P3 Specific Latent Heat (page 194)

Question 1

If you heat up a pan of water on the stove, the water never gets any hotter than what temperature, and what is this called?

Answer 1

never gets any hotter than 100°C, even if you carry on heating it up, it wont get any hotter/rise.

This is to do with latent heat.

Question 2

Does a Change of State require Energy?

Answer 2

Yes

Question 3

Why is it when a substance is melting or boiling, you’re still putting in energy and so increasing the internal energy?

Answer 3

because the energy’s used for breaking bonds between particles rather than raising the temperature.

There are flat spots on the heating graph where energy is being transferred by heating but not being used to change the temperature.

see graph 1 on page 194

Question 4

When a substance is condensing or freezing, what is happening?

Answer 4

bonds are forming between particles, which releases energy.

This means the internal energy decreases, but the temperature doesn’t go down until all the substance has turned to liquid (condensing) or a solid (freezing).

The flat parts of the graph show this energy transfer (see graph 2 on page 194).

Question 5

The energy needed to change the state of a substance is called what?

Answer 5

Latent heat.

Question 6

What is meant by ‘Specifit Latent Heat (SLH) of a substance?

Answer 6

it means the amount of energy needed to change 1 kg of it from one state to another without changing its temperature.

Question 7

What does cooling, specific latent heat mean?

Answer 7

it is the energy released by the change in state.

Question 8

Is Specific Latent heat the same or different for different materials?

Answer 8

it is different, and for different for changing between different states.

Question 9

What is the ‘specific latent heat’ called for changing between a solid and a liquid (melting or freezing)?

Answer 9

it is called the ‘specific latent heat of fusion

Question 10

What is the ‘specific latent heat’ called for changing between a liquid and a gas (evaporating, boiling or condensing)?

Answer 10

it is called ‘specific latent heat of vaporisation’

Question 11

Dont get confused with specific heat capacity on (p.169), which relates to a temperature rise of 1°C, because specific latent heat means?

Answer 11

specific latent heat is about changes of state where there’s no temperature change.

Question 12

What is the formula to work out the energy needed (or released( wen a substance of mass m changes state?

Answer 12

Energy (E( = Mass (m) X Specific Latent Heat (L)

or

E = mL

Energy is given in joules (J), mass is in kg and SLH is in J/kg.

Question 13

The specific Latent heat of vaporisation for water (boiling) is 2 260 000 J/kg. How much energy is needed to completely boil 1.50 kg of water at 100°C? ?

Answer 13

1) Just plug the numbers into the formula
2) The units are joules becasue it’s energy

E = mL
= 1.50 x 260 000
= 3 390 000 J

If you’re finding mass or SLH, you’ll need to rearrange. Here’s the formula triangle (page 194)

                          E
                      \_\_\_\_\_\_\_
                    m   X    L

When it comes to the specific latent hea of vaporisation and fusion, the formula’s the same, but the process is different. Make sure you understand which process you’re actually looking at.

Question 14

The SLH of fusion for a particular substance is 120 000 J/kg. How much energy is needed to melt 250 g of the substance when it is already at its melting temperature? (2 marks)

Answer 14

E = m x L = 0.25 x 120 000 (1 mark)
= 30 000 J (1 mark)