Calorimetry Measurement of Enthalpy Change. Specific heat capacity is the amount of heat needed to raise the temperature of 1g of substance by 1K Specific heat capacity of water = 4.18 KJ kg -1 K -1 or 4.18 J g -1 K 1
Specific heat capacity of water = 4.18 KJ kg-1 K-1
or 4.18 J g-1 K1
Be careful with the units it could also be quoted as KJ g-1 K-1
Ensure you use the correct units in your calculation!
To measure the heat released in a process we arrange for the heat to be transferred to a substance (usually water) then measure the temperature rise.
H = mass of water x specific heat capacity x temp rise
H = m x c x T
Note m = mass of water not mass of any solids present
An insulated container to serve as a calorimeter
Volumetric appaaratus (e.g burette, pipette, measuring cylinder)
2) Thoroughly mix the reactants and record the highest or lowest temperature reached
For a given mass (m kg) of reacting substance the heat energy released is calculated using the equation
Heat = m x c x T
H = m x c x T
= 4.180 KJ
This is for the no of moles of CuSO4 used in the experiment
No of moles of CuSO4 = 0.2 x 100 = 0.02 moles
The reaction is exothermic so we need to put in a negative sign
Note We do not use the standard sign as standard conditions were not used.
To find the heat of combustion of a substance a known mass of the substance is burned, the heat released transferred to water and the enthalpy change found as before
Initial mass of lamp + ethanol = 65.20g
Final mass of lamp = 64.28g
Final temperature of water = 47.1oC
Initial temperature of water = 28.5oC
Mass of the water = 300g
What are the products of complete combustion of ethanol?
What mass of ethanol was burnt? How many moles is this?
What quantity of heat was transferred to the water?
Find Hc of ethanol
Identify any sources of error
Is ethanol a good fuel?
H = 300 x 4.18 KJ kg-1 K-1 x 18.6K = 23.3KJ
Mass of ethanol used = 0.92g
0.92g = 0.92 = 0.02 mol
Heat lost to surroundings (air, can thermometer)
Errors in measuring temperature change (unavoidable error in reading thermometer)
Errors in measuring masses (unavoidable error in reading balance)
The enthalpy change of combustion is high ethanol is a good fuel