Grade 12 Chemistry
Use Hess's Law to find ΔH for the following reactions: Express your answers in kJ/mol of the first reactant on the left in each equation.
1. NH3(g) + O2(g) = N2(g) + H2O(g)
2.NO(g) + O2(g) = NO2(g)
3. H2SO4(I) = SO2(g) + H2O(g) + O2(g)
4. Mg(s) + CO2(g) = MgO(s) + C(s)
5. F2(g) + H2O(I) = HF(g) + O2(g)
6. C6H6(l) + O2(g) = CO2(g) + H2O(g)
7. NH3(g) + O2(g) = NO2(g)+H2O(g)
8. CH4(g) + O2(g) = CO2(g) + H2O(l)
9. H2S(g) + O2(g) = H2O(l) + SO2(g)
10 CaO(s) + H2O(l) = Ca (OH)2(s)
don't give me the wrong answers!
1. Given: NH3(g) + O2(g) = N2(g) + H2O(g)
We can break this equation down into two steps:
Step 1: NH3(g) = 1.5 H2(g) + 0.5 N2(g)
Step 2: 0.5 H2(g) + 0.5 O2(g) = H2O(g)
To find the overall enthalpy change, we need to add the enthalpy changes of the individual steps. Remember to multiply the enthalpy change of each step by a factor if necessary.
Step 1: ΔH1 = ΔHf(N2(g)) + 0.5 ΔHf(H2(g)) - ΔHf(NH3(g))
Step 2: ΔH2 = ΔHf(H2O(g)) - 0.5 ΔHf(H2(g)) - 0.5 ΔHf(O2(g))
Overall enthalpy change: ΔH = ΔH1 + ΔH2
2. Given: NO(g) + O2(g) = NO2(g)
To find the overall enthalpy change, we need to look for the reverse reaction of one of the given reactions. In this case, we can use Reaction 3 from the question.
Reverse of Reaction 3: SO2(g) + H2O(g) + O2(g) = H2SO4(l)
Since the desired reaction is the reverse of Reaction 3, the enthalpy change will be the opposite of the enthalpy change of Reaction 3.
ΔH = -ΔH(Reaction 3)
3. Given: H2SO4(l) = SO2(g) + H2O(g) + O2(g)
To find the overall enthalpy change, we can use the given reaction directly.
ΔH = ΔHf(SO2(g)) + ΔHf(H2O(g)) + ΔHf(O2(g)) - ΔHf(H2SO4(l))
4. Given: Mg(s) + CO2(g) = MgO(s) + C(s)
We can break this equation down into two steps:
Step 1: Mg(s) = MgO(s)
Step 2: C(s) + O2(g) = CO2(g)
To find the overall enthalpy change, we need to add the enthalpy changes of the individual steps.
Step 1: ΔH1 = ΔHf(MgO(s)) - ΔHf(Mg(s))
Step 2: ΔH2 = ΔHf(CO2(g)) - ΔHf(C(s)) - ΔHf(O2(g))
Overall enthalpy change: ΔH = ΔH1 + ΔH2
5. Given: F2(g) + H2O(l) = HF(g) + O2(g)
We can break this equation down into two steps:
Step 1: F2(g) = 2F(g)
Step 2: 2F(g) + H2O(l) = 2HF(g) + O2(g)
To find the overall enthalpy change, we need to add the enthalpy changes of the individual steps. Remember to multiply the enthalpy change of each step by a factor if necessary.
Step 1: ΔH1 = 2ΔHf(F(g)) - ΔHf(F2(g))
Step 2: ΔH2 = 2ΔHf(HF(g)) + ΔHf(O2(g)) - 2ΔHf(F(g)) - ΔHf(H2O(l))
Overall enthalpy change: ΔH = ΔH1 + ΔH2
6. Given: C6H6(l) + O2(g) = CO2(g) + H2O(g)
To find the overall enthalpy change, we can use the given reaction directly.
ΔH = ΔHf(CO2(g)) + ΔHf(H2O(g)) - ΔHf(C6H6(l))
7. Given: NH3(g) + O2(g) = NO2(g) + H2O(g)
To find the overall enthalpy change, we can use the given reaction directly.
ΔH = ΔHf(NO2(g)) + ΔHf(H2O(g)) - ΔHf(NH3(g))
8. Given: CH4(g) + O2(g) = CO2(g) + H2O(l)
To find the overall enthalpy change, we need to look for the reverse reaction of one of the given reactions. In this case, we can use Reaction 6 from the question.
Reverse of Reaction 6: CO2(g) + H2O(g) = C6H6(l) + O2(g)
Since the desired reaction is the reverse of Reaction 6, the enthalpy change will be the opposite of the enthalpy change of Reaction 6.
