A chemistry student needs to determine the enthalpy change for the reaction of nitrogen gas reacting with hydrogen gas to form ammonia gas, given that they have the following information:N2(g) + 3H2(g) --> 2NH3(g) ΔH1 = -92.4 kJ/mol2H2(g) + O2(g) --> 2H2O(l) ΔH2 = -572.0 kJ/molN2(g) + O2(g) --> 2NO(g) ΔH3 = +180.6 kJ/molUse Hess's law to calculate the enthalpy change for the reaction:2NH3(g) + 5O2(g) --> 2HNO3(aq) + 2H2O(l)

Question

A chemistry student needs to determine the enthalpy change for the reaction of nitrogen gas reacting with hydrogen gas to form ammonia gas, given that they have the following information:N2(g) + 3H2(g) --> 2NH3(g)   ΔH1 = -92.4 kJ/mol2H2(g) + O2(g) --> 2H2O(l)    ΔH2 = -572.0 kJ/molN2(g) + O2(g) --> 2NO(g)     ΔH3 = +180.6 kJ/molUse Hess's law to calcu

Answer 1

To use Hess's law, we need to manipulate the given reactions to match the target reaction. We will manipulate the reactions and then add them together to find the enthalpy change for the target reaction.Target reaction: 2NH3 g  + 5O2 g  --> 2HNO3 aq  + 2H2O l Step 1: Reverse reaction 1 and multiply by 2:2NH3 g  --> N2 g  + 3H2 g    H1' = +92.4 kJ/molStep 2: Multiply reaction 2 by 2:4H2 g  + 2O2 g  --> 4H2O l    H2' = -1144.0 kJ/molStep 3: Multiply reaction 3 by 2:2N2 g  + 2O2 g  --> 4NO g    H3' = +361.2 kJ/molStep 4: Add a new reaction to convert 4NO g  to 2HNO3 aq :4NO g  + 6H2O l  --> 2HNO3 aq  + 4H2O l    H4 = unknownNow, we can add the manipulated reactions together:2NH3 g  --> N2 g  + 3H2 g    H1' = +92.4 kJ/mol4H2 g  + 2O2 g  --> 4H2O l    H2' = -1144.0 kJ/mol2N2 g  + 2O2 g  --> 4NO g    H3' = +361.2 kJ/mol4NO g  + 6H2O l  --> 2HNO3 aq  + 4H2O l    H4 = unknown--------------------------------------------------------------2NH3 g  + 5O2 g  --> 2HNO3 aq  + 2H2O l    H = H1' + H2' + H3' + H4Now we need to find H4. We can do this by subtracting the other enthalpy changes from the target reaction:H4 = H -  H1' + H2' + H3' H4 = H -  +92.4 kJ/mol - 1144.0 kJ/mol + 361.2 kJ/mol H4 = H -  -690.4 kJ/mol Now we can plug H4 back into the target reaction:2NH3 g  + 5O2 g  --> 2HNO3 aq  + 2H2O l    H = -690.4 kJ/molSo, the enthalpy change for the target reaction is -690.4 kJ/mol.