Hess's Law and enthalpy cycles
In a nutshell
Understanding Hess's law and how this can be applied to enthalpy cycles allows you to calculate enthalpy changes indirectly. By interpreting different enthalpy values, you can learn to produce enthalpy cycles and determine unknown enthalpy changes.
Enthalpy cycles
Hess's law states that regardless of the route you take, the total enthalpy change will be equal.
This law applies to enthalpy cycles which show alternative routes for a reaction to reach the same product. Any route will result in the same total enthalpy change.
Standard enthalpies of formation
Enthalpy cycles are useful to determine unknown enthalpy changes if all ΔfHθ are known for compounds.
Note: Elements always have an ΔfHθ of zero.
ΔrHθ is the standard enthalpy change of reaction where compounds have their molar quantities in the equation under standard conditions.
ΔfHθ is the standard enthalpy change of formation where one mole of a compound is formed from its individual elements in standard states under standard conditions.
PROCEDURE
1. | Make the two routes in the cycle equal to each other |
2. | Rearrange to find the unknown enthalpy change |
3. | Input reactants and products into equation |
4. | Input known enthalpy values into equation |
Example
ΔfHθ [H2O(l)]=−84 kJ mol−1
ΔfHθ[CO2(g)]=−281 kJ mol−1
What is ΔrHθ?
Following Hess's law, route 1 and route 2 will have an equal enthalpy change. The arrows for ΔfHθ point towards the main reaction as the reactants and products at the top are formed from the elements at the bottom. ΔfHθ is presented for all the compounds present in the chemical reaction. All elements (H2,O2,C) have an ΔfHθ of zero.
If route 1 = route 2:
ΔfHreactantsθ+ΔrHθ=ΔfHproductsθ
This can be rearranged to:
ΔrHθ=ΔfHproductsθ−ΔfHreactantsθ
Now you can input the given values into this equation:
ΔrHθ=[(CO2)+(2×H2)]−[(2×H2O)+(C)]
ΔrHθ=[−281+(2×0)]−[(2×−84)+0]
ΔrHθ=−113 kJ mol−1
ΔrHθ for this reaction is −113 kJ mol−1.
Note: The first zero represents the element H2 in the products and second zero represents the element C in the reactants.
Standard enthalpies of combustion
Similarly, enthalpy cycles can use ΔcHθ to determine unknown enthalpy changes. All reactants and products other than oxygen will have a ΔcHθ value.
Note: Combustion products do not have a ΔcHθ value.
ΔcHθ is the standard enthalpy change of combustion where one mole of a substance is completely combusted in oxygen under standard conditions.
PROCEDURE
1. | Make the two routes in the cycle equal to each other |
2. | Rearrange to find the unknown enthalpy change |
3. | Input reactants and products into equation |
4. | Input known enthalpy values into equation |
Example
ΔcHθ[C(s)]=−389 kJ mol−1
ΔcHθ[H2(s)]=−315 kJ mol−1
ΔcHθ[C4H9OH(l)]=−2479 kJ mol−1
What is ΔfHθ?
As route 1 and 2 have an equal enthalpy change, we can determine ΔfHθ from the ΔcHθ values provided. The arrows for ΔcHθ point from the main reaction to the bottom compounds as the reactants and products at the top are combusted to produce the same combustion products shown at the bottom. The combustion products and oxygen have ΔcHθ of zero.
If route 1 = route 2:
ΔfHθ+ΔcHθ (products)=ΔcHθ (reactants)
This can be rearranged to:
ΔfHθ=ΔcHθ (reactants)−ΔcHθ (products)
Input values for the corresponding compounds:
ΔfHθ=[(4×C)+(5×H2)+(21×O2)]−[C4H9OH]
ΔfHθ=[(4×−389)+(5×−315)+(21×0)]−[−2479]
ΔfHθ =−652 kJ mol−1
ΔfHθ for this reaction is −652 kJ mol−1.
Note: ΔcHθ can be carried out using individual elements or hydrocarbons as the reactants.
Forming enthalpy cycles
Enthalpy cycles (or Hess cycles) can be formed when you cannot directly measure enthalpy changes such as thermal decomposition reactions. These are endothermic so decrease in temperature, but must be heated to begin the reaction. Calorimetry experiments can be carried out to measure the total enthalpy changes of an alternative route that is equal to the route you cannot measure.
PROCEDURE
1. | Write out the chemical equation that will form the top of the cycle |
2. | Determine what type of enthalpy change could be measured and the substances needed at the bottom of the cycle |
3. | Draw the arrows in the correct direction for the new enthalpy change |
4. | Determine the two different routes |
Example
Draw an enthalpy cycle for the thermal decomposition of carbonic acid, H2CO3.
Write out the equation for the thermal decomposition of carbonic acid:
H2CO3(l)→CO2(g)+H2O(l)
This will form the top of the enthalpy cycle which is ΔrHθ. A calorimetry experiment can be carried out to measure ΔfHθ of the reactants and products which would give you:
H2(g)+C(s)+121O2(g)
This will form the bottom of the enthalpy cycle. The arrow towards the reactants of the thermal decomposition will be ΔfHθ (reactants) and the arrow towards the products will be ΔfHθ (products).
Now you can form the enthalpy cycle which will look like this:
Both of the bottom arrows are pointing towards the main reaction as the elements at the bottom are forming the reactants and products of the main reaction. The two routes can be distinguished by the different colour arrows.
ΔrHθ for the thermal decomposition of carbonic acid can be calculated by using the general formula route 1 = route 2:
ΔfH(products)θ+ΔrHθ=ΔfH(reactants)θ
ΔrHθ=ΔfH(products)θ−ΔfH(reactants)θ
These numbers can be determined through calorimetry experiments.