Hess's Law

The enthalpy change for the reaction

C(s, graphite) + 1⁄2O₂(g) --> CO(g) cannot be measured directly since some carbon dioxide is always formed in the reaction.

It can be calculated using Hess’s Law and the enthalpy changes of combustion of graphite and of carbon monoxide.

C(s, graphite) + O₂(g) --> CO₂ ΔH=-394 kJmol^–1

CO(g) + 1⁄2O₂(g) --> CO₂ ΔH=-283 kJmol^–1

The enthalpy change for the reaction of graphite with oxygen to give carbon monoxide is

• -677 kJmol^–1

• -111 kJmol^–1

• +111 kJmol^–1

• +677 kJmol^–1

(Hess's Law)Calculate the ∆H for the following reaction: 2H₂O₂ →  2H₂O  + 1 O₂ 
You are given these two equations:
2H₂  +  O₂ → 2H₂O            ∆H  =  -572 kJ
H₂  +  O₂  →  H₂O₂            ∆H  =  -188 kJ 

• ∆H  =  -948 kJ 
• ∆H  =  -196 kJ 
• ∆H  =  -384 kJ 
• ∆H  =  -188 kJ 

Hess's Law states that...

• all reactions are reversible

• the enthalpy change of a reaction is independent of the route taken

• the direct route results in a greater enthalpy change

• the indirect route results in a lower enthalpy change as energy is lost in the second step

When you multiply a chemical equation by 2, what is done to the heat of reaction?

• multiplied by 1/2

• multiplied by 2

• nothing

When you flip a chemical reaction using Hess's Law, what is done to the heat of reaction value?

• nothing

• the sign is flipped

• 1/delta H

• -150 kJ

• +150 kJ

• -300 kJ

• +450 kJ