Question

6.9

Find ΔHrxn for the following reaction:

2CdS(s)+3O2(g)→2CdO(s)+2SO2(g)

Use the following reactions with known ΔH values:

2H2S(g)S(s,rhombic)CdO(s)+++O2(g)O2(g)H2S(g)→→→2S(s,rhombic)SO2(g)CdS(s)++2H2O(g)H2O(g)ΔH=−442.4 kJΔH=−296.8 kJΔH=−124.7 kJ

Express the energy to one decimal place and include the appropriate units.

ΔHrxn =

Answer 1

2H₂S (g) + O₂(g)         2H₂O(g) + 2S(rhombic)   H = -442.4 kJ ---------------(1)

S (rhombic) + O₂(g) SO₂(g)   H = -296.8 kJ --------------------------------(2)

CdO (s) + H₂S(g) CdS(s) + H₂O(g)   H = -124.7 kJ ------------------------(3)

Multiplying equation 3 by 2 and reversing

2CdS(s) + 2H₂O(g) 2CdO (s) + 2H₂S(g)         H = 249.4 kJ ------------------------(4)

Multiplying equation 2 by 2

2S (rhombic) + 2O₂(g)         2SO₂(g)   H = -593.6 kJ --------------------------------(5)

Adding equations 4 and 5

2CdS(s) + 2S (rhombic) + 2O₂(g)     2CdO (s) + 2H₂S(g) + 2SO₂(g)            H = -344.2 kJ ------------(6)

Adding equations 6 and 1

2CdS(s) + 3O₂(g)     2CdO (s) + 2SO₂(g)            H = -786.6 Kj

Hrxn = -786.6 kJ