answer 15

Exothermic as energy is released.

Question 16

What is an electrochemical cell?

Answer 16

Basic system made up of two different electrodes in contact with an electrolyte.

Question 17

What is the electrolyte?

Answer 17

Liquid that contains ions which react with the electrodes.

Question 18

How is the charge set up in an electrochemical cell?

Answer 18

The chemical reactions between the electrodes and electrolyte sets up the charge difference between the electrodes.

Question 19

The bigger the difference in reactivity of the electrodes, the _______ the voltage of the cell.

Answer 19

Bigger.

Question 20

What other factor, besides the electrodes, will affect the voltage?

Answer 20

The electrolyte since different ions in solution will react differently with the metal electrodes used.

Question 21

How is a battery formed?

Answer 21

Connecting two or more cells together in series.

Question 22

What are non-rechargeable batteries?

Answer 22

eg. alkaline batteries, contain cells which use irreversible reactions. Once one of the reactants are used up, they don’t produce any more charge and you have to replace them.

Question 23

What is a rechargeable cell?

Answer 23

The reaction can be reversed by connecting it to an external power source.

Question 24

What is a fuel cell?

Answer 24

An electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently.

Question 25

What sets up the potential difference in the fuel cell?

Answer 25

When the fuel enters and is oxidised.

Question 26

What is a hydrogen fuel cell?

Answer 26

One that combines hydrogen and oxygen to produce water and release energy.

Question 27

What compartment does hydrogen go into?

Answer 27

The anode (-ve) compartment.

Question 28

What compartment does oxygen go into?

Answer 28

The cathode (+ve) compartment.

Question 29

What happens at the negative electrode in a hydrogen fuel cell?

Answer 29

At the anode, hydrogen loses electrons to produce H+ ions. Oxidation.

Question 30

What happens at the positive electrode?

Answer 30

At the cathode, oxygen gains electrons from the cathode and reacts with H+ ions (from the acidic electrolyte) to make water. This is reduction.

Question 31

What is the half equation for the anode of a hydrogen fuel cell?

Answer 31

H₂ ⇨ 2H⁺ + 2e⁻.

Question 32

What is the half equation for the cathode of a hydrogen fuel cell?

Answer 32

O₂ + 4H⁺ + 4e⁻ ⇨ 2H₂O.