# Unit 5 - Revision Summary

Specific heat capacity is the amount of heat required to change the temperature of 1kg of a substance by 1K.

The amount of heat gained or lost = mass × specific heat capacity × change of temperature

Q  = m × c × ∆q

The unit of c is the joule per kilogram kelvin, J kg -1 K

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When solving problems involving specific heat capacities, you only need to know two equations!
1. Heat  =  mass × specific heat capacity × temperature change
Q         =  m c ∆q

and

2. Heat lost = Heat gained
m c ∆q = m c ∆q

When reading the question, concentrate on what is losing heat and what is gaining heat - then use the above two equations to solve the problem.

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For the Leaving Certificate you only need to know how to measure the specific heat capacity of water OR a metal by a mechanical OR an electrical method.

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Latent heat is the “hidden heat” used to change the state of a substance without a change in temperature.

The specific latent heat (s.l.h.) is the quantity of energy required to change the state of 1 kg of a substance.

Energy added or removed = mass ×  s.l.h.
E = m × l

The unit of s.l.h is the joule per kilogram, J kg -1

The specific latent heat of fusion of ice is the amount of heat required to change 1kg of ice to water without a change in temperature.

The specific latent heat of vaporisation of water is the amount of heat required to change 1kg of water to steam  without a change in temperature.

When solving problems involving specific latent heat, you only need to know three equations!

1.  Heat = mass × specific heat capacity × temperature change
Q = m c ∆q
2.  Heat gained = Heat lost  (a variation is: Heat supplied = Heat gained)
3.  Energy added or removed  = mass ×  s.l.h.
E  = m  ×  l    [use when there is a change of state]

When reading the question, find what substance is changing state and apply Equation 3 to it
E  = m  ×  s.l.h.

Then solve the problem, as before, concentrating on what is losing heat and what is gaining heat.

In the latent heat of fusion of ice experiment, this is the equation that you should use:

 Heat Gained = Heat Lost Heat gained by ice melting + Heat gained by resulting water rising to final temperature = Heat lost by calorimeter cooling to final temperature + Heat lost by warm water cooling to final temperature (ml)ice + (mcDq)melted ice = (mcDq)cal + (mcDq)water

In the latent heat of vaporisation of steam experiment, this is the equation you should use:

Heat Lost    =    Heat Gained
Energy lost by steam    =    Energy gained by water
condensing and                      and calorimeter
water cooling

mSl    +  mScw∆q          =  mWcW∆q    + mCcC∆q

Applications

Specific heat capacity: Storage heaters

Latent heat: Heat pump (e.g. refrigerator), Perspiration

In Latent Heat you need to know the following experiments for the Leaving Certificate:
· Measurement of the specific latent heat of fusion of ice
· Measurement of the specific latent heat of vaporisation of water.

Congratulations! You have completed Unit 5.