STAGE 2 CHEMISTRY

ASSESSMENT TYPE 1: Investigations Folio

Practical Investigation: Preparation of an Ester

Response 1

Skills checklist (Teacher record)

Part A: / Teacher observations:
Performing reflux / Clamped correctly. Vapours reached the top of the condenser.
Use of separating funnel / Careful.
Performing distillation / Clamped correctly. Adaptor joint under strain. Thermometer too low.
Clarity of ester product / Cloudy.
Area cleanliness / Clean and well-organised.
Safe conduct / Good – aware of closeness of other groups.All precautions observed.
Disposed of excess chemicals and prepared ester down the sink.
Cooperation / Generally good communication between partners.
Other / Had to be told to tie hair back.

Student Report

Results

Distillation range: 128 - 135C

Ester has a sweet odour.

Discussion

Reaction equation:

Reflux procedure

The reactants must be heated for an extended period. This is a problem because the liquids are volatile and so will evaporate and this will mean that not much product will form. Safety will also be a problem.By heating under reflux conditions the vapours condense on the sides of the water-cooled condenser so that the liquids fall back into the pear-shaped flask so that no reactants or products are lost during the heating process.

Isolation procedures

1. Initial washing with water

At the end of the heating period all reactants and products are present in the mixture. This is because the system reaches a state of chemical equilibrium and the concentrations of all species remain unchanged. Washing with water will remove most of the water-soluble substances.

The aqueous layer was identified by adding some distilled water and seeing which layer increased in volume. This layer was removed and the ester layer left in the separating funnel.

2. Washing with Na2CO3 solution

After the washing with water there will still be traces of acid left in the ester layer. These can be removed by reaction with Na2CO3.

2H+ + CO32-  CO2 + H2O

3. Final washing with water

This removes any traces of water-soluble materials (such as the salts formed in the last washing) from the separating funnel.

4. Stand over CaCl2

CaCl2 is a dehydrating agent and so removes any traces of water trapped in the ester layer. There was so much water that it formed a new layer.

5. Distillation

Distillation allows the separation of liquids that have different boiling points as each liquid has its own boiling point and the temperature only rises when that liquid has been completely boiled off. The ester had the highest boiling point and so was collected last.

Poor Yield

This method results in a poor yield of ester because:

  • the reaction was not refluxed for long enough
  • the reaction reaches an equilibrium so that 100% product yield cannot be obtained –the concentrations of all species, reactants and products, will remain constant
  • a little bit of ester is lost at every washing – some is let through the tap of the separating funnel, with the aqueous layer.

Modifications to improve yield of ester

  1. Reflux the reaction mixture for longer so that more reactants are converted into products.
  1. An increased concentration of one of the reactants (eg, the acid) could be used to push the equilibrium to the right and so improve the yield of ester.
  1. The separating funnel could be used more carefully so that less ester gets through with the aqueous layers.

Modifications to improve purity of ester

The preparation could be carried out more carefully to get a better boiling point and a clearer ester.

Safety hazards/precautions

  1. Safety glasses, and plastic apron were worn and any heating was done with a tiny flame and constant observation.
  1. When the mixture was shaken with Na2CO3 solution a gas (CO2) formed in the separating funnel and could have caused the tap or stopper to burst out and spill the liquids. The tap of the funnel was opened often to allow the gas to escape before it built up badly. When the tap was opened it wasturned away from people so that no liquids gotonto their faces.

Review of team-work

This preparation would have been difficult to undertake individually as two pairs of hands were definitely an advantage in setting up for the reflux and the distillation. In the formative prac we had discussed ways in which to work most efficiently (eg one partner to handle the glassware and the other to be responsible for the clamps when setting up the distillation) and had come into the lab one lunch time to practice.

We shared the heating during the reflux and distillation. We each heated for about 10 minutes whilst the other partner made and recorded observations, kept the bench uncluttered and got organized for the next part of the prac. Both of us tend to work neatly and to clean up as we go so we worked really well together.

We were both disappointed at getting a cloudy ester. It was a pity we didn’t have more time so that we could have done a second distillation as this would probably have resulted in a better quality ester.

