What happens to the position of equilibrium when more of a reactant is added?

It moves to the right, forming more product.

Use the Kc expression to explain why adding a reactant shifts the equilibrium to the right.

Raising a concentration term on the bottom would make the ratio smaller than Kc, so terms on the top must grow until the original value is restored.

What happens if a product is continually removed from an equilibrium mixture?

The equilibrium keeps shifting to the right as the system replaces what has been taken away.

Explain, using the Kc expression, why removing a product shifts the equilibrium to the right.

Taking away a term from the top drops the ratio below Kc, so more product must be made to bring it back up.

What must happen if a concentration term on the top of a Kc expression is increased?

A term on the bottom must become larger as well so that the ratio still equals Kc, which means the equilibrium position moves to the left.

Why does the value of Kc stay the same while these shifts are taking place?

The temperature has not changed, and temperature is the only thing that alters the value of the constant.

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

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Elements From the Sea

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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

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Polymers and Life

Oceans

Topic overview Solubility in aqueous and non-aqueous solvents Lattice, hydration and solution enthalpies and how they relate Measuring an enthalpy change of solution Charge density and the size of enthalpy terms Entropy: qualitative treatment Total entropy change and the condition for feasibility The limiting temperature, and why a feasible reaction may not happen Calculating the entropy change of the system The solubility product expression, its units and calculations Predicting precipitation and the common ion effect Bronsted-Lowry theory of acids and bases Strong acids and strong bases Weak acids, Ka and pKa pH of strong acids and strong bases pH of weak acids, dilution and the limits of the approximation What a buffer is and how it resists a change in pH Buffer calculations, capacity and the natural buffers How the greenhouse effect works and which gases cause it Why a molecule absorbs, concentrations in ppm, and ocean chemistry

Developing Metals

Topic overview Manganate(VII) titrations and their calculations Thiosulfate titrations, recording results and good technique Shapes of complexes and coordination number Balancing redox equations involving acid-base reactions Simple electrochemical cells Action of an electrochemical cell Standard electrode potentials and calculating E°cell Rusting and how it is prevented Transition metals: definition, oxidation states and colours d-block electron configurations of the atoms and their ions Why transition metals show variable oxidation states Ligands and complex ions Coordinate bonding and what makes a ligand Bidentate and multidentate ligands, and finding a complex's formula Reactions of Fe2+, Fe3+ and Cu2+ with hydroxide and ammonia Catalytic activity of transition metals Origin of colour in transition metal complexes Colorimetry

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

Topic overview Units, standard form and the mole in calculations Yield, atom economy and logarithmic quantities Significant figures, means and handling data Symbols, and rearranging the standard equations Substituting into rearranged equations Reading spectra and kinetics graphs Gradients, calibration lines and the Arrhenius plot Shapes and bond angles from electron pair repulsion Chiral centres, E/Z isomerism and optical activity

Elements From the Sea

Using the Kc expression to explain a shift in position

1 thing to know
Components 01, 02 and 03

Overview

Recall questions on why adding a reactant or removing a product shifts the position while Kc stays fixed, and on the reaction quotient.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Why does Kc stay the same while the position shifts, which way does an H₂ + I₂ ⇌ 2HI mixture with [H₂] = [I₂] = 0.0200 and [HI] = 0.100 mol dm⁻³ move when Kc = 49.0, and what does a ratio larger than Kc tell you?

Questions from this subtopic

Each one with the model answer underneath.

1
What happens to the position of equilibrium when more of a reactant is added?
Model answer
It moves to the right, forming more product.
2
Use the Kc expression to explain why adding a reactant shifts the equilibrium to the right.
Model answer
Raising a concentration term on the bottom would make the ratio smaller than Kc, so terms on the top must grow until the original value is restored.
3
What happens if a product is continually removed from an equilibrium mixture?
Model answer
The equilibrium keeps shifting to the right as the system replaces what has been taken away.
4
Explain, using the Kc expression, why removing a product shifts the equilibrium to the right.
Model answer
Taking away a term from the top drops the ratio below Kc, so more product must be made to bring it back up.
5
What must happen if a concentration term on the top of a Kc expression is increased?
Model answer
A term on the bottom must become larger as well so that the ratio still equals Kc, which means the equilibrium position moves to the left.
6
Why does the value of Kc stay the same while these shifts are taking place?
Model answer
The temperature has not changed, and temperature is the only thing that alters the value of the constant.

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