5. Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

One that gives out energy to the surroundings

Question 2

What is an endothermic reaction?

Answer 2

One that takes in energy from the surroundings

Question 3

In terms of energy, what happens when bonds are broken?

Answer 3

Energy is required (endothermic)

Question 4

In terms of energy, what happens when bonds are formed?

Answer 4

Energy is released (exothermic)

Question 5

How do you remember whether energy is released or required when bonds are being rearranged?

Answer 5

Bendy Mexicans

B(endo)ROKEN
M(exo)ADE

Question 6

What does it mean if the energy of the products is lower than the reactants?

Answer 6

Energy has been released and the reaction is exothermic

Question 7

What does it mean if the energy of the products is higher than the reactants?

Answer 7

Energy has been taken in and the reaction is endothermic

Question 8

How does a catalyst increase the rate of reaction?

Answer 8

By lowering the activation energy

Question 9

How do you work out total energy change?

Answer 9

• total for breaking bonds: no. of bonds x energy it takes to break that type of bond
• total for making bonds: no. of bonds x energy it takes to make that type of bond
• total energy change: breaking bonds - making bonds

Question 10

What does it mean if you get a positive number when working out energy change?

Answer 10

The reaction is endothermic

Question 11

What does it mean if you get a negative number when working out energy change?

Answer 11

The reaction is exothermic

Question 12

In terms of energy change, what do exothermic and endothermic reactions always have?

Answer 12

• endothermic always have positive energy changes

* exothermic always have negative

Question 13

How can a simple cell be made?

Answer 13

By connecting two different metals in contact with an electrolyte

Question 14

What do batteries consist of to provide a greater voltage?

Answer 14

Two or more cells connected

Question 15

What chemicals are cells made from?

Answer 15

• carbon rod with metal cathode
• manganese oxide
• ammonium chloride
• zinc casting anode

Question 16

What is the electrolyte in a cell made of?

Answer 16

Ammonium chloride

Question 17

What is the anode made of in a cell?

Answer 17

Zinc casting

Question 18

What does the voltage produced by a cell depend on?

Answer 18

The type of metals making up the cathode and anode and electrolyte

Question 19

What happens when two metals are dipped in salt solution and joined by a wire?

Answer 19

The more reactive metal will donate electrons to the less reactive metal to form an electrical cell

Question 20

Which metal will lose their electrons in a simple electrical cell?

Answer 20

The more reactive metal

Question 21

What happens when there is a greater difference between reactivity of metals in a cell?

Answer 21

There will be a higher voltage produced

Question 22

Are alkaline batteries rechargeable?

Answer 22

No

Question 23

What happens when one of the reactants has been used up in non-rechargeable batteries?

Answer 23

The chemical reactions stop

Question 24

What happens to the chemical reactions in rechargeable batteries?

Answer 24

They are reversed using an external electrical current

Question 25

How do hydrogen fuel cells work?

Answer 25

• oxygen is supplied to the positive electrode and diffuses through the graphite to react to form hydroxide ions - accepting electrons from the external circuit
• hydrogen is supplied to the negative electrode and diffuses through the graphite electrode and reacts with hydroxide ions to form water
• overall charge left in fuel cell

Question 26

What hydrogen fuel cells offer an alternative to?

Answer 26

Fossil fuels and rechargeable cells and batteries

Question 27

Advantages of hydrogen fuel cells?

Answer 27

• don’t need to be electrically recharged
• no pollutants produced
• can be a range of sizes for different uses
• do not get less efficient as they run
• can be a source of drinking water

Question 28

Disadvantages of hydrogen fuel cells?

Answer 28

• hydrogen is flammable and difficult to store
• hydrogen is sometimes produced for the cell by non renewable sources
• produce a relatively low voltage so several are needed together

Question 29

What are fuel cells useful as an energy source for aircraft?

Answer 29

• provide water
• efficient
• no moving parts
• lightweight

Question 30

What are the main disadvantages of using hydrogen fuel cells for aircraft?

Answer 30

• have poisonous catalysts

* producing the hydrogen and oxygen will use energy elsewhere

Question 31

Half equation for the hydrogen supplied to a hydrogen fuel cell? (anode)

Answer 31

H2 -> 2H+ + 2e-

Question 32

Half equation for the oxygen supplied to a hydrogen fuel cell?

Answer 32

02 + 4H+ + 4e- -> 2H2O

Question 33

Equation for the two electrode reactions in a hydrogen fuel cell?

Answer 33

2H2 + O2 -> 2H2O