Define homogeneous catalysis.

Homogeneous catalysis is catalysis in which the catalyst is in the same physical state, or phase, as the reactants.

Why is the destruction of ozone by chlorine radicals described as homogeneous catalysis?

The chlorine radicals and the ozone are all gases, so catalyst and reactants are in the same phase.

How does a homogeneous catalyst work, in general terms?

The catalyst first reacts with one of the reactants to form an intermediate, and that intermediate then reacts on to give the products and regenerate the catalyst.

How does the enthalpy profile of a homogeneously catalysed reaction differ from the uncatalysed one?

It has two humps with a dip between them where the intermediate sits, and each hump is lower than the single peak of the uncatalysed route.

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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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

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Polymers and Life

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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

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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

The Ozone Story

Homogeneous catalysis and intermediates

2 things to know
Components 01, 02 and 03

Overview

Recall questions on what makes catalysis homogeneous, what an intermediate is, and the two steps of the chlorine radical cycle.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Why is ozone destruction by chlorine radicals homogeneous catalysis, what is an intermediate, and why must every step of a catalysed route have a lower activation enthalpy than the uncatalysed step?

Numbers

What is the step that regenerates the chlorine radical, which species is the catalyst and which the intermediate, and what is the overall equation?

Questions from this subtopic

Each one with the model answer underneath.

1
Define homogeneous catalysis.
Model answer
Homogeneous catalysis is catalysis in which the catalyst is in the same physical state, or phase, as the reactants.
2
Why is the destruction of ozone by chlorine radicals described as homogeneous catalysis?
Model answer
The chlorine radicals and the ozone are all gases, so catalyst and reactants are in the same phase.
3
How does a homogeneous catalyst work, in general terms?
Model answer
The catalyst first reacts with one of the reactants to form an intermediate, and that intermediate then reacts on to give the products and regenerate the catalyst.
4
What is an intermediate?
Model answer
An intermediate is a species that is made in one step of a reaction and used up in a later step, so it does not appear in the overall equation.
5
How does the enthalpy profile of a homogeneously catalysed reaction differ from the uncatalysed one?
Model answer
It has two humps with a dip between them where the intermediate sits, and each hump is lower than the single peak of the uncatalysed route.
6
Why must every step of the catalysed route have a lower activation enthalpy than the uncatalysed step?
Model answer
The slowest step controls the overall rate, so if any one step still had a barrier as high as before there would be no gain in speed.

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