Energy Calculations

Question 1

What type of reaction occurs when food and fuels react with oxygen?

Answer 1

Exothermic

Question 2

How can a simple calorimeter be used to measure the amount of energy released by different fuels or foods?

Answer 2

• The simplest calorimeter is some water in a glass beaker/metal can
• When a substance burns and heats the water, the temperature rise of the water depends on the amount of energy released
• We can measure the temperature change of the water and use this to work out how much energy was transfered to the water using the equation Q = mcΔt

Question 3

What equation can we use to work out the amount of energy transfered to water by a fuel/food?

Answer 3

Q = mcΔt

Q = amount of energy transfered, joules (J)

m = mass of water, grams (g)

c = specific heat capacity of water, J/g/°C

Δt = temperature change, °C

Question 4

Why do simple calorimeters not give accurate results?

Answer 4

Because much of the energy released by the food/fuel is used to heat the surroundings

Question 5

How would you compare the energy released by burning different substances? (What units would you use?)

Answer 5

Either energy change in kJ per gram or in kJ per mole can be used

Question 6

How can the energy change in kJ/mole be calculated?

Answer 6

By multiplying the energy change in kJ/g by the RFM of the substance

(Mass (g) = Moles X RFM)

Question 7

How can the energy lost to the surroundings be reduced when doing a reaction in a solution?

Answer 7

By using an insulated container

Question 8

If using Q = mcΔt, what can you assume about the solution?

Answer 8

That it behaves like water

This means that 1cm³ = 1 g and the specific heat capacity is 4.2 J/g/°C

Question 9

When a solid is added to water or an aqueous solution, what can we assume?

Answer 9

The volume of the solution does not change

Also that 1 cm³ of solution still equals 1 g and the specific heat capacity is still 4.2 J/g/°C

Question 10

What do energy level diagrams show?

Answer 10

The energy changes for chemical reactions

Question 11

What happens when bonds are broken and what does this mean?

Answer 11

Energy is taken in so the reaction is endothermic

Question 12

What happens when bonds are made and what does this mean?

Answer 12

Energy is released so the reaction is exothermic

Question 13

Draw an energy level diagram for an exothermic reaction

Answer 13

Question 14

Draw an energy level diagram for an endothermic reaction

Answer 14

Question 15

What is bond energy?

Answer 15

The energy needed to break a particular chemical bond

Question 16

What are bond energies measured in?

Answer 16

kJ/mol

Question 17

What are the advantages of using hydrogen as a fuel?

Answer 17

• It burns easily
• It releases a large amount of energy per gram
• It produces no CO₂ when burned, only water
• It can be burned in combustion engines or fuel cells
• It can be produced from renewable resources

Question 18

What are the disadvantages of using hydrogen as a fuel?

Answer 18

• Supply problems
• Storage problems
• Safety problems
• Vehicles that use fuel cells need to match the performance, convenience, and costs of petrol and diesel vehicles