Kimia lecture
Transcript of Kimia lecture
Lecture 4: Hess’s Law
Reading: Zumdahl 9.5
Outline:
Definition of Hess’ Law
Using Hess’ Law (examples)
Q: What is Hess’s Law ?
• Recall (lecture 3) Enthalpy is a state function. As such, DH for going from some initial state to some final state is pathway independent.
• Hess’s Law: DH for a process involving the transformation of reactants into products is not dependent on pathway.
(This means we can calculate DH for a reaction by a single step, or by multiple steps)
Using Hess’s Law
When calculating DH for a chemical reaction as a single step, we can use combinations of reactions as “pathways” to determine DH for our “single step” reaction.
2NO2 (g)
N2 (g) + 2O2 (g)
q
2NO2 (g)N2 (g) + 2O2 (g)
The reaction of interest is:
N2(g) + 2O2(g) 2NO2(g) DH = 68 kJ
This reaction can also be carried out in two steps:
N2 (g) + O2 (g) 2NO(g) DH = +180 kJ
2NO (g) + O2(g) 2NO2(g) DH = -112 kJ
• If we take the previous two reactions and add them, we get the original reaction of interest:
N2 (g) + O2 (g) 2NO(g) DH = +180 kJ
2NO (g) + O2 (g) 2NO2(g) DH = -112 kJ
N2 (g) + 2O2 (g) 2NO2(g) DH = + 68 kJ
Note: the important things about this example is that the sum of DH for the two reaction steps is equal to the DH for the reaction of interest.
Big idea: We can combine reactions of known
DH to determine the DH for the overall
“combined” reaction.
Hess’s Law: An Important Detail
One can always reverse the direction of a
reaction when making a combined reaction.
When you do this, the sign of DH changes.
N2(g) + 2O2(g) 2NO2(g) DH = +68 kJ
2NO2(g) N2(g) + 2O2(g) DH = - 68 kJ
One more detail:• The magnitude of DH is directly proportional to the
quantities involved. (This means DH is an “extensive” quantity).
• So, if the coefficients of a reaction are multiplied by a number, the value of DH is also multiplied by the same number.
N2(g) + 2O2(g) 2NO2(g) DH = 68 kJ
2N2(g) + 4O2(g) 4NO2(g) DH = 136 kJ
Using Hess’s Law: tips
• When trying to combine reactions to form a reaction of interest, it is usually best to work backwards from the reaction of interest.
• Example:
What is DH for the following reaction?
3C (gr) + 4H2 (g) C3H8 (g)
3C (gr) + 4H2 (g) C3H8 (g) DH = ?
You’re given the following reactions:
C (gr) + O2 (g) CO2 (g) DH = -394 kJ
C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H2O (l) DH = -2220 kJ
H2 (g) + 1/2O2 (g) H2O (l) DH = -286 kJ
Step 1. Only reaction 1 has C (gr). Therefore, we will multiply by 3 to get the correct amount of C (gr) with respect to our final equation.
(x3) C (gr) + O2 (g) CO2 (g) DH = -394 kJ
3C (gr) + 3O2 (g) 3CO2 (g) DH = -1182 kJ
Step 2: To get C3H8 on the product side of the reaction, we need to reverse reaction 2, and change
the sign of DH.
3CO2 (g) + 4H2O (l) C3H8 (g) + 5O2 (g) DH = +2220 kJ
C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H2O (l) DH = -2220 kJ
Step 3: Add two “new” reactions together to see what
remains:
3C (gr) + 3O2 (g) 3CO2 (g) DH = -1182 kJ
3CO2 (g) + 4H2O (l) C3H8 (g) + 5O2 (g) DH = +2220 kJ2
3C (gr) + 4H2O (l) C3H8 (g) + 2O2 DH = +1038 kJ
• Step 4: Compare previous reaction to final reaction, and determine how to reach final reaction:
3C (gr) + 4H2O (l) C3H8 (g) + 2O2 DH = +1038 kJ
H2 (g) + 1/2O2 (g) H2O (l) DH = -286 kJ
3C (gr) + 4H2 (g) C3H8 (g)
Need to multiply second reaction by 4
Example (cont.)
• Step 4: Compare previous reaction to final reaction, and determine how to reach final reaction:
3C (gr) + 4H2O (l) C3H8 (g) + 2O2 DH = +1038 kJ
4H2 (g) + 2O2 (g) 4H2O (l) DH = -1144 kJ
3C (gr) + 4H2 (g) C3H8 (g)
3C (gr) + 4H2O (l) C3H8 (g) + 2O2 DH = +1038 kJ
4H2 (g) + 2O2 (g) 4H2O (l) DH = -1144 kJ
3C (gr) + 4H2 (g) C3H8 (g) DH = -106 kJ
Which is the one step reaction of interest
Another Example:
• Calculate DH for the following reaction:
H2(g) + Cl2(g) 2HCl(g)
Given the following:
NH3 (g) + HCl (g) NH4Cl(s) DH = -176 kJ
N2 (g) + 3H2 (g) 2NH3 (g) DH = - 92 kJ
N2 (g) + 4H2 (g) + Cl2 (g) 2NH4Cl(s) DH = - 629 kJ
Step 1: Only the first reaction contains the product of interest (HCl), but as a reactant. Therefore, reverse this reaction and multiply by 2 to get stoichiometry correct.
NH3 (g) + HCl (g) NH4Cl(s) DH = -176 kJ
2NH4Cl(s) 2NH3 (g) + 2HCl (g) DH = +352 kJ
Step 2: Need Cl2 as a reactant, therefore, add reaction 3 to result from step 1 and see what is left.
2NH4Cl(s) 2NH3 (g) + 2HCl (g) DH = 352 kJ
N2(g) + 4H2(g) + Cl2(g) 2NH4Cl(s) DH = -629 kJ
N2 (g) + 4H2 (g) + Cl2 (g) 2NH3(g) + 2HCl(g)
DH = -277 kJ
Step 3: Use remaining known reaction in combination with the result from Step 2 to get final reaction.
N2 (g) + 4H2 (g) + Cl2 (g) 2NH3(g) + 2HCl(g) DH = -277 kJ
? ( N2 (g) + 3H2(g) 2NH3(g) DH = -92 kJ)
H2(g) + Cl2(g) 2HCl(g) DH = ?
Key: need to reverse the middle reaction
• Step 3. Use remaining known reaction in combination with the result from Step 2 to get final reaction.
N2 (g) + 4H2 (g) + Cl2 (g) 2NH3(g) + 2HCl(g) DH = -277 kJ
2NH3(g) 3H2 (g) + N2 (g) DH = +92 kJ
H2(g) + Cl2(g) 2HCl(g) DH = -185 kJ
1
This is the desired reaction and resultant DH!