[OCR] 2

Topic: Chemical Bonding 2

Water chemistry plays a critical role in the well-being of fishes in an aquarium. ![img-2.jpeg](img-2.jpeg) ![img-3.jpeg](img-3.jpeg) ![img-4.jpeg](img-4.jpeg) ![img-5.jpeg](img-5.jpeg) [18.0m]

Part 1: (a) (i) Use relevant standard redox potentials from the Data Booklet to show that this reaction is spontaneous under standard conditions. [2m]

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Part 2: (a) (ii) Suggest the role of beneficial bacteria in the nitrification process. [1m]

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Part 3: (b) (i) State the day number that first indicates that beneficial bacteria $A$ is established. [1m]

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Part 4: (b) (ii) State the day number that is first safe for the addition of fish. [1m]

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Part 5: (b) (iii) On day $35,25\%$ of the tank water is replaced with water that is free of $NO_{3}^{-}$. [1m]

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Part 6: (b) (iv) Calculate $z$ for a sample of tank water at pH 7.4 on day 11. [2m]

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Part 7: (c) (i) Write two equations to show how the tank water behaves as a buffer when small amounts of $OH^{-}(aq)$ and $H_{3}O^{+}(aq)$ are separately added to it. [2m]

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Part 8: (c) (ii) Suggest why tank water with a higher carbonate hardness is better at maintaining its pH. [1m]

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Part 9: (d) (i) A $100 cm^{3}$ of sample of aquarium water containing dissolved oxygen required $13.40 cm^{3}$ of $0.0100 mol dm^{-3}$ sodium thiosulfate solution to reach the end point. [2m]

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Part 10: (d) (ii) With the aid of half-equations, write a balanced equation for the reaction between $N_{3}^{-}$ and $NO_{2}^{-}$ under acidic conditions. [2m]

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Part 11: (e) Describe and explain the relative basicities of the three nitrogen-containing groups in compound $P$. [3m]

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[OCR] 2

Part 1: (a) (i) Use relevant standard redox potentials from the Data Booklet to show that this reaction is spontaneous under standard conditions. [2m]

Your Answer

Marking Points

Part 2: (a) (ii) Suggest the role of beneficial bacteria in the nitrification process. [1m]

Your Answer

Marking Points

Part 3: (b) (i) State the day number that first indicates that beneficial bacteria $A$ is established. [1m]

Your Answer

Marking Points

Part 4: (b) (ii) State the day number that is first safe for the addition of fish. [1m]

Your Answer

Marking Points

Part 5: (b) (iii) On day $35,25\%$ of the tank water is replaced with water that is free of $NO_{3}^{-}$. [1m]

Your Answer

Marking Points

Part 6: (b) (iv) Calculate $z$ for a sample of tank water at pH 7.4 on day 11. [2m]

Your Answer

Marking Points

Part 7: (c) (i) Write two equations to show how the tank water behaves as a buffer when small amounts of $OH^{-}(aq)$ and $H_{3}O^{+}(aq)$ are separately added to it. [2m]

Your Answer

Marking Points

Part 8: (c) (ii) Suggest why tank water with a higher carbonate hardness is better at maintaining its pH. [1m]

Your Answer

Marking Points

Part 9: (d) (i) A $100 cm^{3}$ of sample of aquarium water containing dissolved oxygen required $13.40 cm^{3}$ of $0.0100 mol dm^{-3}$ sodium thiosulfate solution to reach the end point. [2m]

Your Answer

Marking Points

Part 10: (d) (ii) With the aid of half-equations, write a balanced equation for the reaction between $N_{3}^{-}$ and $NO_{2}^{-}$ under acidic conditions. [2m]

Your Answer

Marking Points

Part 11: (e) Describe and explain the relative basicities of the three nitrogen-containing groups in compound $P$. [3m]

Your Answer

Marking Points

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