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Question:

Given that: (i) C(graphite) + O2(g) → CO2(g); ΔrHo = x kJ/mol (ii) C(graphite) + 1/2O2(g) → CO(g); ΔrHo = y kJ/mol (iii) CO(g) + 1/2O2(g) → CO2(g); ΔrHo = z kJ/mol Based on the given thermochemical equations, find out which one of the following algebraic relationships is correct?

z=x+y

x=y-z

y=2z-x

x=y+z

Solution:

C(graphite) + O2(g) → CO2(g) ΔrHo = x kJ/mol (1)
C(graphite) + 1/2O2(g) → CO(g) ΔrHo = y kJ/mol. (2)
CO(g) + 1/2O2(g) → CO2(g) ΔrHo = z kJ/mol (3)
Now, (1) = (2) + (3)
x = y + z.