Define a homogeneous equilibrium.

One in which all the species in the equilibrium mixture are in the same physical state.

Write the general expression for Kc for the equilibrium aA + bB ⇌ cC + dD.

Kc = [C]ᶜ[D]ᵈ ÷ ([A]ᵃ[B]ᵇ), with each concentration measured at equilibrium.

Why must the units of Kc be worked out for each equilibrium?

Because they depend on the stoichiometry, which decides how many concentration terms appear above and below the line.

Deduce the units of Kc for N₂O₄(g) ⇌ 2NO₂(g).

Kc = [NO₂]² ÷ [N₂O₄], giving (mol dm⁻³)² ÷ mol dm⁻³, which is mol dm⁻³.

Deduce the units of Kc for H₂(g) + I₂(g) ⇌ 2HI(g).

There are two concentration terms above and two below, so the units cancel and Kc has no units.

Deduce the units of Kc for 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).

Kc = [SO₃]² ÷ ([SO₂]²[O₂]), so the units are mol⁻¹ dm³.

OCR A-Level Chemistry B H433

Development of Practical Skills in Chemistry

Topic overview Planning experiments Implementing: apparatus, units and recording data Significant figures and titration calculations Graphs, gradients and drawing a conclusion Anomalous results, precision, accuracy and types of error Calculating uncertainties and improving a method Hazards, risk and control measures Recording, processing and judging data Measuring pH, mass and volume, and choosing an indicator Reflux, distillation, purification and the standard tests PAG1 to PAG6: moles, enthalpy, qualitative analysis and organic synthesis PAG7 to PAG12: electrochemistry, rates and pH

Elements of Life

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Developing Fuels

Topic overview Molar gas volume and the ideal gas equation Gas volume calculations and measuring a gas in the laboratory Bonding and shape in organic molecules Enthalpy changes: definitions and standard conditions Bond enthalpies q = mcΔT and measuring an enthalpy change in solution Burning a fuel, correcting for heat loss, and the assumptions made Hess' law and enthalpy cycles Catalysts, heterogeneous catalysis and catalyst poisons Cracking and how it is carried out The pollutants from burning fuels and their effects Catalytic converters and cutting emissions Aliphatic and aromatic compounds, functional groups and general formulae Deducing formulae and naming a branched compound Combustion of organic compounds Addition of bromine and hydrogen halides, and the test for unsaturation Hydrogenation and hydration of alkenes Addition polymerisation Electrophilic addition mechanism Displayed, shortened and skeletal formulae Chain, position and functional group isomerism Why alkenes show stereoisomerism, and the E/Z and cis/trans rules Naming and comparing the stereoisomers Sustainable fuels: benefits and risks

Elements From the Sea

Topic overview Atom economy Extraction of the halogens from sea minerals Electrolysis of molten salts and writing electrode half-equations Electrolysis of aqueous solutions and the electrolysis of brine Redox as electron transfer The rules for assigning oxidation states Oxidation state calculations, and defining oxidation and reduction Balancing a redox equation with oxidation states The iodine-thiosulfate titration and its calculations Systematic nomenclature of inorganic compounds Physical properties of the halogens Halogen reactivity and the displacement equations Observing a displacement, and using an organic solvent or starch Silver halide precipitation and tests Preparation of the hydrogen halides Properties of the hydrogen halides Chlorine: risks, benefits and uses Dynamic equilibrium Writing a Kc expression and calculating Kc What the magnitude of Kc tells you, and what changes it Using the Kc expression to explain a shift in position Temperature, pressure and catalysts on an equilibrium

The Ozone Story

Topic overview Electronegativity and bond polarity Intermolecular bonds Boiling points and intermolecular bonds Activation enthalpy and enthalpy profiles Collision theory and the Boltzmann distribution Following a reaction and finding the rate from a graph Catalysts and alternative reaction routes Homogeneous catalysis and intermediates Composition of gas mixtures by volume Recognising and naming haloalkanes Primary, secondary and tertiary haloalkanes, and the amines Properties and nucleophilic substitution of haloalkanes Substitution, nucleophiles and the SN2 mechanism Relative reactivity of the haloalkanes Homolytic and heterolytic fission, radicals and half curly arrows Initiation, propagation and termination in a radical chain reaction Ozone depletion by haloalkanes Ozone in the stratosphere and troposphere Radiation of the Earth and the Sun Effects of UV and visible radiation on molecules Calculations with frequency, wavelength and energy

What's in a Medicine?

