Describe the shape of a graph of volume of gas produced against time for a reaction that slows down and then stops.

It is steepest at the start, then curves and becomes less steep, and finally flattens into a horizontal line.

Describe how the rate of a reaction at one particular moment is found from a curve on a graph of quantity against time.

A tangent is drawn to the curve at that time and the slope of the tangent is used as the rate at that moment.

Explain why a tangent is needed to find the rate at a given time rather than simply reading a value off the curve.

The steepness of the curve changes throughout the reaction, so only the slope at that point gives the rate at that moment rather than an average.

Two reactions are followed on the same axes of gas volume against time. State how you can tell from the curves which reaction is faster at the start.

The reaction whose curve is steeper at the start is the faster one.

AQA GCSE Combined Science Chemistry 8464

Atomic Structure and the Periodic Table

Topic overview Atoms, elements and compounds Word equations, symbols and balanced equations Mixtures From the plum pudding model to the nuclear model Bohr, Chadwick and the modern model of the atom Relative electrical charges of subatomic particles Size and mass of atoms Relative atomic mass and electronic structure The periodic table Development of the periodic table Metals and non-metals Group 0 Group 1 Group 7

Bonding, Structure, and the Properties of Matter

Topic overview Chemical bonds, including metallic bonding Ionic bonding Ionic compounds Covalent bonding and how it is represented The three states of matter and changes of state Predicting state, state symbols, and the limits of the particle model Properties of ionic compounds Properties of small molecules Polymers and giant covalent structures Properties of metals and alloys, and why metals conduct Diamond and graphite Graphene and fullerenes

Chemical Analysis

Topic overview Pure substances Formulations Chromatography Tests for hydrogen, oxygen, carbon dioxide and chlorine

Chemical Changes

Topic overview The reactivity series Metal oxides, extraction, and redox in terms of electrons Acids reacting with metals, neutralisation and salts Soluble salts The pH scale and neutralisation The process of electrolysis, and molten ionic compounds Using electrolysis to extract metals Electrolysis of aqueous solutions

Chemistry of the Atmosphere

Topic overview The Earth's early atmosphere How oxygen increased and carbon dioxide decreased The gases in today's atmosphere, and which are greenhouse gases Human activities which contribute to an increase in greenhouse gases in the atmosphere Global climate change and carbon footprints Atmospheric pollutants from fuels, and their effects

Energy Changes

Topic overview Exothermic and endothermic reactions Everyday uses, and investigating temperature change Reaction profiles

Organic Chemistry

Topic overview Crude oil, hydrocarbons and alkanes Fractional distillation and petrochemicals Properties of hydrocarbons Cracking, and testing alkenes with bromine water Why cracking is useful, and equations for it

Quantitative Chemistry

Topic overview Conservation of mass and balanced chemical equations Relative formula mass and concentration of solutions Mass changes when a gas is involved, and measurement uncertainty

The Rate and Extent of Chemical Change

Topic overview Measuring the mean rate of a reaction Rate from graphs, and the gradient of a tangent Factors which affect the rates of chemical reactions Collision theory and activation energy Catalysts Reversible reactions and dynamic equilibrium

Using Resources

Topic overview Using the Earth's resources and sustainable development Producing potable water in the UK Desalination, and analysing water samples Waste water treatment Life cycle assessment Ways of reducing the use of resources

The Rate and Extent of Chemical Change

Rate from graphs, and the gradient of a tangent

3 things to know
Chemistry Paper 2

Overview

Mean rate calculations, the shape of a volume against time graph and what a horizontal section means, drawing a tangent to find the rate at one moment, comparing rates by steepness and why a curve is steepest at the start, plotting graphs with a line of best fit, y = mx + c, gradients and intercepts of straight lines, and direct proportionality.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

State what a horizontal section on a graph of quantity of product against time tells you about the reaction.

Numbers

A reaction produces 30 cm³ of gas in the first 50 s. Calculate the mean rate over that period.

Process

Explain why a graph of product formed against time is steepest at the very beginning of a reaction.

Questions from this subtopic

Each one with the model answer underneath.

1
Describe the shape of a graph of volume of gas produced against time for a reaction that slows down and then stops.
Model answer
It is steepest at the start, then curves and becomes less steep, and finally flattens into a horizontal line.
2
A reaction produces 30 cm³ of gas in the first 50 s. Calculate the mean rate over that period.
Model answer
30 cm³ ÷ 50 s = 0.6 cm³/s.
3
State what a horizontal section on a graph of quantity of product against time tells you about the reaction.
Model answer
The reaction has finished, so no more product is forming and the rate is zero.
4
Describe how the rate of a reaction at one particular moment is found from a curve on a graph of quantity against time.
Model answer
A tangent is drawn to the curve at that time and the slope of the tangent is used as the rate at that moment.
5
Explain why a tangent is needed to find the rate at a given time rather than simply reading a value off the curve.
Model answer
The steepness of the curve changes throughout the reaction, so only the slope at that point gives the rate at that moment rather than an average.
6
Two reactions are followed on the same axes of gas volume against time. State how you can tell from the curves which reaction is faster at the start.
Model answer
The reaction whose curve is steeper at the start is the faster one.

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Atomic Structure and the Periodic Table

Bonding, Structure, and the Properties of Matter

Chemical Analysis

Chemical Changes

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