Energy changes

Question 1

What happens in an exothermic reaction?

Answer 1

Heat is given out - an exothermic reaction transfers energy to the surroundings, this is shown by a rise in temperature

Question 2

give an example of exothermic reaction:

Answer 2

• combustion
• neutralisation
• many oxidation reactions

Question 3

Give an example of an exothermic reaction being used in day-to-day situations:

Answer 3

• some hand warmers
• self heating cans of hot chocolate and coffee

Question 4

What happens in an endothermic reaction?

Answer 4

Heat is taken in - an endothermic reaction takes in energy from the surroundings. it is shown by a fall in temperature

Question 5

give an example of an endothermic reaction:

Answer 5

• reaction between citric acid and sodium hydrogen-carbonate
• thermal decomposition

Question 6

give an example of an endothermic reaction being used in day-to-day situations:

Answer 6

• sports injury packs

Question 7

What does an endothermic reaction profile look like?

Answer 7

• the products are at a higher energy than the reactants
• the difference in heat represents the overall energy change during the reaction (energy taken in) per mole

Question 8

what does an exothermic reaction profile look like?

Answer 8

• the products are at a lower energy than the reactants.
• the difference in height is the energy change (energy given out) per mole
• the initial rise in energy represents the activation energy

Question 9

what type of reaction is bond breaking?

Answer 9

endothermic - energy must be supplied to break the existing bonds

Question 10

what type of reaction is bond formation?

Answer 10

exothermic - energy is released when new bonds are formed

Question 11

how to calculate overall energy change?

Answer 11

sum of all energies needed to break bonds in reactants - energy released when the new bonds are formed in products

Question 12

how does an electrochemical cell work?

Answer 12

1) an electrochemical cell is a system made of 2 different electrodes in contact with an electrolyte.
2) the electrolyte is a liquid that contains ions which react with the electrodes
3) the chemical reactions set up a charge difference between the electrodes
4) if the electrodes are then connected by a wire, the charge can flow and electricity is produced

Question 13

What does the voltage of a cell depend on?

Answer 13

• type of electrodes
• the bigger the difference in reactivity of the electrodes, the bigger the voltage
• the electrolyte used

Question 14

what happens in non-rechargeable batteries?

Answer 14

the chemical reactions are irreversible so over time the reacting particles get used up and turned into products of the reaction. Once any of the reactants are used up, the battery cannot be used as the reaction cannot happen

Question 15

What is a fuel cell?

Answer 15

An electrical cell that is supplied with a fuel and oxygen (or air)

Question 16

How does a fuel cell work?

Answer 16

• it uses energy from the reaction between the fuel and oxygen to produce electricity efficiently
• when the fuel enters the cell, it becomes oxidised and sets up a potential difference within the cell

Question 17

Give an example of a fuel cell:

Answer 17

hydrogen-oxygen fuel cell
- it combines hydrogen and oxygen to produce clean water and release energy

Question 18

What are the advantages of a hydrogen-oxygen fuel cell in cars?

Answer 18

• doesn’t produce as many pollutants
• cheaper to make than batteries for electric cars
• stores more energy

Question 19

what are the disadvantages of a hydrogen-oxygen fuel cell in cars?

Answer 19

• hydrogen is a gas, so it takes up more space than a battery
• hydrogen is explosive - hard to store safely
• hydrogen fuel is often made from hydrocarbons or electrolysis (both bad for planet as they still burn fossil fuels)