Question

Question 1.) Given the following data:

C2H2(g) + 5/2 O2(g) à2CO2(g) + H2O(l)      ΔH= -1300.kJ

C(s) + O2(g) àCO2(g)                                   ΔH= -394kJ

H2(g) + O2(g) à H2O(l)                                 ΔH= -286kJ

Use Hess’s Law to calculate ΔH for the reaction

2C(s) + H2(g) àC2H2(g)

Question 2.) Calculate ΔH° for each of the following reactions using the data for the enthalpy of

formation in the appendix.

2Na(s) + 2H2O(l)à2NaOH(aq) + H2(g

Answer 1

C2H2(g) + 5/2 O2(g) -----> 2CO2(g) + H2O(l) ΔH= -1300.kJ

C(s) + O2(g) -----> CO2(g)                                   ΔH= -394kJ

H2(g) + O2(g) -----> H2O(l)                                 ΔH= -286kJ

invert (1)

2CO2(g) + H2O(l)  -----> C2H2(g) + 5/2 O2(g) ΔH= +1300.kJ

C(s) + O2(g)  -----> CO2(g)                                   ΔH= -394kJ

H2(g) + O2(g)  ----->  H2O(l)                                 ΔH= -286kJ

multiply (2) by 2

2CO2(g) + H2O(l)  -----> C2H2(g) + 5/2 O2(g) ΔH= +1300.kJ

2C(s) + 2O2(g)  -----> 2CO2(g) ΔH= 2*(-394kJ) = -788

H2(g) + O2(g)  ----->  H2O(l)                                 ΔH= -286kJ

add all

2CO2(g) + H2O(l) + 2C(s) + 2O2(g) + H2(g) + O2(g) -----> C2H2(g) + 5/2 O2(g) + 2CO2(g) + H2O(l)

Cancel common terms

2C(s) + H2(g) -----> C2H2(g)

Add al H = 1300-788-286 = 226 kJ

2)

HRxn = Hprod - Hreact

2Na(s) + 2H2O(l) ---> 2NaOH(aq) + H2(g)

Hprod =(2*−425.93 + 0) = -851.86

Hreact = (2*0 + 2*−285.8) = -571.6

HRxn =  -851.86 - -571.6 = -280.26 kJ