C. |
Energy Calculations |
There are three different
formulas that we use to calculate energy.
All three are listed in Chemistry Reference Table T.
|
1.
Q= mc T |
This formula
is used when the sample changes temperature.
There is no phase change.
That would be the red lines in the
diagram above.
|
|
Q= Quantity
of Heat energy measured in joules (j)
m= Mass of
only the sample measured in grams (g)
C= Specific Heat Capacity
of the sample measured in
......joules/gram x Kelvin or degrees celcius (j/gK) or (j/goC)
T=
change in Temperature measured in kelvin or degrees
celcius (K/oC)
.............................. T
= Tfinal -
Tinitial |
|
2. Q= mHf |
This formula
is used when the sample phase changes
between a solid and a liquid.
There is no temperature change.
That would be the lower blue line
in the diagram
above. |
| |
Q= Quantity
of Heat energy measured in joules (j)
m= Mass of
only the sample measured in grams (g)
Hf =
Heat of Fusion measured in joules/gram (j/g) |
|
3.
Q= mHv |
This formula is used
when the sample phase changes
between a liquid and a gas.
There is no temperature change.
That would be the upper blue line
in the diagram
above. |
| |
Q= Quantity
of Heat energy measured in joules (j)
m= Mass of
only the sample measured in grams (g)
Hv = Heat
of Vaporization measured in joules/gram (j/g) |
| |
4. Water Constants |
There
are specific values we use for water in these equations.
They are listed in Chemistry Reference Table B.
|
|
C =
4.18 or 4.2 (j/gK) or (j/goC)
Hf = 333.6
or 334 (j/g)
Hv =
2259 or 2260 (j/g) |
