Hess's Law states that the total enthalpy change of a reaction is independent of the route taken, as long as the initial and final conditions are the same.

Using Hess's Law, write the equation to calculate ΔH r from standard enthalpy changes of formation.

Δ H r = ΣΔ H θ f ( products ) - ΣΔ H θ f ( reactants ). This is derived from the cycle: ΔH 2 = ΔH 1 + ΔH r , so ΔH r = ΔH 2 - ΔH 1 .

True or False? Hess's Law can be used to calculate enthalpy changes that cannot be measured directly by calorimetry.

True. For example, the enthalpy of formation of propane cannot be measured directly because hydrogen and carbon do not react under standard conditions. Hess's Law allows it to be calculated indirectly.

To calculate Δ H f from combustion enthalpies using Hess's Law: Δ H f = .......... - .......... , where ΔH 1 is the direct combustion of elements and ΔH 2 is the combustion of the compound.

To calculate Δ H f from combustion enthalpies using Hess's Law: Δ H f = ΔH 1 - ΔH 2 , where ΔH 1 is the direct combustion of elements and ΔH 2 is the combustion of the compound.

Define standard enthalpy change of atomisation .

A standard enthalpy change of atomisation (Δ H θ at ) is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state. For example: ½H 2 (g) → H (g).

Calculate Δ H r for 2NaHCO 3 (s) → Na 2 CO 3 (s) + CO 2 (g) + H 2 O (l), given: Δ H θ f [NaHCO 3 ] = -950.8, [Na 2 CO 3 ] = -1130.7, [CO 2 ] = -393.5, [H 2 O] = -285.8 kJ mol -1 .

Δ H r = ((-1130.7) + (-393.5) + (-285.8)) - (2 x -950.8) = +91.6 kJ mol -1 .

True or False? When applying Hess's Law, the number of moles of each compound must be taken into account when using Δ H f values.

True. If two moles of a compound appear in the equation, the Δ H f value for that compound must be multiplied by 2.

The average C-H bond enthalpy in methane is found by calculating Δ H for breaking all four bonds, then .......... by 4. Using Δ H θ f [CH 4 ] = -74.8 and atomisation data, Δ H = +1664.5, giving an average bond enthalpy of .......... kJ mol -1 .

The average C-H bond enthalpy in methane is found by calculating Δ H for breaking all four bonds, then dividing by 4. Using Δ H θ f [CH 4 ] = -74.8 and atomisation data, Δ H = +1664.5, giving an average bond enthalpy of +416 kJ mol -1 .

Calculate Δ H f [C 2 H 6 (g)] from combustion data: Δ H c [C (graphite)] = -393.5, [H 2 ] = -285.8, [C 2 H 6 ] = -1559.7 kJ mol -1 .

Δ H f = ((2 x -393.5) + (3 x -285.8)) - (-1559.7) = -84.7 kJ mol -1 .

A LevelChemistryCambridge (CIE)Flashcards5. Chemical EnergeticsHess’s Law

Hess’s Law (Cambridge (CIE) A Level Chemistry): Flashcards

Exam code: 9701

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Define Hess's Law.

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Define Hess's Law.
Hess's Law states that the total enthalpy change of a reaction is independent of the route taken, as long as the initial and final conditions are the same.
Using Hess's Law, write the equation to calculate ΔH_r from standard enthalpy changes of formation.
ΔH_r = ΣΔH^θ_f (products) - ΣΔH^θ_f (reactants). This is derived from the cycle: ΔH₂ = ΔH₁ + ΔH_r, so ΔH_r = ΔH₂ - ΔH₁.
True or False?
Hess's Law can be used to calculate enthalpy changes that cannot be measured directly by calorimetry.
True.
For example, the enthalpy of formation of propane cannot be measured directly because hydrogen and carbon do not react under standard conditions. Hess's Law allows it to be calculated indirectly.
To calculate ΔH_f from combustion enthalpies using Hess's Law: ΔH_f = .......... - .......... , where ΔH₁ is the direct combustion of elements and ΔH₂ is the combustion of the compound.
To calculate ΔH_f from combustion enthalpies using Hess's Law: ΔH_f = ΔH₁ - ΔH₂, where ΔH₁ is the direct combustion of elements and ΔH₂ is the combustion of the compound.
Define standard enthalpy change of atomisation.
A standard enthalpy change of atomisation (ΔH^θ_at) is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state. For example: ½H₂ (g) → H (g).
Calculate ΔH_r for 2NaHCO₃ (s) → Na₂CO₃ (s) + CO₂ (g) + H₂O (l), given: ΔH^θ_f [NaHCO₃] = -950.8, [Na₂CO₃] = -1130.7, [CO₂] = -393.5, [H₂O] = -285.8 kJ mol^-1.
ΔH_r = ((-1130.7) + (-393.5) + (-285.8)) - (2 x -950.8) = +91.6 kJ mol^-1.
True or False?
When applying Hess's Law, the number of moles of each compound must be taken into account when using ΔH_f values.
True.
If two moles of a compound appear in the equation, the ΔH_f value for that compound must be multiplied by 2.
The average C-H bond enthalpy in methane is found by calculating ΔH for breaking all four bonds, then .......... by 4. Using ΔH^θ_f [CH₄] = -74.8 and atomisation data, ΔH = +1664.5, giving an average bond enthalpy of .......... kJ mol^-1.
The average C-H bond enthalpy in methane is found by calculating ΔH for breaking all four bonds, then dividing by 4. Using ΔH^θ_f [CH₄] = -74.8 and atomisation data, ΔH = +1664.5, giving an average bond enthalpy of +416 kJ mol^-1.
Calculate ΔH_f [C₂H₆ (g)] from combustion data: ΔH_c [C (graphite)] = -393.5, [H₂] = -285.8, [C₂H₆] = -1559.7 kJ mol^-1.
ΔH_f = ((2 x -393.5) + (3 x -285.8)) - (-1559.7) = -84.7 kJ mol^-1.

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