Background

In order to fulfil the Internal Assessment criteria your students will need to do at least two Design practicals. In reality this means doing about four so that they can learn from their mistakes and have a good chance of achieving high marks for at least two of them.

Design practicals are totally different to all the practicals listed under Details of individual practicals as no instructions can be given to the students. Essentially all you do is give them an open-ended problem to investigate. The subject guide gives two clear instructions on this. Either you may suggest the general research question only. For example, you could ask them to investigate some property of a reaction system such as nucleophilic substitution. The student would then focus this to come up with a specific research question such as ‘How does changing the halogen affect the rate of substitution of primary halogenoalkanes with hydroxides ions?’ Alternatively the teacher may suggest a specific research question and specify the dependent variable. For example, 'Investigate one factor affecting the voltage in electrochemical cells’. The student would then need to come up with a focused research question such as, ‘How does altering the temperature of the two half-cells affect the voltage in a cell made of a Cu(s)/Cu2+(aq) half-cell connected by a salt bridge to a Zn(s)/Zn2+(aq) half-cell?’

Personally I find this a little superficial as it is not normally what chemists do. Usually chemists have a clear idea what the problem is – the challenge is devise a solution to solve it. Forensic scientists, for example, may need to be able to show that the paint left on a child’s bicycle came from a car suspected of being involved in a hit-and run accident. In industry chemists may need to understand the mechanism of a particular reaction in order to be able to suggest a more efficient and economical catalyst. Analytical chemists may need to know the exact nature and quantity of pollutants in a sample of water etc.

So how do we go about devising a good Design experiment for our students where the problem is not known?

 

 

Introducing Design practicals

 

1. Selecting the variables and research question

Because the idea is new to students you will need to go through one exercise carefully with them to show them how to do it. Students need to be clear that all experiments will have variables and these can be sub-divided into three basic types.

1. Controlled variables. These are variables that remain constant throughout all the experiments.

2. The Independent variable. This is the variable that is manipulated by the student.

3. The Dependent variable. This is the variable that is measured.

Once they understand this I then give them a general research question and ask them to list all the variables. One example I give to illustrate this is ‘Investigate an aspect of esterification’.

The list of variables could include:

The alcohol used
The organic acid used
The temperature of the reaction
The time they are allowed to react
The nature of the catalyst
The smell of the product
The rate of the reaction
The equilibrium constant for the reaction

What students will then need to do is identify which one(s) they can easily measure, i.e. the Dependent variable. I give esterification as an example for two reasons. Firstly, it is not actually very easy to assess for all three criteria D, DCP and CE but it illustrates the point well and it means you can keep other good examples which you can assess easily for all three criteria. Secondly, the experiments can be done very quickly in a test-tube as the students have already done one simple experiment to give them the general method in the practical on Organic functional groups. The reason why esterification is not very suitable for assessment is that it is not really that easy to measure the rate of the reaction or the equilibrium constant in the time available (as it takes about a week for the reaction to reach equilibrium at room temperature). However it is very easy to detect the distinctive smell of the product. The problem is how can smell be measured? One way is for the student to devise their own scale such as 1 to 5 where 1 is revolting through to 5 which is extremely pleasant. Once they have identified smell as the Dependent variable they then need to choose one of the other variables to be their Independent variable and keep all the others constant. For example, they may choose to make the alcohol the independent variable and come up with a focused research question such as, ‘How does increasing the length of the hydrocarbon chain in primary alcohols, ROH, affect the smell of the ester produced by reacting the alcohol with ethanoic acid? The trick is to get each student individually to consider which variable they can manipulate easily and then get them to come up with their own research question. A good Design practical is one that has many variables and one in which all the students in the class come up with a different research question. Some of the actual research questions that my students came up with (after I guided them away from looking at the rate of reaction or value of equilibrium constant) were:

1.  To determine whether changing the type of alcohol from primary to secondary to tertiary has any effect on the odour of the ester produced with propanoic acid..

2.   How do different types of structural isomerism within the alcohols with molecular formula C4H10O affect the smell of esters produced with the same acid?

3.    Is it the nature of the carboxylic acid or the alcohol that influences the smell of an ester? Or both?

4.   To determine the different physical properties of esters produced when cis-but-2-ene-1,4-dioic acid and trans-but-2-ene-1,4-dioic acid react with butan-1-ol. I.e. does cis/trans isomerism affect the smell of an ester?

5. To investigate whether there is a trend in the smells of the esters produced between salicylic acid and a homologous series of alcohols.

This of course is only a training exercise so it can be discussed with the class (although it is good to let them actually do the practical work with different acids and alcohols as they enjoy it and come up with some interesting smells). If it was for real students must come up with their own research question with no help from you. To ensure fairness it is good to ask them to do this under your supervision. If you set it for homework then the chances are that other people (parents, other students etc.) and the Internet may also play a part and you cannot be certain it is all the student’s own work. Specifically you must not:

  • Give the students a focused research question
  • Tell them the outcome of the investigation
  • Tell them which Independent variable to select
  • Tell them which variables to keep constant.

 

 

2. Designing the method

Once the students have a clear research question they must then design a method which enables them to alter the Independent variable, measure the Dependent variable and control all the other variables. They must specifically state how they are controlling the controlled variables. If this is genuinely not possible to do for all the variables then they should make some effort to monitor them. An example of this can be found in some rate studies where the temperature is controlled. If the reaction is exothermic then the temperature may be varying throughout the reaction. Although it may be difficult to control this, even with a water bath, the temperature can at least be monitored to see whether it does actually change.

Students may either design their own method or make use of a standard measurement technique to solve their research question. If they do use a standard technique (such as the Winkler method in order to identify the factors affecting the amount of dissolved oxygen in water) then they should give the reference to identify its source. Clearly they need access to sources to do this and I think it is reasonable that once they have determined their own research question they should be allowed to use the Internet to aid them in the design of their method,

The method the student devises must be capable of providing sufficient information, i.e. sufficient data so that the aim of the investigation can be suitably addressed and its reliability evaluated. This means allowing for repeat measurements, consistent results in titrations, and a minimum of five data points for a best-fit straight line to be made if a trend line is obtained from a scatter graph (for example, a calibration curve obtained using a spectrometer).

For the method described by the student to control the variables and collecting sufficient data you must not tell them:

  • Which apparatus to select
  • The experimental method
  • How to collect the data
  • How much data to collect.

 

 

Suggestions for design experiments

Once you have digested all this and given them a trial run you will need to choose about four actual Design practicals to give them. Most teachers tend to give these in the second year when students have already done some DCP and CE assessed practicals. You may give all the students the same general investigation or you may give a list and ask them to select one.  This second option has the advantage that there is less chance that students in the same class will come up with similar research questions. A list of about fifteen possibilities has been prepared which does the round at IB workshops. I’ll give this below but at the workshops for experienced teachers that I’ve run there is general agreement that there are four or five which are definitely reliable and work well in terms of providing many different possible research questions. For four of these, which I’ve put first in bold with links, I have given a separate page with some examples of possible research questions which they can lead to.

  

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