Topic overview Formulae of oxygen-containing homologous series Primary, secondary and tertiary alcohols The acidity of phenol compared with alcohols and carboxylic acids The iron(III) chloride test and esterifying a phenol Esterification and oxidising primary, secondary and tertiary alcohols Dehydration, halogenation and planning a two-step synthesis Preparing aspirin, filtering under reduced pressure and recrystallisation Thin layer chromatography, Rf values and percentage yield Reflux and distillation in an organic preparation Washing, drying and redistilling a liquid product The aims of green chemistry and atom economy Catalysts, renewable feedstocks and measuring greenness Elimination reactions The molecular ion and fragment ions Identifying a compound from its fragments and the M+1 peak How infrared absorption works and the characteristic absorptions Reading a spectrum, the fingerprint region and combining techniques

The Chemical Industry

Topic overview Rate equations, order and the rate constant Deducing the units of k and rate calculations Half-lives Continuous monitoring, quenching and the initial rates method Reading order from concentration-time and rate-concentration graphs The Arrhenius equation and what each symbol means The ln k against 1/T plot and finding the activation enthalpy Rate-determining step and mechanism Effect of conditions on the magnitude of the equilibrium constant Economic operating conditions for an industrial process Writing a Kc expression and deducing its units ICE-table calculations and measuring Kc experimentally Chemical reactions in industrial processes Nitrogen, ammonia and the ammonium ion Oxidation states of nitrogen and the tests for nitrate and ammonium Costs, benefits and risks of industrial processes

Polymers and Life

Oceans

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Developing Metals

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Colour by Design

Topic overview How dyes attach to fibres Structure of a dye molecule Fats and oils Arenes and delocalisation Representations of benzene and enthalpy of hydrogenation Tests for the common functional groups Identifying a polyfunctional molecule from a set of test results Halogenation and nitration of benzene and the electrophiles involved The nitration mechanism, sulfonation and the Friedel-Crafts reactions Diazotisation and why it is done cold Coupling reactions, azo dyes and the route from benzene Oxidising an aldehyde and the three carbonyl tests Cyanohydrin formation and its use in synthesis Using the Data Sheet reactions to plan a route Choosing between routes on yield and atom economy Nucleophilic addition mechanism Classifying organic reactions and unfamiliar mechanisms Origins of colour in organic molecules How gas-liquid chromatography separates a mixture Retention time, peak area and the limits of the technique

Chemical Literacy

Topic overview Extracting and using data and information from a scientific text Written communication and comprehension

Mathematical Skills in Chemistry

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The Chemical Industry

Writing a Kc expression and deducing its units

3 things to know
Components 01, 02 and 03

Overview

Recall questions on homogeneous equilibria, writing the expression from a balanced equation, and deducing the units for each case.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What is a homogeneous equilibrium, what is the general Kc expression, and what are the units for the dinitrogen tetroxide equilibrium?

Numbers

What are the units of Kc for the hydrogen iodide, sulfur trioxide and ammonia equilibria?

Numbers

When does Kc have no units, how is the change in amount fixed by the stoichiometry, and why is the flask volume not needed to find Kc for ethanoic acid + ethanol ⇌ ethyl ethanoate + water?

Questions from this subtopic

Each one with the model answer underneath.

1
Define a homogeneous equilibrium.
Model answer
One in which all the species in the equilibrium mixture are in the same physical state.
2
Write the general expression for Kc for the equilibrium aA + bB ⇌ cC + dD.
Model answer
Kc = [C]ᶜ[D]ᵈ ÷ ([A]ᵃ[B]ᵇ), with each concentration measured at equilibrium.
3
Why must the units of Kc be worked out for each equilibrium?
Model answer
Because they depend on the stoichiometry, which decides how many concentration terms appear above and below the line.
4
Deduce the units of Kc for N₂O₄(g) ⇌ 2NO₂(g).
Model answer
Kc = [NO₂]² ÷ [N₂O₄], giving (mol dm⁻³)² ÷ mol dm⁻³, which is mol dm⁻³.
5
Deduce the units of Kc for H₂(g) + I₂(g) ⇌ 2HI(g).
Model answer
There are two concentration terms above and two below, so the units cancel and Kc has no units.
6
Deduce the units of Kc for 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).
Model answer
Kc = [SO₃]² ÷ ([SO₂]²[O₂]), so the units are mol⁻¹ dm³.

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