What is meant by the rate of a chemical reaction?

It is the amount of reactant used up, or of product formed, in a given time.

Suggest a method for measuring the rate of a reaction that gives off a gas, using a gas syringe.

Connect the flask to a gas syringe and record the volume of gas collected at fixed time intervals, such as every 10 seconds.

Suggest a method for measuring the rate of a reaction that produces a cloudy precipitate.

Stand the flask over a marked cross and time how long it takes for the cross to become invisible through the mixture, as happens when sodium thiosulfate reacts with hydrochloric acid.

Suggest how the rate of a reaction in which the colour of the solution changes could be followed.

Use a colorimeter to measure how much light the solution absorbs at set times, since the reading changes as the coloured substance is used up or formed.

OCR GCSE Chemistry A J248

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions State symbols in equations Conservation of mass, and changes of mass in open systems Exothermic and endothermic reactions, and reaction profiles Activation energy and why reactions need it Oxidation and reduction in terms of oxygen Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids The pH scale, acidity and alkalinity Measuring pH with indicators and pH meters Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures Nanoparticles: size and surface area to volume Uses and risks of nanoparticulate materials

Global Challenges

Monitoring and Controlling Chemical Reactions

Topic overview Carrying out an acid–alkali titration Theoretical mass and percentage yield Atom economy and its calculation Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Transition metals Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine Tests for aqueous cations and anions Flame tests, and identifying metal ions Identifying a species from test results Instrumental methods of analysis

Monitoring and Controlling Chemical Reactions

Measuring the rate of a reaction, and rate graphs

3 things to know
Paper 2

Overview

What rate means, following a reaction with a gas syringe, a balance, a disappearing cross, a colorimeter or a pH probe, the units of rate, and reading the gradient of a rate graph.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

On a rate graph, why does the line flatten, what does a horizontal line show, and how do you calculate the rate at one instant?

Key term

In general terms, what two quantities can be followed over time to measure the rate of a reaction?

Key term

Suggest how a balance can be used to measure the rate of a reaction that releases a gas, and explain why the mass changes.

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by the rate of a chemical reaction?
Model answer
It is the amount of reactant used up, or of product formed, in a given time.
2
In general terms, what two quantities can be followed over time to measure the rate of a reaction?
Model answer
How quickly a reactant is used up, or how quickly a product is formed.
3
Suggest a method for measuring the rate of a reaction that gives off a gas, using a gas syringe.
Model answer
Connect the flask to a gas syringe and record the volume of gas collected at fixed time intervals, such as every 10 seconds.
4
Suggest how a balance can be used to measure the rate of a reaction that releases a gas, and explain why the mass changes.
Model answer
Stand the open flask on a balance and record the mass at fixed intervals; the mass falls because the gas produced escapes from the flask.
5
Suggest a method for measuring the rate of a reaction that produces a cloudy precipitate.
Model answer
Stand the flask over a marked cross and time how long it takes for the cross to become invisible through the mixture, as happens when sodium thiosulfate reacts with hydrochloric acid.
6
Suggest how the rate of a reaction in which the colour of the solution changes could be followed.
Model answer
Use a colorimeter to measure how much light the solution absorbs at set times, since the reading changes as the coloured substance is used up or formed.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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AThe products sit at a higher energy level than the reactants.

BThe activation energy, the energy that must be supplied before the reaction can happen.

CThe reactants, the products, the activation energy and the overall energy change.

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Sodium ions give a bright yellow flame colour.

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