Energy Changes

Question 1

What is an exothermic reaction

Answer 1

• its a reaction that transfers energy to the surroundings, usually by heating
• this is shown by a raise in temperature

Question 2

Examples of exothermic reactions

Answer 2

• combustion
• neutralisation
• oxidation

Question 3

Every day uses of exothermic reactions

Answer 3

• some hand warmers use oxidation of iron in air to release energy
• self heating cans of hot chocolate and coffee

Question 4

What is an endothermic reaction

Answer 4

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

Question 5

Examples of endothermic reactions

Answer 5

• the reaction between citric acid and sodium hydrogencarbonate
• 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

Question 7

How is energy moved around in chemical reactions

Answer 7

• if the products of a reaction store more energy than the original reactants then they must have taken the difference in energy from the surroundings
• if the products store less energy,then the excess energy was transferred to the surroundings during the reaction
• the overall amount of energy doesn’t change because energy is conserved in reactions

Question 8

How to measure energy transfer

Answer 8

1. Take temperature of reagents
2. Mix them in a polystyrene cup
3. Measure temperature of solution at the end of the reaction
4. Make sure to put polystyrene cup into a beaker of cotton wool to give more insulation and put a lid on the cup to reduce energy lost by evaporation

Question 9

Experiment to find neutralisation between HCl and NaOH

Answer 9

1. Pu 25cm^3 of 0.25mol/dm^3 of HCl and NaOH in separate beakers
2. Place the beakers in a water bath set to 25 degrees C until they’re both at the same temp
3. Add the HCl followed by the NaOH to a polystyrene cup with a lid
4. Take the temperature of the mixture every 30 seconds and record the highest temperature
5. Repeat steps 1-4 using 0.5mol/dm^3 and then 1mol/dm^3 of hydrochloric acid

Question 10

Exothermic reaction profile

Answer 10

1. Higher line for reactants
2. Lower line for products
3. Draw activation energy (upwards arrow)
4. Draw energy released (downwards arrow)

Question 11

Activation energy

Answer 11

• the minimum amount of energy reactants need to collide with eachother and react
• the greater the activation energy, the more energy needed to start the reaction

Question 12

Endothermic reactions profile

Answer 12

1. Lower line for reactants
2. Higher line for products
3. Big activation energy (big arrow upwards)
4. Energy absorbed arrow upwards

Question 13

Breakage of bonds

Answer 13

• energy is supplied to break existing bonds
• endothermic process

Question 14

Bond forming

Answer 14

• energy is released when new bonds are formed
• exothermic process

Question 15

Bond energy equation

Answer 15

1. Find energy required to break original bonds
2. Find energy released by forming the new bonds
3. Find overall energy change