A2 Live Session Spring 2024

Mrs Nerys Stephens�Chemistry Teacher and �Head of Years 9, 10 & 11� Ysgol Bro Pedr

Spring 2024 A2 Sessions:

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Session 1 – Accurate Enthalpy Change Calculation (PB)

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Session 2 – Enthalpy and Born Haber Cycles (NS)

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Session 3 – Iodine Clock Reaction – discussing order and mechanism (PB)

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Session 4 – Approach to Unit 5 Assessment (NS)

�Enthalpy and Born Haber Cycles

�A2 level Chemistry (* part of Topic 3.6)�

�Approach to Unit 5 Assessment

�A2 level Chemistry �

This resource puts focus on the core and compulsory practical exercises as highlighted in the WJEC’s A level specifications. It is intended to help pupils to strengthen their practical skills and to create their ‘laboratory book’. It is also extremely useful for revision and is ideal preparation for answering questions on practical work in the written examination papers. It is designed to be used for independent learning but is also regularly used to support and develop learners’ practical skills within lessons.

Chemistry | Experiments on Film (science.cymru)

MAIN THEMES OR TOPIC AREAS FOR ASSESSMENT IN UNIT 5 IN SUMMER 2024

A Thermometric Titration – a typical example

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In this experiment, students titrate sodium hydroxide solution with hydrochloric acid. By measuring the temperature change each time a portion of acid is added, students can determine the endpoint of the titration, indicated by the highest temperature. They then use this information to calculate the concentration of the hydrochloric acid.

The practical usually takes about one hour.

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- don't forget to carry out a risk assessment, for your own safety

Procedure:

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* set up the apparatus as follows

1. Stand an insulated cup in a beaker for support.
2. Using a pipette and safety filler, transfer 20 cm³ (or 25 cm³) of the sodium hydroxide solution into the cup, and measure the steady temperature.
3. Using the burette, add a small portion (3–5 cm³) of dilute hydrochloric acid to the solution in the cup, noting down the actual volume reading. Stir by swirling the cup and measure the highest temperature reached.
4. Immediately add a second small portion of the dilute hydrochloric acid, stir, and again measure the highest temperature and note down the volume reading.
5. Continue in this way until there are enough readings to decide the maximum temperature reached during this experiment. You will need to add at least 30 cm³ of the acid.
6. Plot a graph of temperature against the volume of acid added and use extrapolation of the two sections of the graph to deduce the maximum temperature reached without heat loss.
7. Use your results to calculate the concentration of the hydrochloric acid.

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The main concern in this experiment is the heat loss.

- if possible, a lid should be used. More reliable results can be achieved using two polystyrene cups (one inside the other).

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* it is essential that you understand the importance of extrapolating the cooling curve to estimate the maximum temperature reached without heat loss.

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Identification of Unknown Solutions by Qualitative Analysis

- to plan and carry out a method to identify six inorganic salts by the interactions between their solutions.

Solutions were:

• Ba(NO₃)₂
• Pb(NO₃)₂
• MgSO₄
• KI
• Na₂CO₃
• Zn(NO₃)₂

The type pf practical tests your knowledge of Topic 1.6, and centres on the varying solubility of certain salts, as well as their colour.

Solubility of Group 2 salts:

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Hydroxides – Mg(OH)2 is insoluble in water, but solubility increases down the group.

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Sulphates – BaSO4 is insoluble, but solubility decreases going down the group.

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Carbonates – all Group 2 carbonates are insoluble.

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Nitrates - all Group 2 nitrates are soluble.

Planning A Sequence Of Tests To Identify Organic Compounds

- This exercise is designed to enhance your understanding of functional group behaviour and to strengthen the linkage between theory and experiment.

You are required to devise a plan to identify six different organic compounds, from the following list of eight compounds:

• 1-aminobutane (an amine)
• benzenecarbaldehyde (an aldehyde)
• benzenecarboxylic acid (a carboxylic acid)
• butanone (a methyl ketone)
• diphenylmethanone (a ketone, that is not a methyl ketone)
• ethanamide (an amide)
• ethylbenzenecarboxylate (an ester)
• benzenecarbonitrile (a nitrile)

Sample Plan of Attack

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1. Test all compounds with 2,4-DNPH reagent:

- The aldehyde and both ketones will react to give a yellow/red ppt.,

other compounds do not react.

2. Further test those that gave a positive test to 1. with Tollens' reagent:

- The aldehyde reacts to give a silver mirror, whilst the 2 ketones do not react.

3. Test the ketones with iodoform reagent:

- the methyl ketone reacts to give a yellow ppt., other ketone does not react.

Sample Plan of Attack

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4. Test the remaining compounds with test papers:

- The amine turns red litmus blue, the carboxylic acid (in solution) turns blue litmus red, others no indications.

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* the amine should also be confirmed by the evolution of N₂, on the addition of nitrous acid. Nitrous acid is obtained by the action of dilute H₂SO₄ on sodium nitrate(III)

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* the carboxylic acid should also be confirmed by the evolution, from aqueous solution, of CO₂ with NaHCO₃

Sample Plan of Attack

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5. Test the remaining compounds by boiling with sodium hydroxide solution:

- The amide and nitrile evolve NH₃ (red litmus blue), the remaining compound does not.

6. For the amide and nitrile (gave +ve for NH₃), acidify solutions resulting from the reaction with NaOH. and cool:

- The nitrile gives a white ppt. of benzoic acid, whilst the amide does not.

Sample Plan of Attack

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7. Test the remaining solution (gave -ve for NH₃), by acidifying and cooling:

- The ester gives a white ppt. of benzoic acid

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Following the 7 steps outlined should enable the identification of all unknown substances.