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This question is about ammonia, NH₃ Complete the dot and cross diagram for the ammonia molecule shown in Figure 6 - AQA - GCSE Chemistry - Question 5 - 2019 - Paper 1
Question 5
This question is about ammonia, NH₃ Complete the dot and cross diagram for the ammonia molecule shown in Figure 6. Show only the electrons in the outer shell of each... show full transcript
Worked Solution & Example Answer:This question is about ammonia, NH₃ Complete the dot and cross diagram for the ammonia molecule shown in Figure 6 - AQA - GCSE Chemistry - Question 5 - 2019 - Paper 1
Step 1
Complete the dot and cross diagram for the ammonia molecule shown in Figure 6.
Answer
The dot and cross diagram for ammonia (NH₃) consists of nitrogen (N) in the center with three hydrogen (H) atoms arranged around it. Nitrogen has five outer shell electrons, and we represent the bonding pairs of electrons as shared between the nitrogen and each hydrogen atom. The diagram will show a pair of electrons (dot and cross) for each N-H bond, resulting in:
- N: •• (3 bonding pairs with H)
- H: • (for each hydrogen atom)
This indicates the electron sharing and bonding nature of the molecule.
Step 2
Give one limitation of using a dot and cross diagram to represent an ammonia molecule.
Answer
One limitation of using a dot and cross diagram is that it does not represent the three-dimensional shape of the molecule. It gives a two-dimensional view, which may mislead regarding the actual spatial arrangement and angles between the hydrogen atoms in the ammonia molecule.
Step 3
Explain why ammonia has a low boiling point.
Answer
Ammonia has small molecular mass and weak intermolecular forces, specifically hydrogen bonds. The energy required to break these intermolecular attractions is low compared to covalent bonds. Therefore, little energy is needed to overcome these forces, resulting in a low boiling point.
Step 4
Step 5
Calculate the overall energy change for the reaction.
Answer
To calculate the overall energy change, we use the bond energies:
-
Bonds broken:
- N-H (3 bonds): 3 × 391 kJ/mol = 1173 kJ/mol
- O=O (3 bonds): 3 × 498 kJ/mol = 1494 kJ/mol
- Total = 1173 + 1494 = 2667 kJ/mol
-
Bonds formed:
- N≡N (2 bonds): 2 × 945 kJ/mol = 1890 kJ/mol
- O-H (6 bonds): 6 × 464 kJ/mol = 2784 kJ/mol
- Total = 1890 + 2784 = 4674 kJ/mol
Overall energy change = Bonds broken - Bonds formed = 2667 kJ/mol - 4674 kJ/mol = -2007 kJ/mol.
Step 6
Explain why the reaction between ammonia and oxygen is exothermic.
Answer
The reaction is exothermic because the total energy released in forming the bonds in the products (water and nitrogen) is greater than the energy required to break the bonds in the reactants (ammonia and oxygen). Given that the overall energy change calculated is negative (-2007 kJ/mol), it indicates that energy is released to the surroundings.
Step 7
Complete Figure 8 by labelling the: activation energy and overall energy change.
Answer
In Figure 8, the activation energy can be identified as the energy required to initiate the reaction, which is the height of the peak of the energy profile. The overall energy change should be labeled as a drop from the reactants' energy level to products' energy level, indicating the energy released during the reaction.