Question 5.5.1: Use Hess’s law to calculate enthalpy change . Given the foll......
Use Hess’s law to calculate enthalpy change.
Given the following two reactions,
Reaction 1: SnCl_{2}(s) + Cl_{2}(g) → SnCl_{4}(ℓ) ∆H(1) = −195 kJ
Reaction 2: TiCl_{2}(s) + Cl2(g) → TiCl_{4}(ℓ) ∆H(2) = −273 kJ
calculate the enthalpy change for the following chlorine exchange reaction.
Reaction 3: SnCl_{2}(s) + TiCl_{2}(ℓ) → SnCl_{4}(ℓ) + TiCl_{2}(s) ∆H_{net} = ?
You are asked to calculate the enthalpy change for a given reaction.
You are given the enthalpy change for two other reactions.
In order to use Hess’s law, the sum of two or more chemical reactions must result in the net reaction. In this example, reversing the second reaction and adding it to the first reaction results in the net reaction. Notice that reversing the second reaction requires changing the sign of ∆H(2). Because the net reaction can be expressed as the sum of two chemical reactions, the enthalpy change for the net reaction is equal to the sum of the enthalpy changes for the individual steps.
SnCl_{2}(s) + Cl_{2}(g) → SnCl_{4}(ℓ) ∆H(1) = −195 kJ
TiCl_{4}(ℓ) → TiCl_{2}(s) + Cl_{2}(g) ∆H(2)′ = −∆H(2) = +273 kJ
SnCl_{2}(s) + TiCl_{4}(ℓ) → SnCl_{4}(ℓ) + TiCl_{2}(s) ∆H_{net} = ∆H(1) + ∆H(2)′ = −195 kJ + 273 kJ
= 78 kJ