Designing in Chemistry (DP IB Chemistry): Revision Note
Designing in Chemistry
This is where you turn your well-explored research question into a practical, step-by-step plan or methodology.
Your goal is to design a valid procedure that allows you to collect sufficient, high-quality data to properly answer your research question.
The design must be explained clearly, considering variables, measurements, and all safety aspects.
Designing your methodology
Identify and justify the choice of variables
At the start of your design, you must clearly list and explain your choice of variables.
Independent variable (IV): The single variable you deliberately change to see its effect.
Dependent variable (DV): The variable you measure to see how it is affected by the change in the IV.
Controlled variables (CVs): All other factors that could plausibly affect the outcome. You must explain how you will keep these constant to ensure a fair test.
Justify the range and quantity of measurements
It is not enough to just state your measurements; you must justify them.
Range of the independent variable:
You should plan to collect data across a suitable range.
A minimum of five different values for the independent variable is recommended to establish a clear trend.
Justify your choice.
For a kinetics experiment, you might state: "A temperature range of 20°C to 60°C was chosen because temperatures below this result in a reaction that is too slow, while temperatures above 60°C can cause the reactant solution to evaporate significantly."
Quantity of measurements:
You must repeat the experiment for each value of the independent variable to ensure the results are reliable.
A minimum of three trials is recommended.
Repeating trials allows you to calculate the mean average, which:
Reduces the effect of random error.
Helps you to identify and discard any anomalous results.
Design and explain a valid methodology
This is the detailed, step-by-step procedure of your experiment.
It must be a logical sequence of instructions that is clear enough for another chemist to follow and replicate your experiment exactly.
Include precise details of the apparatus used.
For example, "Measure the solution using a 50.0 cm3 burette" not "add some solution".
Creativity in design can be shown in how you solve a measurement problem.
For example, using a colorimeter to track the rate of a reaction that produces a coloured product is more creative and reliable than timing a colour change by eye.
When writing your report, a good way to structure your methodology is:
Materials and apparatus:
A bulleted list of all chemicals (with concentrations) and equipment (with sizes and precision).
Safety, ethical, and environmental:
A brief risk assessment identifying key hazards and outlining specific precautions and waste disposal methods.
Procedure:
The numbered, step-by-step instructions.
Develop investigations using different approaches
While hands-on laboratory work is most common, your investigation could be based on other sources of data:
Databases
Using an established chemical database (e.g. NIST Chemistry WebBook (opens in a new tab)) to find data for a property (like boiling points of a homologous series) and then processing and analysing it to find a trend.
Simulations
Using a simulation (e.g. from PhET (opens in a new tab)) to collect data for a process that is too difficult or dangerous to conduct in a school lab, such as investigating the behaviour of ideal gases.
Pilot methodologies
A pilot study is a small-scale trial run of your experiment.
This is extremely useful to check that your planned methodology works.
For example, a quick pilot (or rough) titration will tell you the approximate volume needed for neutralisation.
This means that in your proper, recorded titrations, you will know when to slow down the rate of addition to get a very precise endpoint.
Worked Example
Designing a titration investigation
Research question:
"What is the concentration of ethanoic acid in commercially sold vinegar?"
Identifying and justifying variables:
Independent variable:
Volume of 0.100 mol dm-3 sodium hydroxide (the titrant) added from the burette.
Dependent variable:
The point of neutralisation, identified by the first permanent colour change of the phenolphthalein indicator.
Control variables:
The concentration of the NaOH standard solution must be known and constant.
The volume of the vinegar sample must be measured precisely with a volumetric pipette.
The temperature should be kept constant by conducting all trials in the same location away from heat sources, as temperature can affect indicator performance.
Justifying the methodology:
A 25.00 cm3 volumetric pipette will be used to measure the initial vinegar sample
It has a very low uncertainty (e.g. ±0.03 cm3), ensuring the initial amount of acid is consistent.
The vinegar will first be diluted by a factor of 10.
This is to ensure the volume of titrant needed is large enough to be measured accurately by the burette.
The chosen indicator will be phenolphthalein
Its endpoint pH range (8.2–10.0) aligns with the steep equivalence point region for a weak acid-strong base titration.
Worked Example
Designing a calorimetry investigation
Research question:
"How does the strength of a monoprotic acid affect the standard enthalpy of neutralisation?"
Identifying and justifying variables:
Independent variable:
The type of acid used (strong: 0.5 M HCl vs. weak: 0.5 M CH3COOH).
Dependent variable:
The maximum temperature change (ΔT) of the reaction mixture, used to calculate the enthalpy of neutralisation.
Control variables:
The volume and concentration of all acid and base solutions must be kept constant.
The initial temperature of the solutions must be the same before mixing.
The total volume of the final mixture must be consistent across all trials.
Justifying the methodology:
A polystyrene cup will be used as the calorimeter and placed inside a glass beaker for stability.
This is because polystyrene is a better thermal insulator than glass, which minimises the most significant systematic error: heat loss to the surroundings.
A lid will be used on the cup.
This is to reduce heat loss by evaporation.
The temperature will be recorded every 15 seconds for 2 minutes after mixing.
This allows for an accurate extrapolation back to the point of mixing, correcting for heat loss as the reaction proceeds.
Worked Example
Designing a rates of reaction investigation
Research question:
"What is the effect of temperature on the rate of reaction between aqueous sodium thiosulfate and hydrochloric acid?"
Identifying and justifying variables:
Independent variable:
Temperature of the reaction mixture (°C).
Dependent variable:
The initial rate of reaction, determined by measuring the time taken (s) for a set amount of sulfur precipitate to form and obscure a cross drawn beneath the flask. Rate is calculated as 1/time.
Control variables:
The concentrations and volumes of both the sodium thiosulfate and hydrochloric acid solutions must be kept constant.
The total volume of the solution must also be constant.
Justifying the methodology:
A range of five temperatures from 20°C to 60°C will be investigated.
This range is wide enough to show a significant change in rate.
It avoids low temperatures that may result in an impractically slow reaction.
It also avoids high temperatures where the evaporation of water becomes a significant issue.
A water bath will be used to bring the reactant solutions to the desired temperature before mixing.
This is to maintain this temperature during the reaction.
Also, this is more accurate than heating the flask on a tripod and gauze and is crucial for controlling the key variable of temperature.
Examiner Tips and Tricks
Detail is key:
A vague method cannot be replicated and will not score well.
Instead of "Add some acid to the base," write:
Use a 25.00 cm3 volumetric pipette to transfer 25.00 cm3 of the 1.0 M HCl solution into the 250 cm3 conical flask.
Safety is not an afterthought:
Your design must include a dedicated safety section.
Be specific.
Instead of "Handle chemicals with care," write:
Wear safety goggles and a lab coat at all times.
1.0 M HCl is a corrosive irritant; handle with gloves in a well-ventilated area.
Dispose of all waste in the correct chemical waste container.
Justify your choices:
For top marks, you must explain why you chose a specific piece of apparatus or a certain range of values.
This shows the assessor you are thinking like a chemist.
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