A scientist seals some liquid nitrogen inside a container. The liquid is already at the temperature where it boils.
Transferring 9950 J to the boiling liquid turns 50 g of it into gas. Calculate nitrogen's specific latent heat of vaporisation.
Our own questions on this topic, marked straight away, so you see what to revise. Free now.
Show model answer for 08.1Hide model answer for 08.1
50 g = 0.050 kg.
Use this equation:. E = m × L. Put in the values:. 9950 = 0.050 × L. Rearrange for the latent heat:. L = 9950 over 0.050. = 199 000 J/kg
How the marks are earned
- One mark: change grams to kilograms.
- One mark: substitute into energy = mass × specific latent heat.
- One mark: rearrange to find the latent heat.
- One mark: the correct value. The unit, joules per kilogram, is not needed for the mark here, but always write it.
Why it scores
The equation needs the mass in kilograms. If this conversion goes wrong you lose only this mark, because later marks follow from your own mass.
Showing the substitution and the rearranged form picks up the working marks, and the division gives the final answer. The unit is not needed for the mark here, but always write it.
Revise Specific Latent HeatExplain the change in the nitrogen's internal energy as it boils from liquid to gas.
See how you would score on States of MatterShow model answer for 08.2Hide model answer for 08.2
Internal energy is the total kinetic and potential energy of all the particles. While the liquid boils, the energy supplied pulls the particles apart, so their potential energy increases.
Boiling happens at a fixed temperature, so the particles keep the same average speed and kinetic energy. Potential energy has risen and kinetic energy has not, so the overall internal energy has risen.
How the marks are earned
- One mark: the particles' potential energy goes up.
- One mark: the particles' kinetic energy does not change. The reason, a steady temperature, is extra detail and is not required.
- One mark: the internal energy therefore goes up. This mark needs both earlier points.
Why it scores
Naming the correct store that changes is the key first mark.
The constant temperature shows kinetic energy is unchanged, and combining both stores gives the conclusion.
Revise States of MatterThe nitrogen has all boiled, and now the gas gets hotter. Explain what happens to the pressure in the sealed container.
See how you would score on Gas Pressure & TemperatureShow model answer for 08.3Hide model answer for 08.3
Heating the gas raises the mean kinetic energy of its particles, so they move faster.
Faster particles hit the walls of the container harder, so each collision exerts a larger force. They also hit the walls more often, because they move faster and the volume is fixed.
More force from more frequent collisions on the same wall area means the pressure of the gas increases.
How the marks are earned
- One mark: the particles' mean kinetic energy goes up, or their mean speed goes up. Either one is enough.
- One mark: each collision with a wall exerts a larger force.
- One mark: collisions with the walls happen more often.
- One mark: so the pressure increases. This depends on at least one other mark.
Why it scores
This starts the chain at the correct place: temperature relates to particle energy.
Two separate effects each earn a mark. Force per collision and frequency of collisions are both needed.
The final mark links the particle effects back to pressure, so it only counts if earlier points are present.
Revise Gas Pressure & Temperature