Solution: From the enthalpies of reaction2C(s) + O2 (g) → 2CO(g)                          ΔH   rxn = -221.0 kJ2C(s) + O2 (g) + 4H2 (g) → 2CH3OH(g)    ΔHrxn = -402.4 kJcalculate ΔH for the reaction:CO(g) + 2H2 (g) → CH3OH(g)(a) -623.4 kJ (b) 623.4 kJ (c) 181.4 kJ (d) -90.7 kJ (e) -181.4 kJ

Problem

From the enthalpies of reaction

2C(s) + O2 (g) → 2CO(g)                          ΔH   rxn = -221.0 kJ
2C(s) + O2 (g) + 4H2 (g) → 2CH3OH(g)    ΔHrxn = -402.4 kJ

calculate ΔH for the reaction:

CO(g) + 2H2 (g) → CH3OH(g)

(a) -623.4 kJ 

(b) 623.4 kJ 

(c) 181.4 kJ 

(d) -90.7 kJ 

(e) -181.4 kJ

Solution

We’re being asked to determine the enthalpy change (ΔHrxn) for the chemical reaction:

From the enthalpies of reaction


CO(g) + 2H2 (g) → CH3OH(g)


We can use Hess’s Law to determine the enthalpy change of the overall reaction from the given reactions:


1. 2 C(s) + O2 (g) → 2 CO(g)                                             ΔH1 = -221.0 kJ

2. 2 C(s) + O2 (g) + 4H2 (g) → 2CH3OH(g)                       ΔH2 = -402.4 kJ



We now need to find a combination of reactions that when added up, gives us the overall reaction.


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