Required Practical 5 - Rates of Reaction + Turbidity & Improving Rate Experiments & Rate Graphs

How can gas production be used to measure reaction rate?

Add a measured volume/concentration of reactant to a flask. -- Connect the flask to a gas syringe. -- Add the second reactant and immediately start the timer. -- Record the volume of gas produced at regular time intervals. -- Repeat using different concentrations while keeping other variables constant.

What is the independent variable? (RP - Gas volume)

Concentration of a reactant.

What is the dependent variable? (RP - Gas volume)

Volume of gas produced over time / rate of gas production.

Give important control variables. (RP - Gas volume)

Depending on the experiment: temperature, volume of solution, mass/surface area of solid reactant and apparatus used.

Why is a gas syringe useful?

It allows the volume of gas produced to be measured directly.

Why should the apparatus be airtight? (RP - Gas volume)

To prevent gas escaping, which would cause the measured gas volume to be too low.

How can turbidity be used to investigate reaction rate?

Place sodium thiosulfate solution in a flask over a marked cross. -- Add hydrochloric acid and start the timer immediately. -- Observe the cross through the solution. -- Stop the timer when the cross can no longer be seen. -- Repeat using different concentrations of sodium thiosulfate while controlling other variables.

Why does the cross disappear? (RP - Turbidity)

Solid sulfur precipitate forms, making the solution increasingly cloudy/turbid.

How can rate be estimated from the disappearing-cross experiment?

rate ∝ 1 / time -- therefore, a shorter time means a faster reaction.

Why is the disappearing-cross method subjective?

Different people may judge the point at which the cross disappears differently.

How could this be improved?

Use a light sensor/data logger to measure the change objectively rather than relying on human judgement.

Why should the same observer be used if no light sensor is available?

To make the judgement of the endpoint more consistent.

Why should experiments be repeated?

To identify anomalous results and calculate a mean, improving reliability.

Why should temperature be controlled?

Temperature affects reaction rate, so allowing it to change would introduce another variable.

Why should only one variable be changed?

So any change in rate can be attributed to the independent variable.

Why might the first gas-volume reading be inaccurate?

Some reaction may occur before the bung is fitted/apparatus sealed, allowing gas to escape.

How could this be improved?

Set up the apparatus so it can be sealed before the reactants are mixed, where appropriate.

Two experiments produce the same final volume of gas but one graph is steeper. What does this mean?

The steeper experiment has a faster reaction rate, but the same final volume means the same total amount of product was formed.

Why might two different concentrations eventually produce the same amount of gas?

If the reactant whose concentration changed was in excess, the same amount of the limiting reactant was present, so the same total amount of product forms.