Energy Changes (5)

Question 1

What is an exothermic reaction

Answer 1

• one which transfers energy to the surroundings, usually by heating, this can be shown by a rise in temp
• the best example is burning fuels
• some everyday uses are : hand warmers, self heating cans
• if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants

Question 2

What is an Endothermic Reaction

Answer 2

• one which takes in energy from the surroundings, this is shown by a fall in temperature
• much less common than exothermic, thermal decomposition is the best example
• if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants

Question 3

How do you measure the amount of energy released by two solutions

Answer 3

1) Place the polystyrene cup inside the glass beaker to make it more stable.
2) Measure an appropriate volume of each liquid, eg 25 cm3.
3) Place one of the liquids in a polystyrene cup.
4) Record the temperature of the solution.
5) Add the second solution and record the highest or lowest temperature obtained.
6) Change your independent variable and repeat the experiment. Your independent variable could be the concentration of one of the reactants, or the type of acid/alkali being used, or the type of metal/metal carbonate being used.

Question 4

What hazards can there be with this practical

Answer 4

• Dilute acids and alkalis (could irritate the skin or eyes), Avoid contact with skin, rinse off skin if necessary, wear eye protection
• Solutions of metal salts used in displacement reactions (Dangerous to the environment), Dispose of metal salt solutions as advised by teacher, some metal salts are collected for safe disposal rather than being poured down the normal drain

Question 5

What are reaction profile diagrams

Answer 5

• Chart showing the energy in the reactants and products, and the difference in energy between them.

Question 6

What is a profile reaction of an exothermic reaction

Answer 6

• The energy level decreases in an exothermic reaction. This is because energy is given out to the surroundings.

Question 7

What is a profile reaction of an endothermic reaction

Answer 7

• The energy level increases in an endothermic reaction. This is because energy is taken in from the surroundings.

Question 8

What are the feature of reaction profile diagrams

Answer 8

• The activation energy is shown as a ‘hump’ in the line, which:
• starts at the energy of the reactants
• is equal to the difference in energy between the top of the ‘hump’ and the reactant
• difference between reactants and products is overall change in energy

Question 9

What equipment do you need for the practical

Answer 9

• polystyrene cup
• thermometer
• 250 cm3 glass beaker
• measuring cylinder
• top pan balance

Question 10

How do you calculate energy change

Answer 10

• add together the bond energies for all the bonds in the reactants - this is the ‘energy in’
• add together the bond energies for all the bonds in the products - this is the ‘energy out’
• energy change = energy in - energy out