Question 4
HAmmonia is made by reacting nitrogen with hydrogen.
N2+3H2→2NH3 \mathrm{N_{2} + 3H_{2} \rightarrow 2NH_{3}} N2+3H2→2NH3The energy change for this reaction is −93 kJ/mol.
The diagram shows the equation using displayed formulae.
The table shows some bond energies.
| Bond | Bond energy (kJ/mol) |
|---|---|
| H–H | 436 |
| N–H | 391 |
| N–N | 158 |
| N≡N | ? |
a.[4]
Calculate the bond energy of the N≡N triple bond.
b.[2]
The reaction between nitrogen and hydrogen has a very high activation energy.
Use the bond energies to suggest why.
c.[3]
Hydrazine, N₂H₄, can decompose to make nitrogen and hydrogen.
N2H4→N2+2H2 \mathrm{N_{2}H_{4} \rightarrow N_{2} + 2H_{2}} N2H4→N2+2H2The diagram shows the equation using displayed formulae.
Calculate the energy change for this reaction. Use your answer to (a).
d.[2]
A student says:
"The reaction to make ammonia is exothermic because more bonds are made than are broken."
Evaluate the student's statement. Use your answers to (a) and (c).
3.2 Energetics Questions
51 exam-style questions on OCR GCSE Chemistry 3.2 Energetics, covering 3.2.1 Exothermic and endothermic reactions and reaction profiles and 3.2.2 Calculating energy change from bond energies. Each one has a worked solution and a mark scheme showing where the marks go.