Energy Changes

Question 1

What is an Exothermic reaction?

Answer 1

A reaction that transfers energy to the surroundings, usually by heating
- shown in a rise in temperature

Question 2

What is are examples of an Exothermic reaction?

Answer 2

Combustion- burning fuels
- gives out lots of energy

Neutralisation reactions (acid + alkali)

Many oxidation reactions

Question 3

What are everyday uses of Exothermic reactions?

Answer 3

• handwarmers
• self heating cans of hot chocolate or coffee
• freezing

Question 4

What is an Endothermic reaction?

Answer 4

A reaction that takes in energy from the surroundings
- shown by fall in temperature

Question 5

What is are examples of an Endothermic reaction?

Answer 5

• citric acid + sodium hydrogencarbonate
• thermal decomposition

Question 6

What are everyday uses of Endothermic reactions?

Answer 6

• Sports injury packs
• melting

Question 7

What is a Reaction Profile?

Answer 7

Diagrams that show relative energies of reactants and products in a reaction
- show how energy changes over the course of the reaction

Question 8

What is Activation energy?

Answer 8

The minimum amount of energy the reactants need to collide with eachother and react
-The energy needed to start the reaction

Question 9

Is bond breaking an endothermic or exothermic reaction?

Answer 9

Endothermic- energy must be supplied

Question 10

Is bond formation an endothermic or exothermic reaction?

Answer 10

Exothermic- energy is released

Question 11

What happens to bonds in a chemical reaction?

Answer 11

Old bonds are broken and new bonds are formed

Question 12

What is the equation to calculate Overall Energy Change using known Bond energies?

Answer 12

Overall energy change= energy required to break bonds - energy released by forming bonds

Question 13

What is an electrochemical cell?

Answer 13

A basic system made up of 2 different electrodes in contact with an electrolyte

• electrodes need to conduct electricity-> usually metal
  -charge difference between electrodes, charge flows through wire, electricity is produced
• connected voltmeter, measures voltage of the cell.

Question 14

What factors does the voltage of a cell depend on?

Answer 14

• type of electrode (different metals react differently with the same electrolyte)
• difference of reactivity between electrodes (bigger the difference, bigger the cell voltage)
• electrolyte used (reacts differently with the electrodes)
• battery formed by connecting 2 or more cells in series (combined voltage)

Question 15

How do non-rechargeable batteries work?

Answer 15

• reactions that happen at the electrodes are irreversible
• reacting particles get used up and turned into products over time (electrolyte ions + metal ions on electrode)
• once reactants are used up, reactions can’t happen -> no electricity is produced
• reactants can’t reverse back into products, cell cannot be recharged (alkaline batteries)

Question 16

What is a fuel cell?

Answer 16

An electrical cell supplied with a fuel and oxygen, uses energy from the reaction to produce electrical energy

Question 17

How does a fuel cell work?

Answer 17

• Fuel enters a cell, becomes oxidised and sets up PD within the cell

Question 18

How does a Hydrogen-Oxygen fuel cell work?

Answer 18

Electrolyte- acid
Electrode- porous carbon with catalyst

• Hydrogen goes to the anode, Oxygen goes to the cathode
  -Hydrogen looses electrons at anode (-ve), producing H+ ions (oxidation)
  -H+ ions move to the cathode (+ve)
  -Oxygen gains electrons from the cathode (+ve), reacts with H+ ions (from electrolyte) to make H2O. (reduction)
• Electrons flow through external circuit (anode to cathode) (-ve to +ve), creating electrical current
• Overall reaction: hydrogen + oxygen-> water

Question 19

What charge does an anode have in fuel cells?

Answer 19

Negative (opposite to electrolysis)

Question 20

What charge does a cathode have in fuel cells?

Answer 20

Positive (opposite to electrolysis)

Question 21

What are advantages of Hydrogen-Oxygen Fuel Cells?

Answer 21

• produce less pollutants, only water and heat
• Electric vehicle batteries are polluting to dispose of
• Rechargeable batteries
• Less expensive than Batteries to produce
• Batteries store less energy, charge more often

Question 22

What are disadvantages of Hydrogen-Oxygen Fuel Cells?

Answer 22

-Hydrogen is a gas, takes more storage space than rechargeable battery
- Hydrogen is explosive (in air), hard to store safely
- Hydrogen fuel often made from hydrocarbons (fossil fuels) or electrolysis (fuelled by fossil fuels)