Measuring Rates of Reaction

How can gas production be used to measure reaction rate?

Measure the volume of gas produced at regular time intervals.

What apparatus can be used to measure gas volume?

A gas syringe.

Why is a gas syringe useful?

It allows direct measurement of gas volume over time.

How can a measuring cylinder be used to collect gas?

Gas can be collected over water by displacement, if the gas is suitable.

How can mass loss be used to measure reaction rate?

Measure the decrease in mass as gas escapes from the apparatus.

What apparatus is used for mass-loss measurements?

A balance.

Why does the mass decrease during some reactions?

A gaseous product escapes from the reaction vessel.

Why is cotton wool sometimes placed in the neck of a flask?

It reduces loss of liquid by splashing while allowing gas to escape.

Why must the gas be able to escape in a mass-loss experiment?

Otherwise the balance measures the mass of the gas as well as the remaining contents.

How can a colour change be used to measure reaction rate?

Measure the time taken for a visible change to occur.

What is the disappearing-cross method?

Measuring the time taken for a cross beneath a flask to become obscured by a precipitate.

Which reaction is commonly used for the disappearing-cross method?

Sodium thiosulfate reacting with hydrochloric acid.

What causes the cross to disappear in the sodium thiosulfate reaction?

Formation of a solid sulfur precipitate makes the mixture cloudy.

What is one limitation of the disappearing-cross method?

Deciding when the cross disappears is subjective.

Why should gas-volume readings be taken at regular intervals?

To produce consistent data for plotting a rate graph.

Why should the apparatus be sealed quickly when measuring gas production?

To prevent loss of gas before measurement begins.

Why should experiments be repeated?

To identify anomalies and improve reliability.

What is a control variable?

A factor kept constant to ensure a fair test.

What is the independent variable?

The factor deliberately changed.

What is the dependent variable?

The factor measured in response to the independent variable.