Energy Cycle Calculations (Cambridge (CIE) A Level Chemistry): Revision Note
Energy Cycle Calculations
The energy cycle involving the enthalpy change of solution (ΔHsolθ), lattice energy (ΔHlattθ), and enthalpy change of hydration (ΔHhydθ) can be used to calculate the different enthalpy values
According to Hess’s law, the enthalpy change of the direct and of the indirect route will be the same, such that:
ΔHhydθ = ΔHlattθ + ΔHsolθ
This equation can be rearranged depending on which enthalpy value needs to be calculated
For example, ΔHlattθ can be calculated using:
ΔHlattθ = ΔHhydθ - ΔHsolθ
Remember: the total ΔHhydθ is found by adding the ΔHhydθ values of both anions and cations together
Remember: take into account the number of each ion when completing calculations
For example, MgCl2 has two chloride ions, so when completing calculations this will need to be accounted for
In this case, you would need to double the value of the hydration enthalpy, since you are hydrating 2 moles of chloride ions instead of 1
Worked Example
Calculate the ΔHθ of the chloride ion in potassium chloride using the following data:
ΔHlattθ [KCl] = -711 kJ mol-1
ΔHsolθ [KCl] = +26 kJ mol-1
ΔHhydθ [K+] = -322 kJ mol-1
Answer:
Step 1: Draw the energy cycle of KCl
Step 2: Apply Hess’s law to find ΔHhydθ [Cl-]
ΔHhydθ = (ΔHlattθ[KCl]) + (ΔHsolθ[KCl])
(ΔHhydθ[K+]) + (ΔHhydθ[Cl-]) = (ΔHlattθ[KCl]) + (ΔHsolθ[KCl])
(ΔHhydθ[Cl-]) = (ΔHlattθ[KCl]) + (ΔHsolθ[KCl]) - (ΔHhydθ[K+])
Step 3: Substitute the values to find ΔHhydꝋ [Cl-]
ΔHhydθ [Cl-] = (-711) + (+26) - (-322) = -363 kJ mol-1
Worked Example
Calculate the ΔHθhyd of the magnesium ion in the magnesium chloride using the following data:
ΔHlattθ [MgCl2] = -2592 kJ mol-1
ΔHsolθ [MgCl2] = -55 kJ mol-1
ΔHhydθ [Cl–] = -363 kJ mol-1
Answer:
Step 1: Draw the energy cycle of MgCl2
Step 2: Apply Hess’s law to find ΔHhydꝋ [Mg2+]
ΔHhydθ = (ΔHlattθ[MgCl2]) + (ΔHsolθ [MgCl2])
(ΔHhydθ[Mg2+]) + (2ΔHhydθ [Cl-]) = (ΔHlatθ [MgCl2]) + (ΔHsolθ [MgCl2])
(ΔHhydθ[Mg2+]) = (ΔHlattθ[MgCl2]) + (ΔHsolθ[MgCl2]) - (2ΔHhydθ[Cl-])
Step 3: Substitute the values to find ΔHhydθ[Mg2+]
ΔHhydθ[Mg2+] = (-2592) + (-55) - (2 x -363) = -1921 kJ mol-1
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?