A student investigates how the rate of reaction between magnesium and hydrochloric acid depends on acid concentration. Identify the independent, dependent and three control variables.

The independent variable is the concentration of the acid and the dependent variable is the rate, for example the volume of hydrogen collected in a fixed time. Control variables include the temperature, the mass and surface area of the magnesium, and the total volume of acid used.

Why must the surface area of a solid reactant be kept the same across a set of rate experiments?

A larger surface area exposes more particles to collision, so it increases the rate independently of the variable being studied. Using ribbon of the same width and length, or powder of the same grade, keeps this factor from confusing the results.

Which control variables must be kept constant between runs of an iodine clock experiment?

The temperature, the total volume of the mixture, and the amount of thiosulfate and of starch added. The concentrations of all reagents except the one being varied must also be held constant, which is why water is added to keep the total volume the same.

Why is water added to some mixtures in a rate experiment so that every run has the same total volume?

Changing the volume of one reagent changes its concentration in the mixture, but it also changes the concentrations of everything else if the total volume varies. Topping up with water keeps the total volume fixed, so only the intended concentration changes.

WJEC AS-Level Chemistry 1410QS

Formulae and Equations

Topic overview Charges on common ions Building formulae for ionic and molecular compounds Assigning oxidation numbers Using oxidation numbers to follow redox changes Balancing equations and using state symbols Ionic and half equations

Basic Ideas About Atoms

Topic overview Radioactive decay and nuclear equations Alpha, beta and gamma radiation Half-life, dating and the uses of radioisotopes: half-life of radioactive decay Half-life, dating and the uses of radioisotopes: adverse consequences for living cells Ionisation energies and electronic structure Orbitals and the filling of sub-shells Atomic spectra and the energy of light: atomic emission spectrum of the hydrogen atom Atomic spectra and the energy of light: relationship between energy and frequency Atomic spectra and the energy of light: significance of the frequency

Chemical Calculations

Topic overview Relative masses and the mass spectrometer: relative atomic Relative masses and the mass spectrometer: how a mass spectrometer works Relative masses and the mass spectrometer: molecular ion The mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectra The mole, the Avogadro constant and molar mass: moles and percentage composition by mass Empirical and molecular formulae Concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3 Concentration, solubility and titration calculations: stoichiometry and the limiting reagent Concentration, solubility and titration calculations: titration calculations and estimating a titre Gas volumes and the ideal gas equation Yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yield Yield, atom economy and experimental uncertainty: how to estimate the percentage error

Bonding

Topic overview Ionic bonds and covalent bonds: ionic bonding in terms of ion formation Ionic bonds and covalent bonds: covalent bonding and forces Electronegativity, polarity and in-between bonds: concepts of electronegativity and bond polarity Electronegativity, polarity and in-between bonds: forces Forces between molecules Hydrogen bonding Shapes of molecules and ions: VSEPR principle and its use Shapes of molecules and ions: bond angles associated with linear

Solid Structures

Topic overview Ionic crystals: sodium chloride and caesium chloride Giant covalent structures: diamond and graphite Simple molecular solids: iodine and ice Metals and the electron sea Using structure to explain physical properties

The Periodic Table

Topic overview How the Periodic Table is organised Oxidation and reduction as electron transfer Trends across periods and down groups Group 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acid Group 2: reactions, trends and the reasons behind them — trends down the group Group 2: reactions, trends and the reasons behind them — thermal stability of the carbonates Group 2: reactions, trends and the reasons behind them — oxides and hydroxides Identifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxide Identifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acid Group 7: the halogens — physical properties and the trend in volatility Group 7: the halogens — reactions of the halogens with metals Group 7: the halogens — displacement reactions and the trend Group 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium

Simple Equilibria and Acid-Base Reactions

Topic overview Reversible reactions and dynamic equilibrium Le Chatelier's principle and shifting the position of equilibrium The equilibrium constant Kc Acids, bases, salts and neutralisation pH, strong acids and weak acids Acid-base titrations and their calculations

Thermochemistry

Topic overview Enthalpy changes and their definitions Calculating enthalpy changes from experimental measurements Determining enthalpy changes in the laboratory Hess's law and energy cycles Average bond enthalpies

Rates of Reaction

Topic overview Collision theory and the conditions that change rate Measuring rate and relating it to concentration Energy profiles and activation energy Temperature and the Boltzmann distribution Catalysts and how they work Following reactions in the laboratory

The Wider Impact of Chemistry

Topic overview Social and economic consequences of chemical manufacture and energy supply Environmental consequences of chemical processes and energy production Green chemistry and sustainable practice

