Hess’s Law (CIE A Level Chemistry)

Propanol has the molecular formula C₃H₈O. Use the following information to calculate the enthalpy change for the formation of propanol.

The enthalpy change of combustion of hydrogen is –286 kJ mol^-1

The enthalpy change of combustion of carbon is –394 kJ mol^-1

The enthalpy change of combustion of propanol is – 1816 kJ mol^-1

The first stage in the industrial production of nitric acid from ammonia can be represented by the following equation.

4NH₃ (g) + 5O₂ (g) ⇌ 4NO (g) + 6H₂O (g)

Using the following standard enthalpy change of formation data, what is the value of the standard enthalpy change ∆H_f^θ for this reaction?

Compound	∆Hfθ /kJ mol–1
NH3 (g)	-51.3
NO (g)	+92.2
H2O(g)	-239.6

Titanium occurs naturally as the mineral rutile, TiO₂. One possible method of extraction of titanium is to reduce the rutile by heating with carbon.

TiO₂ (s) + 2C (s) → Ti (s) + 2CO (g)

The standard enthalpy changes of formation of TiO₂ (s) and CO (g) are –890 kJ mol^-1 and –105 kJ mol^-1 respectively.

What is the standard enthalpy change of the extraction of titanium?

The combustion of ethanol (C₂H₅OH) is increasingly being used to fuel cars. 

The standard enthalpy change of formation of carbon dioxide is –382 kJ mol^–1. 

The standard enthalpy change of formation of water is –275 kJ mol^–1.

The standard enthalpy change of formation of ethanol is −266kJ mol^–1. 

What is the standard enthalpy change of combustion of ethanol?

The equations below show the formation of sulfur oxides from sulfur and oxygen.

S (s) + O₂ (g) → SO₂ (g)     ∆H_f^θ = –297 kJ mol^–1

S (s) + 1½ O₂ (g) → SO₃ (g)    ∆H_f^θ = –395 kJ mol^–1

What is the enthalpy change of reaction, ∆H_r^θ, of 2SO₂ (g) + O₂ (g) → 2SO₃ (g)?

A student calculated the standard enthalpy change of formation of propane, C₃H₈, using a method based on standard enthalpy changes of combustion.

He used correct values for the standard enthalpy change of combustion of propane (–2220 kJ mol^–1) and hydrogen (–286 kJ mol^–1) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was –158 kJ mol^–1.

What did he use for the standard enthalpy change of combustion of carbon?

Given the following enthalpy changes,

                            I₂ (s) → I₂ (g)                          ∆H^Ɵ = +38 kJ mol^–1

                            I₂ (g) + 3Cl₂ (g) → 2ICl₃ (s)    ∆H^Ɵ = –214 kJ mol^–1

What is the correct value for ∆H_f^θ of iodine trichloride, ICl₃ (s)?

Using the following information:

                       CO (g) +  O₂ (g)   → CO₂ (g)            ∆H^θ  = –283 kJ mol^–1

                       H₂ (g) +  O₂ (g)   → H₂O (I)              ∆H^θ  = –286 kJ mol^–1

                       H₂O (g)   → H₂O (I)                              ∆H^θ = –44 kJ mol^–1

What is the enthalpy change, ∆H^θ, for the following reaction? 

CO₂ (g) + H₂ (g) → CO (g) + H₂O (g)

Iodine trichloride, ICl₃, is made by reacting iodine with chlorine.

I₂ (s) + Cl₂ (g)  2ICl (s) ;    ∆H^o = +14 kJ mol^–1

ICl (s) + Cl₂ (g)  ICl₃ (s) ;   ∆H^o = –88 kJ mol^–1

By using the data above, what is the enthalpy change of the formation for solid iodine trichloride?