C7 (Exothermic And Endothermic Reactions)

Question 1

Exothermic reaction definition

Answer 1

Energy transfers from the reactants to the surroundings, and the energy of the products is less than the reactants
- decrease temperature

Question 2

Endothermic definition

Answer 2

Energy transfers from the surroundings to the reactions and products have more energy than the reactants
- increase temperature

Question 3

3 examples of Exothermic reactions

Answer 3

• combustion (burning fuels)
• neutralisation (acid + alkali)
• oxidation (eg. sodium + water)

Question 4

2 everyday uses of exothermic reactions

Answer 4

• hand warmers uses oxidation of iron in air

- self heating cans rely on the chemicals in their bases

Question 5

2 examples of endothermic reactions (less common)

Answer 5

• reaction of citric acid and sodium hydrogen carbonate

- thermal decomposition (calcium carbonate to calcium oxide)

Question 6

1 example of the everyday use of endothermic reactions

Answer 6

Sports injury pack, chemical reactions make it cooler

Question 7

How do you measure the energy transfer?

Answer 7

• Take the temperature of the solution before, putting in polystyrene cup with cotton wool and taking the temperature after

Question 8

What are the 3 types of reactions the polystyrene cup test works for?

Answer 8

• neutralisation
• reaction between metal and acids
• reaction between carbonates and acids

Question 9

What is a reaction profile?

Answer 9

Diagrams to show relative energies of reactants and products

Question 10

What is the reaction profile for an exothermic reaction?

Answer 10

• increase for activation energy
• products less energy than reactant
• height is the energy released per mole

Question 11

What is the reaction profile for an endothermic reaction?

Answer 11

• increases due to activation energy
• slight drop but products energy more than reactants
• height is the energy absorbed per mole

Question 12

What happens to the bonds in an endothermic reaction?

Answer 12

• energy supplies that breaks strong bond

- energy input greater than output

Question 13

What happens to the bonds in an exothermic reaction?

Answer 13

• energy released which forms the formation of bonds

- output energy greater than input

Question 14

Overall energy change =

Answer 14

Energy required to break bonds - energy released from bonds

Question 15

What is an electrochemical cell?

Answer 15

• basic system with two metal electrodes that conduct in contact with an electrolyte
• flow of charge created, use voltmeter to measure

Question 16

3 factors the voltage of cell depends on

Answer 16

1) metals react differently with the electrolyte
2) bigger difference in reactivity of the electrodes, bigger the voltage
3) electrolyte used, as different ions react differently with metals

Question 17

What is a battery?

Answer 17

• This is two or more cells together in a series

- cells combined for more voltage

Question 18

What is a non-rechargeable battery?

+ example

Answer 18

• battery contains irreversible reactions, once one of the reactants is used up
• alkaline batteries

Question 19

What is a fuel cell?

Answer 19

An electrical cell which uses fuel and oxygen, and uses the energy from the reaction to produce electricity efficiently

Question 20

What happens when the fuel enters the cells?

Answer 20

• oxidised and sets up potential difference

Question 21

What is the difference between fuel cells and electrolysis?

+ what does it mean for OILRIG

Answer 21

• In fuel cells the anode is negative and the cathode is positive
• Oxidation is at the positive and reduction at the negative

Question 22

What is a hydrogen-oxygen fuel cell?

Answer 22

• This is where the electrolyte is potassium hydroxide

- Hydrogen is at the anode and the oxygen is at the cathode

Question 23

What is the overall reaction of a fuel cell (equation)

Answer 23

2H2 + O2 -> 2H20

Question 24

Advantages of fuel cells

Answer 24

Adv:

• no pollutants
• rechargeable
• cheaper than batteries
• store more energy than batteries

Question 25

Disadvantages of Fuel Cells

Answer 25

• hydrogen takes up more space than rechargeable battery
• hydrogen explosive when reacts with air
• uses fossil fuels to create the hydrogen fuel