C5: Energy Changes

Question 1

What is the principle of conservation of energy?

Answer 1

• Energy is conserved in chemical reactions.
• The amount of energy at the end of a chemical reaction is the same as before the reaction takes place

Question 2

what is an exothermic reaction? Give 3 example and sign of energy change

Answer 2

• a reaction where energy is transferred to the surroundings so that the surroundings temperature increases
• negative sign of energy change
  e.g.
• combustion
• oxidation
• neutralisation

Question 3

What is an endothermic reaction and give two examples

Answer 3

• a reaction where energy is taken in from the surroundings so the surroundings temperature decreases
  e.g:
• photosynthesis
• thermal decomposition
• reaction of citric acid and sodium hydrogen carbonate

Question 4

define ‘ activation energy’

Answer 4

minimum amount of energy that particles need to react

Question 5

What is a reaction profile?

Answer 5

• a graph which shows the relative energies of reactants and product as well as activation energy of the reaction

Question 6

Describe exothermic and endothermic reactions in terms of bond breaking/ forming.

Answer 6

exothermic: energy released from forming bonds is greater than that needed to break the bonds
endothermic: energy needed to break bonds is greater than energy released making them

Question 7

What occurs in a chemical reaction in terms of bond energies?

Answer 7

Bond breaking: energy needs to be taken in from the surroundings to break the chemical bonds
Bond making : energy is transferred to the surroundings as the new bond is formed

Question 8

What is the equation to find enthalpy change in terms of bond energies?

Answer 8

energy of reaction : sum of bonds broken - sum of bonds made

Question 9

what is a cell

Answer 9

• a cell is composed of two electrodes dipped in an electrolyte solution.
• it produces electricity from a chemical reaction

Question 10

What is a battery?

Answer 10

• a battery consists of two or more cells connected in series

Question 11

what determines the voltage obtained from a cell?

Answer 11

• identities of metals used as electrodes
• the identity and concentration of an electrolyte

Question 12

State advantages and disadvantages of using cells and batteries (2/1)

Answer 12

(+)
- some are rechargeable,
- a convenient source of electrical energy
(-)
- harmful chemicals

Question 13

Describe the difference between rechargeable and non rechargeable cells

Answer 13

rechargeable - chemical reactions are reversed when an external current is supplied
non-rechargeable - reactants are used up, cannot be recharged

Question 14

What is a fuel cell?

Answer 14

• fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity

Question 15

What is the overall word
reaction in a hydrogen
fuel cell?

Answer 15

Hydrogen + Oxygen –> Water

Question 16

What is the overall symbol reaction in a hydrogen fuel cell? What are the half equations?

Answer 16

negative electrode
2H² → 4H⁺ + 4e⁻
positive electrode: O2 + 4H⁺ + 4e⁻ → 2H2O
Overall: → 2H² + O2 → 2H²O

Question 17

what ae the advantages and disadvantages of hydrogen fuel cells? (2/4)

Answer 17

adv : no pollutants, no recharging
disadv: flammable, fossil fuel production, toxic chemicals , expensive production of H² by electrolysis

Question 18

Give 3 advantages of
using hydrogen fuel cells
over rechargeable cells
and batteries

Answer 18

● Do not need to be electrically recharged
● No pollutants are produced
● Can be a range of sizes for different uses

Question 19

Give 3 disadvantages
of using hydrogen fuel
cells over rechargeable
cells and batteries

Answer 19

● Hydrogen is highly flammable
● Hydrogen is sometimes produced for the cell
by non-renewable sources
● Hydrogen is difficult to store

Question 20

How can you create a simple cell using
electrodes and an electrolyte?

Answer 20

Connect two different metals (electrodes) and dip them into an electrolyte

Question 21

How does flashcard 20
generate electricity?

Answer 21

• The more reactive metal gets oxidised.
• The electrons flow through the wire from the more reactive metal to the less reactive metal.
• the rate of flow of electrons is an electric current

Question 22

State 4 factors that
affect the voltage of a
cell

Answer 22

● The type of electrode used
● The electrolyte used
● The concentration of the electrolyte
● The temperature

Question 23

Why can a rechargeable battery be
recharged?

Answer 23

The chemical reactions are reversed using an external electric current