The forward reaction in the Haber process is exothermic. Explain why a lower temperature would give a higher yield of ammonia.

Lowering the temperature shifts the equilibrium in the exothermic forward direction, so more ammonia is formed.

Why is a temperature of about 450 °C used in the Haber process rather than a much lower one?

A low temperature gives a good yield but a very slow rate, so a compromise temperature is used to reach an acceptable yield quickly.

Explain why a high pressure increases the yield of ammonia in the Haber process.

There are fewer gas molecules on the product side, so raising the pressure shifts the equilibrium towards ammonia.

Why is the pressure in the Haber process not raised far above about 200 atmospheres?

Very high pressures need stronger, more expensive equipment, cost more energy to produce and are more dangerous.

Explain why adding an iron catalyst to the Haber process does not increase the yield of ammonia.

A catalyst speeds up the forward and reverse reactions equally, so it changes the rate but not the position of equilibrium.

OCR GCSE Chemistry A J248

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions Ionic equations, and state symbols The mole and the Avogadro constant Conservation of mass, and changes of mass in open systems Stoichiometry, and calculating reacting masses Exothermic and endothermic reactions, and reaction profiles Activation energy, and bond energy calculations Oxidation and reduction, by oxygen and by electrons Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids Dilute and concentrated, weak and strong The pH scale, and hydrogen ion concentration pH as a factor of ten, and measuring pH Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures Nanoparticles: size and surface area to volume Uses and risks of nanoparticulate materials

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Biological methods of metal extraction Rate against yield in industry, and reaction conditions The Haber process and fertiliser production Nitrogen, phosphorus and potassium in agriculture Life cycle assessment Recycling, and the decisions behind it Alloys, and the corrosion of metals Comparing glass, ceramics, polymers, composites and metals Functional groups and homologous series Predicting the products of organic reactions Addition polymerisation Condensation polymerisation, and making a polymer DNA and other natural polymers Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking Chemical cells and fuel cells The Earth's early atmosphere, and how oxygen developed The greenhouse effect, and evidence for climate change Effects of carbon dioxide and methane, and air pollutants Producing potable water

Monitoring and Controlling Chemical Reactions

Topic overview Concentration in mol/dm³, and titration Titration calculations, and the mass of a solute Molar gas volume and the volumes of gases Theoretical mass and percentage yield Atom economy, and choosing a reaction pathway Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Changing the conditions of an equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state Limitations of the particle model How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Transition metals Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine Tests for aqueous cations and anions Flame tests, and identifying metal ions Identifying a species from test results Instrumental methods of analysis

Global Challenges

Rate against yield in industry, and reaction conditions

2 things to know
Paper 4

Overview

The Haber and Contact equilibria, how temperature and pressure change yield and rate, the compromise conditions, what a catalyst does to yield, reading rate and yield graphs, and where the nitrogen and hydrogen come from.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Why would a lower temperature give more ammonia in the Haber process, why is 450 °C used, and why not far above 200 atmospheres?

Process

Give the balanced symbol equation for the reversible reaction between nitrogen and hydrogen in the Haber process.

Questions from this subtopic

Each one with the model answer underneath.

1
Give the balanced symbol equation for the reversible reaction between nitrogen and hydrogen in the Haber process.
Model answer
N₂ + 3H₂ ⇌ 2NH₃.
2
The forward reaction in the Haber process is exothermic. Explain why a lower temperature would give a higher yield of ammonia.
Model answer
Lowering the temperature shifts the equilibrium in the exothermic forward direction, so more ammonia is formed.
3
Why is a temperature of about 450 °C used in the Haber process rather than a much lower one?
Model answer
A low temperature gives a good yield but a very slow rate, so a compromise temperature is used to reach an acceptable yield quickly.
4
Explain why a high pressure increases the yield of ammonia in the Haber process.
Model answer
There are fewer gas molecules on the product side, so raising the pressure shifts the equilibrium towards ammonia.
5
Why is the pressure in the Haber process not raised far above about 200 atmospheres?
Model answer
Very high pressures need stronger, more expensive equipment, cost more energy to produce and are more dangerous.
6
Explain why adding an iron catalyst to the Haber process does not increase the yield of ammonia.
Model answer
A catalyst speeds up the forward and reverse reactions equally, so it changes the rate but not the position of equilibrium.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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