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This question is about ammonia, NH₃ - 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 ea... show full transcript
Worked Solution & Example Answer:This question is about ammonia, NH₃ - 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
In the dot and cross diagram for the ammonia molecule (NH₃), the nitrogen atom has five electrons in its outer shell while the hydrogen atoms have one electron each. The diagram should illustrate a nitrogen atom at the center with three hydrogen atoms surrounding it. Draw a dot for each unshared electron on nitrogen and cross marks for the bonding pairs with the hydrogen atoms. Thus, the final diagram shows:
- One nitrogen atom (N) in the center.
- Three hydrogen atoms (H) surrounding the nitrogen.
- Two non-bonding electrons marked on nitrogen.
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 accurately represent the three-dimensional shape of the ammonia molecule. The diagram may give the impression that all atoms are in a plane rather than reflecting the pyramidal shape that occurs due to the presence of lone pairs.
Step 3
Explain why ammonia has a low boiling point.
Answer
Ammonia (NH₃) has a low boiling point due to its molecular structure and bonding characteristics. While ammonia can form hydrogen bonds because of the presence of nitrogen, the overall size of the ammonia molecules is small. As a result, the intermolecular forces are weak compared to larger molecules. Consequently, less energy is required to overcome these intermolecular forces, leading to the relatively low boiling point of ammonia.
Step 4
Step 5
Calculate the overall energy change for the reaction.
Answer
To calculate the overall energy change for the reaction:
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Calculate the energy required to break the bonds in the reactants:
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Bonds broken:
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N-H: 3 bonds
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O=O: 1 bond
Total energy = (3 × 391) + (1 × 498) = 1173 + 498 = 1671 kJ
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Calculate the energy released in making the bonds in the products:
- Bonds formed in products:
- N≡N: 1 bond
- O-H: 6 bonds
Total energy = (1 × 945) + (6 × 464) = 945 + 2784 = 3729 kJ
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Now calculate the overall energy change: Overall energy change = Energy released - Energy used = 3729 - 1671 = 2058 kJ.
Step 6
Explain why the reaction between ammonia and oxygen is exothermic.
Answer
The reaction between ammonia and oxygen is exothermic because the total energy released in forming the bonds of the products (N₂ and H₂O) is greater than the total energy consumed in breaking the bonds of the reactants (NH₃ and O₂). In this case, as calculated, the overall energy change is negative, confirming that energy is released to the surroundings during the reaction.
Step 7
Complete Figure 8 by labelling the activation energy and overall energy change.
Answer
On Figure 8, the activation energy can be labelled as the height of the energy barrier that must be overcome for the reaction to proceed. The overall energy change can be labelled as the difference in energy between the reactants and products, indicating that energy is released during the reaction.