Use equations to show that, when dissolved in water,
(i) HCl acts as a Brønsted-Lowry acid,
(ii) NH₃ acts as a Brønsted-Lowry base - Leaving Cert Chemistry - Question 7 - 2019
Question 7
Use equations to show that, when dissolved in water,
(i) HCl acts as a Brønsted-Lowry acid,
(ii) NH₃ acts as a Brønsted-Lowry base.
Explain why
(iii) HCl has a wea... show full transcript
Worked Solution & Example Answer:Use equations to show that, when dissolved in water,
(i) HCl acts as a Brønsted-Lowry acid,
(ii) NH₃ acts as a Brønsted-Lowry base - Leaving Cert Chemistry - Question 7 - 2019
Step 1
Use equations to show that, when dissolved in water,
(i) HCl acts as a Brønsted-Lowry acid.
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Answer
When HCl is dissolved in water, it dissociates completely to form H₃O⁺ and Cl⁻ ions:
ightleftharpoons H_3O^+ + Cl^-$$
This shows that HCl donates a proton (H⁺) to water, acting as a Brønsted-Lowry acid.
Step 2
(ii) NH₃ acts as a Brønsted-Lowry base.
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Answer
When NH₃ is dissolved in water, it accepts protons from water:
ightleftharpoons NH_4^+ + OH^-$$
This reaction demonstrates that NH₃ acts as a Brønsted-Lowry base by accepting a proton.
Step 3
(iii) HCl has a weak conjugate base.
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Answer
The conjugate base of HCl is Cl⁻, which does not easily accept protons to revert back to HCl. Therefore, Cl⁻ is considered a weak conjugate base.
Step 4
(iv) NH₃ has a strong conjugate acid.
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Answer
The conjugate acid of NH₃ is NH₄⁺, which readily donates protons, making it a strong conjugate acid compared to Cl⁻.
Step 5
(i) Write the self-ionisation constant (Kₕ) expression for water.
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Answer
The self-ionisation constant of water is expressed as:
Kw=[H3O+][OH−]
Step 6
(ii) Name the data in the table consistent with the self-ionisation of water being an exothermic process? Justify your answer.
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Answer
As temperature increases, the value of Kₕ increases, indicating that more water molecules are ionising at higher temperatures. This is consistent with the process being endothermic.
Step 7
(iii) Plot a graph of Kₕ versus temperature (°C).
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Answer
To plot the graph, use the Kₕ values from the table for the corresponding temperatures. Ensure that both axes are labeled correctly.
Step 8
(iv) Use your graph to predict the value of Kₕ at body temperature, 37 °C.
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Answer
From the graph, interpolate the Kₕ value at 37 °C based on the data points surrounding this temperature.
Step 9
(v) Use the value to calculate the H₃O⁺ ion concentration of pure water at 37 °C.
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Answer
Use the relation:
[H_3O^+] = rac{K_w}{[OH^-]}
assuming [OH⁻] is equal to [H₃O⁺] in pure water.
Step 10
(vi) The pH of pure water is close to 6.77 at one of the temperatures in the table.
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Answer
Calculate the H₃O⁺ concentration using the pH of 6.77:
[H3O+]=10−6.77
Step 11
(vii) Find, by calculation, this temperature.
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Answer
Calculate the temperature corresponding to Kₕ value using Kₕ relation and known values.
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