Calculating Heat Change

Heat change in a chemical reaction is directly proportional to the number of mole of reactant that take part in a reaction or number of mole of product been produced.


Example 1:
N(g) + ½ O2(g) → NO2(g) ∆H = +66 kj mol-1
Calculate the heat change when 0.1 mole of nitrogen dioxide is formed in the reaction that shown above.

Answer:
Heat change = 0.1 x 66kJ = 6.6 kJ


Example 2:
C(s) + O2(g) → CO2 (g)   ∆H=  393.5kJ mol-1
How much energy is released when 4g of carbon combust completely in excess oxygen. (Relative atomic mass of carbon = 12 )

Answer:
Number of mole of carbon = 4g 12gmo l 1 = 1 3 mol

Total heat been released = 1 3 ×393.5kJ=131.2kJ



Example 3:
CH3OH(l) +  O2 (g) → CO2 (g) + 2H2O (l)     ∆H =  560 kJ mol-1
Find the mass of alcohol that need to be combusted, in excess of oxygen, to release 140 kJ of heat energy.
[Relative atomic mass: H=1; C=12; 0=16]

Answer:
Number of mole of alcohol = 140kJ 560kJmo l 1 =0.25mol

Relative molecular mass of CH3OH = 12 + 3(1) + 16 + 1 = 32

Mass of alcohol = 0.25mol x 32g/mol = 8g


Example 4:
Calculate the heat change when excess zinc powder is added into 50cm³ of copper(II) sulphate solution 0.2 mol dm-3
CuSO4(ak) + Zn(p) → ZnSO4(ak) + Cu(p)
∆H =  190 kJ mol-1 

Answer:
Number of mole of copper(II) sulphate solution
n= MV 1000 n= (0.2)(50) 1000 =0.01mol

Heat change = 0.01 x 190kJ = 1.9kJ


Example 5:
The heat of combustion of carbon to CO2 is -393.5kJ/mol. Calculate the heat released upon formation of 35.2g of CO2 from carbon and oxygen gas.

Answer:
Relative molecular mass of carbon dioxide = 12 + 2(16) = 44

Number of mole of carbon dioxide = 35.2/44 = 0.8 mol

C + O2  CO2

Number of mole of carbon = 0.8 mol

Heat released = 0.8 x 393.5kJ = 314.8 kJ



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