Specific Latent Heat Questions

Specific Latent Heat Questions

Specific latent heat is the amount of thermal energy needed to change the state of a unit mass of a substance at constant temperature.

Q = mL (used for changes of state at constant temperature)

You will also be using Q = mcΔT (used for changes of temperature)

Latent heat of fusion (melting) of ice = 334 000 J/kg

Specific heat capacity of ice = 2110 J/kg/C

Specific heat capacity of water = 4200 J/kg/C

EASY

1.  Calculate the heat energy needed to melt 0.230 kg of ice at 0°C.

2.  Explain why putting ice at 0°C into a cocktail is much more effective at cooling the drink than adding cold water at 0°C

MEDIUM

3.  A sample of lead has a mass of 11.74kg and is at its melting point of 327 °C. Energy is supplied to the mass at a rate of 1500 W. After heating for 3 minutes all the lead becomes liquid. Assuming all of the heat energy supplied goes into the lead, calculate the latent heat of fusion of lead.

IB LEVEL

4.  When a runner runs she/he produces heat energy. Assume the runner has a mass of 70 kg and is mainly made of water (not a bad assumption!). The runner generates heat at a rate of 1200 W and runs for 30 minutes.

i.  Show the thermal energy generated is 2.2 x 106 J.

ii.  Calculate the temperature rise of the runner (water) assuming no energy loss (The specific heat capacity of water is 4200 Jkg-1K-1).

iii.  This temperature rise would be dangerous. Outline 3 ways in which heat energy is lost by the runner.

iv.  Some energy is lost as evaporation of sweat. Describe, in terms of the energy of molecules, how evaporation causes cooling.

v.  The percentage of energy lost by sweating is around 50%. The specific latent heat of vaporization of sweat is 2.26 x 106 Jkg-1. Use this information to calculate the mass evaporated from the runner.

vi.  State and explain two factors which affect the rate at which sweat is evaporated from the runner.