ΔH = -ΔH(Reaction 6)
9. Given: H2S(g) + O2(g) = H2O(l) + SO2(g)
To find the overall enthalpy change, we can use the given reaction directly.
ΔH = ΔHf(H2O(l)) + ΔHf(SO2(g)) - ΔHf(H2S(g))
10. Given: CaO(s) + H2O(l) = Ca(OH)2(s)
To find the overall enthalpy change, we can use the given reaction directly.
ΔH = ΔHf(Ca(OH)2(s)) - ΔHf(CaO(s)) - ΔHf(H2O(l))
To find ΔH for the given reactions using Hess's Law, we'll use the principle that the overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps.
1. NH3(g) + O2(g) = N2(g) + H2O(g)
We can break this reaction into two steps:
a) NH3(g) + 3/2 O2(g) = NO2(g) + 3/2 H2O(g)
b) NO2(g) + O2(g) = N2(g) + 2H2O(g)
ΔH1 = ΔH2 + ΔH3
2. NO(g) + O2(g) = NO2(g)
This reaction is already balanced as it is. ΔH2 = ΔH2
3. H2SO4(I) = SO2(g) + H2O(g) + O2(g)
We can break this reaction into two steps:
a) H2SO4(I) = 2H2O(l) + SO2(g) + O2(g)
b) 2H2O(l) = 2H2O(g)
ΔH3 = ΔH4 + ΔH5
4. Mg(s) + CO2(g) = MgO(s) + C(s)
This reaction is already balanced as it is. ΔH4 = ΔH4
5. F2(g) + H2O(I) = HF(g) + O2(g)
We can break this reaction into two steps:
a) F2(g) + H2O(I) = HF(g) + OF2(g)
b) OF2(g) + H2O(I) = 2HF(g) + O2(g)
ΔH5 = ΔH6 + ΔH7
6. C6H6(l) + O2(g) = CO2(g) + H2O(g)
This reaction is already balanced as it is. ΔH6 = ΔH6
7. NH3(g) + O2(g) = NO2(g) + H2O(g)
This reaction is already balanced as it is. ΔH7 = ΔH7
8. CH4(g) + O2(g) = CO2(g) + H2O(l)
We can break this reaction into two steps:
a) CH4(g) + 2O2(g) = CO2(g) + 2H2O(g)
b) 2H2O(g) = 2H2O(l)
ΔH8 = ΔH8 + ΔH9
9. H2S(g) + O2(g) = H2O(l) + SO2(g)
We can break this reaction into two steps:
a) 2H2S(g) + 3O2(g) = 2H2O(g) + 2SO2(g)
b) 2H2O(g) = 2H2O(l)
ΔH9 = ΔH10 + ΔH11
10 CaO(s) + H2O(l) = Ca (OH)2(s)
This reaction is already balanced as it is. ΔH10 = ΔH10
Now, you'd need the individual enthalpy changes (ΔH values) for the steps involved in each reaction in order to calculate ΔH for the respective reactions. I don't have access to a database containing those values, but you can find the necessary ΔH values in a chemistry textbook or reliable source to calculate ΔH for each reaction.
To find ΔH for each reaction using Hess's Law, you need to follow these steps:
Step 1: Write down the balanced chemical equations for all the given reactions.
Step 2: Identify other reactions or equations with known ΔH values that can be combined to give the target equation.
Step 3: Manipulate the given equations to cancel out the common substances on both sides and add up the equations to obtain the target equation.
Step 4: Add up the corresponding ΔH values of the manipulated equations to find the overall ΔH for the target equation.
Now, let's go through each of the given reactions and find their corresponding ΔH using Hess's Law:
1. NH3(g) + O2(g) = N2(g) + H2O(g)
This equation is already balanced and represents the target equation itself.
2. NO(g) + O2(g) = NO2(g)
The target equation is already given.
3. H2SO4(I) = SO2(g) + H2O(g) + O2(g)
This equation is already balanced and represents the target equation itself.
4. Mg(s) + CO2(g) = MgO(s) + C(s)
The target equation is already given.
5. F2(g) + H2O(I) = HF(g) + O2(g)
The target equation is already given.
6. C6H6(l) + O2(g) = CO2(g) + H2O(g)
The target equation is already given.
7. NH3(g) + O2(g) = NO2(g) + H2O(g)
This equation is already balanced and represents the target equation itself.
8. CH4(g) + O2(g) = CO2(g) + H2O(l)
The target equation is already given.
9. H2S(g) + O2(g) = H2O(l) + SO2(g)
The target equation is already given.
10. CaO(s) + H2O(l) = Ca(OH)2(s)
The target equation is already balanced.
Please note that without the provided ΔH values for other known reactions, we cannot calculate the final values for each reaction using Hess's Law.