What is a reversible reaction?

A reversible reaction is one in which the products can react together to re-form the reactants, so the reaction can proceed in both directions under the same conditions.

What does the ⇌ sign in an equation tell you?

It tells you the reaction is reversible, so a forward and a backward reaction are both taking place in the same vessel.

Define dynamic equilibrium.

Dynamic equilibrium is the state reached in a closed system when the forward and backward reactions are proceeding at the same rate, so the concentrations of all reactants and products stay constant.

Why is equilibrium described as dynamic rather than static, when nothing appears to be changing?

Nothing appears to change because the concentrations are constant, but both reactions are still going on; reactants are still turning into products and products back into reactants, simply at equal rates.

State the two conditions that must be met before a reversible reaction can settle at a genuine equilibrium.

The system must be closed, so that no substance can enter or leave, and the temperature must be held constant.

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

Simple Equilibria and Acid-Base Reactions

6 subtopics
18 things to know
AS Unit 1

What Simple Equilibria and Acid-Base Reactions covers

Simple Equilibria and Acid-Base Reactions is examined on AS Unit 1 of WJEC Chemistry, The Language of Chemistry, Structure of Matter and Simple Reactions. 105 recall questions across 6 subtopics.

Where it is assessed

AS Unit 1

The Language of Chemistry, Structure of Matter and Simple Reactions.

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

Define a reversible reaction and dynamic equilibrium, and explain why nothing appears to change at equilibrium.

Key term

Describe how the rates of the forward and backward reactions change from the moment two reactants are mixed until equilibrium is established.

Key term

Why can a reversible reaction never reach equilibrium in an open beaker if one of the products is a gas?

Key term

Explain the colour change when concentrated hydrochloric acid is added to a pink solution containing the equilibrium [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l).

Key term

Why does adding more of a reactant increase the yield of product without changing the value of Kc?

Key term

A hydrogen ion is simply a proton. Why is it always written as H⁺(aq) in aqueous solution?

Key term

What is the difference between a base and an alkali?

Key term

What is meant by a monoprotic acid, and give an example.

Numbers

Calculate the pH of 2.00 mol dm⁻³ hydrochloric acid, and comment on the answer.

Key term

What is a strong acid, how is its dissociation written, and how does concentration differ from strength?

Numbers

0.100 mol dm⁻³ ethanoic acid has a pH of 2.88. Calculate the percentage of the acid molecules that have dissociated.

Key term

Why is a pipette used to measure the solution placed in the conical flask, rather than a measuring cylinder?

Key term

Describe how to carry out a titration of sodium hydroxide solution against hydrochloric acid of known concentration.

Key term

Why is the inside of the conical flask rinsed down with distilled water as the end point is approached?

Numbers

Write the expression for Kc for N₂(g) + 3H₂(g) ⇌ 2NH₃(g) and deduce its units.

Key term

State Le Chatelier's principle.

Key term

What effect does adding a catalyst have on the position of an equilibrium?

Key term

Write the expression for Kc for the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g).

Take Simple Equilibria and Acid-Base Reactions further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

A strong acid vs A concentrated acid
A strong acid: an acid fully dissociated in aqueous solution. A concentrated acid: an acid with a large amount of solute in a given volume.

Questions from this topic

Each one with the model answer underneath.

1
What is a reversible reaction?
Model answer
A reversible reaction is one in which the products can react together to re-form the reactants, so the reaction can proceed in both directions under the same conditions.
2
What does the ⇌ sign in an equation tell you?
Model answer
It tells you the reaction is reversible, so a forward and a backward reaction are both taking place in the same vessel.
3
Define dynamic equilibrium.
Model answer
Dynamic equilibrium is the state reached in a closed system when the forward and backward reactions are proceeding at the same rate, so the concentrations of all reactants and products stay constant.
4
Why is equilibrium described as dynamic rather than static, when nothing appears to be changing?
Model answer
Nothing appears to change because the concentrations are constant, but both reactions are still going on; reactants are still turning into products and products back into reactants, simply at equal rates.
5
State the two conditions that must be met before a reversible reaction can settle at a genuine equilibrium.
Model answer
The system must be closed, so that no substance can enter or leave, and the temperature must be held constant.
6
Describe how the rates of the forward and backward reactions change from the moment two reactants are mixed until equilibrium is established.
Model answer
At the start the reactant concentrations are highest, so the forward rate is at its maximum and the backward rate is zero. As reactants are used up the forward rate falls, and as products build up the backward rate rises. The two rates meet, and from that point onwards they remain equal and equilibrium is established.

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Formulae and Equations

Basic Ideas About Atoms

Chemical Calculations

Bonding

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An element is zero.

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AVO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O.

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Simple Equilibria and Acid-Base Reactions revision questions

Is Simple Equilibria and Acid-Base Reactions examined at AS in WJEC Chemistry?: Yes. It sits in AS Unit 1, 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 Simple Equilibria and Acid-Base Reactions cover in WJEC AS-Level Chemistry?: 6 subtopics: 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 and acid-base titrations and their calculations. Each one has its own page with questions and model answers.
How is Simple Equilibria and Acid-Base Reactions organised?: By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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