Organic Compounds

Topic overview Drawing and reading organic structures Naming organic compounds Chain length, functional groups and physical properties Structural isomerism Electrophiles, nucleophiles, radicals and bond fission

Hydrocarbons

Topic overview Alkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefits Alkane bonding, combustion and fossil fuels: combustion calculations and acid rain Radical substitution of alkanes The C=C bond and why alkenes are reactive E–Z isomerism in alkenes Electrophilic addition, Markovnikov's rule and testing for alkenes: mechanism of electrophilic addition Electrophilic addition, Markovnikov's rule and testing for alkenes: Markovnikov's rule and the bromine water test Electrophilic addition, Markovnikov's rule and testing for alkenes: testing for a C=C bond Hydrogenation and addition polymers: conditions required Hydrogenation and addition polymers: nature of addition polymerisation

Halogenoalkanes

Topic overview Nucleophilic substitution by hydroxide ions Elimination to form alkenes Reactivity of the carbon–halogen bond and the silver nitrate test Uses of halogenoalkanes, CFCs and the ozone layer: uses of halogenoalkanes and CFCs Uses of halogenoalkanes, CFCs and the ozone layer: radical mechanism Uses of halogenoalkanes, CFCs and the ozone layer: replacements for CFCs Heating under reflux in the laboratory

Alcohols and Carboxylic Acids

Topic overview Two routes to ethanol: hydration and fermentation — hydration of ethene Two routes to ethanol: hydration and fermentation — fermentation Two routes to ethanol: hydration and fermentation — comparing the two routes on rate Classifying alcohols and dehydrating them Oxidising alcohols to aldehydes, ketones and acids Chemical tests for alcohols and carboxylic acids Carboxylic acids, bases and salts Making an ester and separating it by distillation: esterification Making an ester and separating it by distillation: carrying out the preparation Making an ester and separating it by distillation: separating and purifying Making an ester and separating it by distillation: yield

Instrumental Analysis

Topic overview Reading a mass spectrum Infrared spectra and functional groups NMR spectra and atomic environments Putting the three techniques together

Practical Skills and Techniques

Topic overview Measuring mass, volume, time and temperature Standard solutions and titration technique Indicators and acid–base titration curves Heating, refluxing, distilling and purifying: Heating Heating, refluxing, distilling and purifying: purifying Melting temperature, chromatography and pH measurement Electrochemical cells, rate methods and working safely: Electrochemical cells Electrochemical cells, rate methods and working safely: rate methods and working safely

Planning, Analysis and Evaluation

Topic overview Planning an investigation and controlling variables Recording observations and presenting data Graphs, gradients and processing data Errors, uncertainty, accuracy and precision Evaluating a method and suggesting improvements Research and referencing

Core Experiments and Their Chemistry

Topic overview Gravimetric and acid–base analysis Enthalpy and rate experiments: Enthalpy Enthalpy and rate experiments: rate experiments Redox titrations and electrochemical cells Equilibrium constants and following a titration with a pH probe Organic synthesis, separation and identification: Organic synthesis Organic synthesis, separation and identification: identification tests and chromatography

Mathematical Skills in Chemistry

Topic overview Units, standard form and proportion in chemical calculation: volume, equilibrium, rate and energy units Units, standard form and proportion in chemical calculation: standard form Units, standard form and proportion in chemical calculation: proportion Units, standard form and proportion in chemical calculation: checking answers, buffers, rate constants and units Significant figures, means and uncertainty in experimental data: significant figures in multiplication Significant figures, means and uncertainty in experimental data: relative atomic mass as a weighted mean Significant figures, means and uncertainty in experimental data: absolute and percentage uncertainty Symbols, rearranging equations and logarithms: chemical symbols and what each one asserts Symbols, rearranging equations and logarithms: rearranging an equation Symbols, rearranging equations and logarithms: substituting into equations for Kc, enthalpy and pH Symbols, rearranging equations and logarithms: logarithms, the Arrhenius equation and rearranging formulae Graphs: plotting, gradients, intercepts and tangents — reading a spectrum Graphs: plotting, gradients, intercepts and tangents — reaction order from data and plotting good graphs Graphs: plotting, gradients, intercepts and tangents — Arrhenius plots, zero-order graphs and tangents Graphs: plotting, gradients, intercepts and tangents — initial rates from tangents and interpreting graphs Shape, symmetry and three-dimensional structure: Shape Shape, symmetry and three-dimensional structure: symmetry

Planning, Analysis and Evaluation

6 subtopics
18 things to know
All units

What Planning, Analysis and Evaluation covers

Planning, Analysis and Evaluation is assessed throughout WJEC Chemistry and in the unit 5 practical examination rather than in one unit of its own. 101 recall questions across 6 subtopics.

