Enthalpy calculations 


We can deduce the ΔH of an overall reaction given energy profiles or given the ΔH of related reactions.  
The ΔH of any process depends on the initial and final stages of that process. We can illustrate this simply by looking at the ΔH of the sublimation of water, that is water going from a solid straight to the gaseous state.  
Lets take the following equations 1)  H_{2}O(s) => H_{2}O(l) ΔH = 6.00 kJ/mol Adding 1) and 2) we get H_{2}O(s) + H_{2}O(l) => H_{2}O(l) + H_{2}O(g) ΔH = 50.01 kJ/mol Now since H_{2}O(l) appears on both sides of the equation we can cancel it out to get the equation below H_{2}O(s) => H_{2}O(g) ΔH = 50.01 kJ/mol 

Lets try some examples. But before we do remember: 

Example 1 Find the ΔH of the reaction below CO_{ 2}(g) + H_{ 2}(g) => CO(g) + H_{ 2}O(g)
given the two equations below. 2)  2CO(g) + O_{ 2}(g) =>2CO_{ 2}(g) ΔH =  566 kJ/mol 

Step 1 Reverse equation 2) 3)  2CO_{ 2}(g) => 2CO(g) + O_{ 2}(g) ΔH = 566 kJ/mol 

Step 2 Add equations 1) and 3) and dont forget to also add their ΔH 4)  2CO_{ 2}(g) + 2H_{ 2}(g) => 2CO(g) + 2H_{ 2}O(g) ΔH = (566  484) = 82 kJ/mol 

Step 3 Divide equation 4) by 2 and do the same to its ΔH CO_{ 2}(g) + H_{ 2}(g) => CO(g) + H_{ 2}O(g) ΔH = 82/2 = 41 kJ/mol 

Example 2 Calculate the ΔH of the reaction 2S(s) + 3O_{ 2}(g) =>2SO_{3}(g) given 1)  S(s) + O_{ 2}(g) => SO_{2}(g) ΔH =  297 kJ/mol 2)  2SO_{3}(g) =>O_{ 2}(g) + 2SO_{2} (g) ΔH = 196 kJ/mol 

Step 1 Reverse equation 2) to eliminate the SO_{2} and don't forget to change the sign of the ΔH . 3)  O_{2}(g) + 2SO_{2} => 2SO_{3}(g) ΔH =  196 kJ/mol 

Step 3 Multiply equation 1) by 2 and don't forget to multiply the ΔH 4)  2S(s) + 2O_{ 2}(g) => 2SO_{2}(g) ΔH =  594 kJ/mol 

Step 4 Add equations 3) and 4) and cancel any substance that appears on both sides, as shown on the right. Also add their ΔH 3O_{2}(g)+2S(s) + 2O_{ 2}(g) => 2SO_{3}(g) ΔH =  790 kJ/mol


1) Calculate the ΔH of the direct synthesis of methanol according to the equation 2C(s) + O_{ 2}(g) + 4H_{2}(g) => 2CH_{3}OH (g) ΔH = ? given the ΔH of the following reactions. 

2) Calculate the ΔH of the following reaction 2CO(g) + O_{2}(g) => 2CO_{2} (g) given


3) Calculate the ΔH of the following reaction 2N_{2} (g) + 5O_{2}(g) => 2N_{2}O_{5} (g) given


4) Consider the exercise on the left. Solution Step 1 Step 2 Step 3 

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