C3.3 Calculating and Explaining Energy Change

Question 1

What does calorimetry measure?

Answer 1

The relative amounts of energy released when fuels or food burn

Question 2

Explain the method of calorimetry.

Answer 2

• Put a certain amount of water (e.g 50g) in a copper/glass container (copper/glass conducts electricity well)
• Record the start temperature
• Weigh the spirit (or whatever fuel you’re measuring) burner and lid
• Put the spirit burner under the container and light the wick.
• Heat the water, stirring constantly, until the temperature reaches about 50 degrees C
• Put the flame out using the burner lid, and measure the final temperature of the water
• Weigh the spirit burner and lid again
• Repeat

Question 3

What is the equation for energy transferred/released?

Answer 3

Q = mc∆t
Energy transferred (J) = mass of water (g) x specific heat capacity of water (4.2) x temperature change (degrees C)

Question 4

If 0.9g of fuel x produces enough energy to heat up 50g of water from 11 degrees c to 42 degrees c, how much does energy does 1g of the same fuel produce?

Answer 4

50g x 4.2 x 31 = 6510 joules
6510/0.9 = 7233 J

Answer = 7233 J / 7.233 KJ

Question 5

How can the amount of energy released or absorbed by a chemical reaction in a solution be calculated?

Answer 5

• From the measured temperature change of the solution when the reactants are mixed in an insulated container.
• This can be used for reactions of solids with water and neutralisation reactions

Question 6

How are energy levels affected when bonds are broken?

Answer 6

They decrease as energy is absorbed.

Question 7

How are energy levels affected when bonds are made?

Answer 7

They increase as energy is released.

Question 8

Why is energy released in exothermic reactions?

Answer 8

The energy released making bonds is greater than the energy needed to break existing bonds.

Question 9

Why is energy taken in in endothermic reactions?

Answer 9

The energy needed to break existing bonds is greater than the energy needed to make new bonds.

Question 10

How do catalysts speed up reactions?

Answer 10

They provide a different pathway for a chemical reaction that has a lower activation energy.

Question 11

What is the word equation for a reaction with hydrogen and oxygen?

Answer 11

hydrogen + oxygen ====> water

Question 12

What type of reaction is the one between hydrogen and oxygen?

Answer 12

Exothermic

Question 13

What are two uses of a reaction between hydrogen and oxygen?

Answer 13

• As a fuel in the combustion engines of cars

- As a fuel in a fuel cell

Question 14

What is a fuel cell?

Answer 14

An electrical cell supplied with oxygen and a fuel, it uses energy from the reaction between them to generate electricity.

Question 15

What are the advantages of using hydrogen as a fuel?

Answer 15

• It just forms water so it is very clean

- Less of a need for fossil fuels

Question 16

Why are fuel cells useful?

Answer 16

Unlike a battery, a fuel cell doesn’t need recharging and can be used as long as fuel is supplied

Question 17

What are the disadvantages of using hydrogen as a fuel?

Answer 17

• Engines need to be converted (expensive)
• As a gas it takes more space to store than liquid fuels
• It is very explosive so it is difficult to store safely
• It is made from either hydrocarbons or electrolysis, both procedures use energy