When is the rate of a reaction fastest?

At the start, when the reactants are at their highest concentration.

What is the mean rate?

The total change divided by the total time.

How can you measure the rate of a reaction that gives off a gas?

Collect the gas and measure its volume at regular times, or measure the loss in mass.

Why is a loose cotton wool plug used in a mass loss experiment?

It lets the gas escape but stops liquid spraying out.

What is the disappearing cross method?

Timing how long it takes for a cross to be hidden by the cloudy precipitate formed in the reaction.

What does a horizontal line on the graph show?

The reaction has stopped because a reactant is used up.

How do you find the rate at one moment from a curve?

Draw a tangent and calculate its gradient.

Why is the disappearing cross method subjective?

Different people judge when the cross disappears differently.

Measuring rates of reactionIB MYP Chemistry: Flashcards

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What is the formula for rate of reaction?

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What is the formula for rate of reaction?: Rate = amount of reactant used or product made ÷ time taken.
Give a unit for rate when a gas volume is measured.: cm³ s⁻¹
When is the rate of a reaction fastest?: At the start, when the reactants are at their highest concentration.
What is the mean rate?: The total change divided by the total time.
How can you measure the rate of a reaction that gives off a gas?: Collect the gas and measure its volume at regular times, or measure the loss in mass.
Name a piece of apparatus for measuring gas volume accurately.: A gas syringe.
Why is a loose cotton wool plug used in a mass loss experiment?: It lets the gas escape but stops liquid spraying out.
Which gas is lost when marble chips react with acid?: Carbon dioxide.
What is the disappearing cross method?: Timing how long it takes for a cross to be hidden by the cloudy precipitate formed in the reaction.
How do you get a relative rate from a time?: Relative rate = 1 ÷ time.
What does a steep line on a volume against time graph show?: A fast rate of reaction.
What does a horizontal line on the graph show?: The reaction has stopped because a reactant is used up.
How do you find the rate at one moment from a curve?: Draw a tangent and calculate its gradient.
Why is the disappearing cross method subjective?: Different people judge when the cross disappears differently.

Exam questions on Measuring rates of reaction

A student in Cape Town reacts magnesium ribbon with excess dilute hydrochloric acid in a conical flask connected to a gas syringe. She records the volume of hydrogen collected at regular intervals and plots volume against time.
Between 0 and 20 s she collects 30 cm³ of gas, and between 60 s and 80 s she collects 6 cm³. Calculate the mean rate of reaction, in cm³ s⁻¹, for each period.2 marks
A technician in Manchester places a conical flask containing marble chips and dilute hydrochloric acid on a top-pan balance. A loose plug of cotton wool is fitted in the neck of the flask, and the balance reading is recorded every 30 seconds as the reaction proceeds.
The first reading is 152.40 g and the reading after 60 seconds is 151.92 g. Calculate the mean rate of mass loss, in g s⁻¹, over the first 60 seconds.2 marks
In a school in Mumbai, a student mixes sodium thiosulfate solution with dilute hydrochloric acid in a flask standing on a cross drawn on paper. A yellow solid forms and makes the mixture cloudy. The student times how long it takes until the cross can no longer be seen when looking down through the mixture.
Explain how the time taken for the cross to disappear can be used to compare the rates of reaction in different experiments.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).