Page 1 of 5Stage 2 Chemistry annotated response

Ref: A107234 (revised January 2013)

© SACE Board of South Australia 2013

Performance Standards for Stage 2 Chemistry – Practical Investigation: Preparation of an Ester

Investigation / Analysis and Evaluation / Application / Knowledge and Understanding
A / Designs logical, coherent, and detailed chemistry investigations.
Critically and logically selects and consistently and appropriately acknowledges information about chemistry and issues in chemistry from a range of sources.
Manipulates apparatus and technological tools carefully and highly effectively to implement well-organised safe and ethical investigation procedures.
Obtains, records, and displays findings of investigations using appropriate conventions and formats accurately and highly effectively. / Critically and systematically analyses data and their connections with concepts, to formulate logical and perceptive conclusions and make relevant predictions.
Critically and logically evaluates procedures and suggests a range of appropriate improvements. / Applies chemistry concepts and evidence from investigations to suggest solutions to complex problems in new and familiar contexts.
Uses appropriate chemical terms, conventions, formulae, and equations highly effectively.
Demonstrates initiative in applying constructive and focused individual and collaborative work skills. / Consistently demonstrates a deep and broad knowledge and understanding of a range of chemistry concepts.
Uses knowledge of chemistry perceptively and logically to understand and explain social or environmental issues.
Uses a variety of formats to communicate knowledge and understanding of chemistry coherently and highly effectively.
B / Designs well-considered and clear chemistry investigations.
Logically selects and appropriately acknowledges information about chemistry and issues in chemistry from different sources.
Manipulates apparatus and technological tools carefully and mostly effectively to implement organised safe and ethical investigation procedures.
Obtains, records, and displays findings of investigations using appropriate conventions and formats mostly accurately and effectively. / Clearly and logically analyses data and their connections with concepts, to formulate consistent conclusions and make mostly relevant predictions.
Logically evaluates procedures and suggests some appropriate improvements. / Applies chemistry concepts and evidence from investigations to suggest solutions to problems in new and familiar contexts.
Uses appropriate chemical terms, conventions, formulae, and equations effectively.
Applies mostly constructive and focused individual and collaborative work skills. / Demonstrates some depth and breadth of knowledge and understanding of a range of chemistry concepts.
Uses knowledge of chemistry logically to understand and explain social or environmental issues.
Uses a variety of formats to communicate knowledge and understanding of chemistry coherently and effectively.
C / Designs considered and generally clear chemistry investigations.
Selects with some focus, and mostly appropriately acknowledges, information about chemistry and issues in chemistry from different sources.
Manipulates apparatus and technological tools generally carefully and effectively to implement safe and ethical investigation procedures.
Obtains, records, and displays findings of investigations using generally appropriate conventions and formats with some errors but generally accurately and effectively. / Analyses data and their connections with concepts, to formulate generally appropriate conclusions and make simple predictions, with some relevance.
Evaluates some procedures in chemistry and suggests some improvements that are generally appropriate. / Applies chemistry concepts and evidence from investigations to suggest some solutions to basic problems in new or familiar contexts.
Uses generally appropriate chemical terms, conventions, formulae, and equations with some general effectiveness.
Applies generally constructive individual and collaborative work skills. / Demonstrates knowledge and understanding of a general range of chemistry concepts.
Uses knowledge of chemistry with some logic to understand and explain one or more social or environmental issues.
Uses different formats to communicate knowledge and understanding of chemistry with some general effectiveness.
D / Prepares the outline of one or more chemistry investigations.
Selects and may partly acknowledge one or more sources of information about chemistry or an issue in chemistry.
Uses apparatus and technological tools with inconsistent care and effectiveness and attempts to implement safe and ethical investigation procedures.
Obtains, records, and displays findings of investigations using conventions and formats inconsistently, with occasional accuracy and effectiveness. / Describes basic connections between some data and concepts, and attempts to formulate a conclusion and make a simple prediction that may be relevant.
For some procedures, identifies improvements that may be made. / Applies some evidence to describe some basic problems and identify one or more simple solutions, in familiar contexts.
Attempts to use some chemical terms, conventions, formulae, and equations that may be appropriate.
Attempts individual work inconsistently, and contributes superficially to aspects of collaborative work. / Demonstrates some basic knowledge and partial understanding of chemistry concepts.
Identifies and explains some chemistry information that is relevant to one or more social or environmental issues.
Communicates basic information to others using one or moreformats.
E / Identifies a simple procedure for a chemistry investigation.
Identifies a source of information about chemistry or an issue in chemistry.
Attempts to use apparatus and technological tools with limited effectiveness or attention to safe or ethical investigation procedures.
Attempts to record and display some descriptive information about an investigation, with limited accuracy or effectiveness. / Attempts to connect data with concepts, formulate a conclusion, and make a prediction.
Acknowledges the need for improvements in one or more procedures. / Identifies a basic problem and attempts to identify a solution in a familiar context.
Identifies some chemical terms or formulae.
Shows emerging skills in individual and collaborative work. / Demonstrates some limited recognition and awareness of chemistry concepts.
Shows an emerging understanding that some chemistry information is relevant to social or environmental issues.
Attempts to communicate information about chemistry.

Page 1 of 5Stage 2 Chemistry annotated response

Ref: A107234 (revised January 2013)

© SACE Board of South Australia 2013