CAIE Chemistry (9701)

A student is asked to find the enthalpy change for the reaction between anhydrous magnesium sulfate and water. MgSO

+ 7H O

This enthalpy change cannot be measured directly.

Predict whether the enthalpy change for this reaction is positive or negative. Explain the reason for your prediction. prediction explanation

The student decided to do two separate experiments. Experiment 1 To find the enthalpy change of solution of anhydrous magnesium sulfate, MgSO

, 0.0250 moles of MgSO distilled water. Experiment 2 To find the enthalpy change of solution of hydrated magnesium sulfate, MgSO O

, 0.0250 moles of MgSO distilled water. The results for Experiment 1 are shown in the graph of temperature against time on page 3.

Draw and extrapolate the cooling curve back to 180 seconds. Determine the temperature change during the reaction. temperature change = °C

The anhydrous magnesium sulfate was not added when the timing started. Explain why. 600 120 180 240 time / s 300 360 420 480 29.0 28.0 27.0 26.0 25.0 24.0 23.0 22.0 21.0 20.0 19.0 temperatur e Outline the next steps that should be taken in order to obtain the results in Experiment 1. Write your answer using a series of numbered steps.

The student realised that when dissolving 0.0250 moles MgSO O

, the amount of water present in the compound alters the total volume of water used in Experiment 2. in Experiment 2. : O, 16.0; H, 1.0] volume of distilled water = cm

State which piece of apparatus should be used to measure the volume of distilled water in

(i). Explain your answer. apparatus explanation

The temperature change when 0.0250 moles of MgSO O

is added to the water is very small. Suggest how the student should modify the experimental procedure to make the temperature change larger.

(i) The energy released by 0.0250 moles of MgSO energy absorbed by 0.0250 moles of MgSO Include a sign in your answer. Give your answer to one decimal place. MgSO

+ 7H O(s)

The student noticed that some MgSO

in Experiment 1 was left undissolved. State and explain the effect this would have on the value of the enthalpy change for the reaction in Experiment 1. effect . explanation [1] [Total: 14]

A student is asked to determine the acid dissociation constant, K , for butanoic acid, COOH. [CHCOO][H [CH COOH] The student is told to measure the pH of eight buffer solutions containing CH COOH and the salt sodium butanoate, CHCOO . The salt provides butanoate ions, CHCOO Each buffer solution contains a different ratio of [CH COOH] to [CHCOO Each buffer solution is prepared by mixing different volumes of distilled water,COO

Pure butanoic acid must be kept away from naked flames. Explain why. of CHCOO

? : Na, 23.0; O, 16.0; C, 12.0; H, 1.0] volume = cm [1] COO

in a beaker. solution of the volume calculated in (b)(i). Give the name and capacity of any key apparatus which should be used. Write your answer as a series of numbered steps.

Complete the table to show the volumes of CHCOO

and distilled water that would be needed to provide the stated number of moles of CH COOH andCOO buffer solution. moles of COOH volume of COOH moles ofCOO volume ofCOO volume of distilled water 0.050 25.0 0.005 0.050 25.0 0.008 0.050 25.0 0.010 0.050 25.0 0.025 0.030 15.0 0.050 0.010 5.0 0.050 0.006 3.0 0.050 0.004 2.0 0.050

The pH of each buffer solution is measured. The value of the pH is recorded in the table along with the number of moles of the COOH and CHCOO solution of buffer.

Complete the table. Give your values of -log [acid] [base] to two decimal places. moles of acid, COOH moles of base,COO ratio of [acid]/[base] -log [acid] [base] 0.050 0.005 10.00 3.82 0.050 0.008 6.25 4.01 0.050 0.010 5.00 4.12 0.050 0.025 2.00 4.70 0.030 0.050 0.60 5.04 0.010 0.050 0.20 5.52 0.006 0.050 0.12 5.74 0.004 0.050 0.08 5.91

Plot a graph of pH (y-axis) against -log [acid] [base] (x-axis) on the grid on page 9.

Circle the point on the graph you consider to be most anomalous. Suggest one reason why this anomaly may have occurred during this experimental procedure. Assume no error was made in recording the pH.

When the concentration of acid is equal to the concentration of base, pH = pK Use this information and your graph to find the value for the pKp K = [1] -1.4-1.2-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.2 6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 -log [acid] [base]

Use your answer to (c)(iv) to calculate the value of K . State the units of K . Give your answer to three significant figures . = -log K = units =

The value of pK is lower when the experiment is repeated at a higher temperature. What does this tell you about the enthalpy of dissociation of CH COOH? Explain your answer. [Total: 16] Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every UHDVRQDEOHH?RUWKDVEHHQPDGHE\WKHSXEOLVKHU8&/(6WRWUDFHFRS\ULJKWKROGHUVEXWLIDQ\LWHPVUHTXLULQJFOHDUDQFHKDYHXQZLWWLQJO\EHHQLQFOXGHGWKH publisher will be pleased to make amends at the earliest possible opportunity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