(Done) Energy changes (Paper 1)

Question 1

Define exothermic reaction

Answer 1

• A reaction which transfers energy to the surroundings
• This is shown by a rise in temperature

Question 2

Examples of exothermic reactions

Answer 2

• Combustion
• Neutralisation reactions
• Many oxidation reactions

Question 3

Everyday uses of exothermic reactions

Answer 3

• Hand warmers
• Self heating drinks

Question 4

Define endothermic reaction

Answer 4

• A reaction which takes in energy from the surroundings
• This is shown by a fall in temperature

Question 5

Examples of endothermic reactions

Answer 5

• Citric acid and sodium hydrogen carbonate
• Thermal decomposition - heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide

Question 6

Everyday uses of endothermic reactions

Answer 6

• Sports injury packs - allow the pack to be instantly cold

Question 7

Define reaction profiles

Answer 7

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

Question 8

Define activation energy

Answer 8

• The minimum amount of energy the reactants need to collide with each other and react
• This has to be supplied by heating

Question 9

Features of an exothermic reaction profile

Answer 9

• Small amount of activation energy
• More energy released than activation energy

Question 10

Features of an endothermic reaction profile

Answer 10

• Large amount of activation energy
• Less energy released then activation energy

Question 11

What energy change occurs when breaking bonds

Answer 11

• Energy is used
• Endothermic

Question 12

What energy change occurs when forming bonds

Answer 12

• Energy is released
• Exothermic

Question 13

How do you calculate bond energies

Answer 13

• The sum of the energies needed to break bonds in the reactants minus the energy released when the bonds are formed

Question 14

Define an electrochemical cell

Answer 14

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

Question 15

How does an electrochemical cell cause charge to flow

Answer 15

• The chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes
• If the electrodes are connected by a wire, the charge is able to flow and electricity is produced

Question 16

Define electrolyte

Answer 16

• A liquid that contains ions which react with the electrodes

Question 17

How do the electrodes affect the voltage of an electrochemical cell

Answer 17

• The type of electrode affects voltage as different types of electrode will react differently with the same electrolyte
• The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell

Question 18

How can you calculate and predict the voltages of cells

Answer 18

• If electrode A is less reactive than electrode B, the voltage will be positive and vice versa

Question 19

How does the electrolyte affect the voltage of an electrochemical cell

Answer 19

• The electrolyte used affects voltage as different ions in a solution will react differently with the metal electrodes used

Question 20

What happens to the voltage of a circuit when cells are joined in series

Answer 20

• The voltages of the cells get added together and are now counted as a battery

Question 21

Why are non-rechargeable batteries non-rechargeable

Answer 21

• The chemical reactions that happen at the electrodes are irreversible
• Over time, the reacting particles - the ions in the electrolyte and the metal ions in the electrodes - get used up and turned into the products of the reaction
• Once all the reactants are used up, there are no more reactions between the particles therefore there is no longer a charge difference between the particles and charge is unable to flow
• The products are unable to be converted back into the reactants therefore the reaction can no longer occur and the battery is dead

Question 22

How are rechargeable batteries recharged

Answer 22

• By reversing the reactions by connecting them to an external electrical current

Question 23

Define a fuel cell

Answer 23

• An electrical cell that’s supplied with a fuel and oxygen and uses energy from this reaction to produce electrical energy efficiently
• when the fuel enters the cell it becomes oxidised and sets up a potential difference within the cell

Question 24

Reactants and products of a Hydrogen-Oxygen fuel cell

Answer 24

• Combines hydrogen and oxygen to produce water and release energy

Question 25

Properties of the electrolyte and electrodes in Hydrogen-Oxygen fuel cells

Answer 25

- Electrolyte is often an acid, such as phosphoric acid - Electrodes are often porous carbon with a catalyst

Question 26

How does a Hydrogen-Oxygen fuel cell function

Answer 26

- Hydrogen goes in the negative electrode compartment and oxygen goes in the positive electrode compartment - At the negative electrode, hydrogen loses electrons and is oxidised to produce H+ ions - H+ ions in the electrolyte move towards the positive electrode - At the positive electrode, oxygen gains electrons, is reduced and reacts with the H+ ions to produce water - The electrons flow through an external current from the negative to the positive electrode - this is the electrical current - The overall reaction is hydrogen plus oxygen which gives oxygen

Question 27

Advantages of fuels cells in vehicles

Answer 27

- Don't produce as many pollutants as other fuels - Fuel cells are infinitely rechargeable compared to electrical vehicles which are finitely rechargeable - Batteries store less energy than fuel cells and need to be recharged more often

Question 28

Disadvantages of fuel cells in vehicles

Answer 28

- Hydrogen is as gas to will take up loads more space than a rechargeable battery - Hydrogen is explosive when mixed with air so is difficult to store safely - The hydrogen fuel is often made from hydrocarbons (Fossil fuels) or from electrolysis of water which requires electricity usually generated from unclean options