INTERNATIONAL A-LEVEL

CHEMISTRY (9620)

Unit 5: Practical and synoptic

Section A

Answer all questions in the spaces provided.

A student has samples of three organic compounds. The samples are not labelled but are known to be propanoic acid, butanal and pentan-1-ol.

Describe three test-tube reactions that can be used to identify the three organic compounds. Use a different test-tube reaction for each compound.

Your answer should include the reagent(s) used and the expected observation for each test.

[6 marks]

Propanoic acid

Butanal

Pentan-1-ol

The student mixed together samples of pentan-1-ol and propanoic acid and added a few drops of concentrated sulfuric acid.

The mixture was left in a warm place for a reaction to occur.

Write an equation for the reaction that occurs.

State an observation that would be made at the end of the reaction.

Give the IUPAC name of the organic product.

[3 marks]

Equation

Observation

IUPAC name

An alcohol, ROH, reacts with hydrochloric acid according to the equation

ROH + HCl → RCl + H2O

The kinetics of this reaction were investigated using this method.

• An aqueous mixture was prepared containing equal volumes of alcohol and hydrochloric acid.

• The concentration of HCl in the mixture, [HCl], was monitored by removing 5.00 cm3 samples at known times. The samples were diluted and titrated with aqueous sodium hydroxide.

• A graph was plotted of [HCl] against time.

A 5.00 cm3 sample was removed from the reaction mixture. This sample was then diluted to a volume of 25.0 cm3 using deionised water.

This diluted sample needed 11.50 cm3 of 0.255 mol dm–3 aqueous sodium hydroxide for complete neutralisation.

Calculate [HCl] in the sample removed from the reaction mixture.

Give your answer to the appropriate number of significant figures.

[3 marks]

[HCl]                                                mol dm–3

Figure 1 shows how [HCl] varies with time during the reaction.

Figure 1

Determine the rate of reaction at t = 4.0 minutes by drawing a tangent to the curve in Figure 1.

[3 marks]

rate of reaction                                                      mol dm–3 min–1

A graph of rate of reaction against [HCl] was plotted. The graph was a straight line through the origin.

The reaction between ROH and HCl is first order with respect to ROH

Deduce the rate equation for the reaction between ROH and HCl

[1 mark]

In one calculation of [HCl] the volume of the sample removed was recorded as 5.00 cm3 but 10.00 cm3 were incorrectly removed.

Deduce the effect, if any, of this mistake on the calculated value of [HCl]

[1 mark]

The kinetics of the reverse reaction (RCl + H2O → ROH + HCl) were also investigated.

The reaction was found to be first order with respect to RCl and zero order with respect to H2O

In one experiment, the results obtained were

[RCl] = 0.107 mol dm–3

rate = 0.562 mol dm–3 min–1

Calculate the value of the rate constant, k, for this reverse reaction.

Deduce its units.

[2 marks]

k                                      

units                                 

Hydrolysis reactions such as that between RCl and H2O occur by different mechanisms that can be represented as shown.

Mechanism 1              RCl + H2O → ROH + HCl (slow)

Mechanism 2              RCl → R+ + Cl – (slow)

                                 R+ + H2O → ROH + H+ (fast)

Deduce which mechanism (1 or 2) is supported by the information given in Question 02.5

Explain your choice.

[2 marks]

This question is about electrochemical cells.

Figure 2 shows a diagram of an electrochemical cell.

Figure 2

Explain how the salt bridge provides an electrical connection between the two half-cells.

[1 mark]

Suggest why potassium chloride would not be a suitable salt for the salt bridge in this cell.

[1 mark]

Suggest why the current in the external circuit of this cell falls to zero after the cell has operated for some time.

[1 mark]

A different electrochemical cell was made from a copper half-cell and a manganese half-cell.

Cu2+(aq) + 2e– → Cu(s)          Eo = +0.34 V

Mn2+(aq) + 2e– → Mn(s)         Eo = –1.18 V

Give the conventional representation for this cell.

Calculate the EMF of this cell.

[2 marks]

Cell representation

EMF                                            V

Table 1 shows some standard electrode potential data.

Table 1

Use these data to answer the following questions.

Iron filings are added to a solution that contains the ions Cu2+, Fe3+ and Zn2+, all with concentration 1.00 mol dm–3

Give the formula of the metal ion in the solution that does not react when iron filings are added.

Give a reason for your answer.

[2 marks]

Deduce the equation for each of the two reactions that occur.

[2 marks]