Where it is assessed

All units

Skills assessed throughout, and in the unit 5 practical examination.

Check the WJEC specification ↗

The 6 subtopics

Each one has its own page. WJEC AS Chemistry students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Distinguish between the independent variable, the dependent variable and a control variable.

Key term

Why is temperature the control variable that most often needs a thermostatic water bath in rate work?

Key term

How would you distinguish a secondary alcohol from a tertiary alcohol?

Key term

What must the heading of a column in a results table contain?

Key term

Distinguish between an observation and an inference, using the addition of barium chloride solution to an unknown as an example.

Key term

Why should the concentration and source of each reagent be noted in a practical record?

Key term

Why is a smooth line of best fit drawn through plotted points rather than joining them dot to dot?

Key term

What graph would you plot to confirm that a reaction is second order with respect to a reactant, and what would it look like?

Evaluation

Distinguish between a random error and a systematic error.

Key term

How could a spirit burner experiment for determining an enthalpy change of combustion be improved?

Key term

Why is an expanded polystyrene cup used for a neutralisation experiment rather than a glass beaker?

Key term

When would carrying out more repeats not improve an experiment?

Key term

Why must the conditions be quoted alongside a literature value?

Key term

What must a citation of an online source include that a citation of a book does not, and why?

Key term

Why should the details of a source be recorded at the moment it is used rather than at the end of a project?

Evaluation

Why is an uncertainty quoted to only one or at most two significant figures?

Evaluation

Why is the tangent drawn as long as the grid allows?

Key term

Distinguish between the accuracy and the precision of a set of measurements.

Take Planning, Analysis and Evaluation further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

Resolution of an instrument vs Accuracy of an instrument
Resolution of an instrument: the smallest change it can display. Accuracy of an instrument: how close its readings are to the truth.

Questions from this topic

Each one with the model answer underneath.

1
Distinguish between the independent variable, the dependent variable and a control variable.
Model answer
The independent variable is the one deliberately changed by the experimenter, and the dependent variable is the one measured in response to it. A control variable is any other factor that could affect the dependent variable and is therefore deliberately held constant.
2
A student investigates how the rate of reaction between magnesium and hydrochloric acid depends on acid concentration. Identify the independent, dependent and three control variables.
Model answer
The independent variable is the concentration of the acid and the dependent variable is the rate, for example the volume of hydrogen collected in a fixed time. Control variables include the temperature, the mass and surface area of the magnesium, and the total volume of acid used.
3
Why must the surface area of a solid reactant be kept the same across a set of rate experiments?
Model answer
A larger surface area exposes more particles to collision, so it increases the rate independently of the variable being studied. Using ribbon of the same width and length, or powder of the same grade, keeps this factor from confusing the results.
4
Why is temperature the control variable that most often needs a thermostatic water bath in rate work?
Model answer
Rate is extremely sensitive to temperature, since a small rise gives a large increase in the proportion of collisions with energy above the activation energy. Even a drift of a few degrees during a series of runs would mask the effect of the variable being investigated.
5
Which control variables must be kept constant between runs of an iodine clock experiment?
Model answer
The temperature, the total volume of the mixture, and the amount of thiosulfate and of starch added. The concentrations of all reagents except the one being varied must also be held constant, which is why water is added to keep the total volume the same.
6
Why is water added to some mixtures in a rate experiment so that every run has the same total volume?
Model answer
Changing the volume of one reagent changes its concentration in the mixture, but it also changes the concentrations of everything else if the total volume varies. Topping up with water keeps the total volume fixed, so only the intended concentration changes.

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Formulae and Equations

Basic Ideas About Atoms

Chemical Calculations

Bonding

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An element is zero.

uncombinedionisedoxidised

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AVO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O.

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COne electron is gained.

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Assemble a full-mark answer from its marking points, in the order an examiner wants them.

A gas syringe measures all the gas produced.

TrueFalse

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Spot the fault in a statement. The traps are the mistakes students actually make.

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Short focus games to reset attention, so the next set lands better than the last.

Planning, Analysis and Evaluation revision questions

Is Planning, Analysis and Evaluation examined at AS in WJEC Chemistry?: Yes. It sits in All units, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Planning, Analysis and Evaluation cover in WJEC AS-Level Chemistry?: 6 subtopics: planning an investigation and controlling variables, recording observations and presenting data, graphs, gradients and processing data, errors, uncertainty, accuracy and precision, evaluating a method and suggesting improvements and research and referencing. Each one has its own page with questions and model answers.
How is Planning, Analysis and Evaluation organised?: By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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