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KS4 Chemistry. Reversible Reactions. Contents. Reversible reactions. Comparing reactions. Dynamic equilibrium. Changing conditions. The Haber process. Summary activities.

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ks4 chemistry

KS4 Chemistry

Reversible Reactions

contents
Contents

Reversible reactions

Comparing reactions

Dynamic equilibrium

Changing conditions

The Haber process

Summary activities

irreversible reactions
Most chemical reactions are considered irreversible in that products are not readily changed back into reactants.

Mg + 2HCl  MgCl2 + H2

Wood reacting with oxygen

Irreversible reactions
  • When magnesium reacts with acid it is not easy to unreact it and get back the magnesium.
  • When wood burns it is pretty difficult to un-burn it back into wood again!
reversible reactions
Although most chemical reactions are difficult to reverse, it is possible to find reactions ranging from irreversible to the fully reversible.

Many biochemical reactions are reversible

Reversible reactions
  • Indeed, many of the biochemical reactions that take place in living things are reversible.
  • There are also some very important industrial reactions, like the Haber process, that are reversible.
heating copper sulfate
The change from blue hydrated copper sulphate to white anhydrous copper sulphate is one of the most commonly known reversible reactions. Heating copper sulfate

heat

hydrated copper sulphate

anhydrous copper sulphate

steam

CuSO4.5H20 CuSO4 + 5H2O

heating ammonium chloride
Ammonium salts are made by reacting ammonia with an acid but some of these salts will decompose back into reactants when heated.

Heat makes the solid disappear as it changes into gases.

Solid reappears as it changes back again in the cool part of the tube.

hydrogen chloride

ammonium chloride

ammonia

heat

NH4Cl(s) NH3(g) + HCl(g)

Heating ammonium chloride
contents1
Contents

Reversible reactions

Comparing reactions

Dynamic equilibrium

Changing conditions

The Haber process

Summary activities

dynamic equilibrium
A reversible reaction is where products can, under appropriate conditions, turn back into reactants.

There will be a range of conditions over which both the forward and backward reaction will take place, and this can lead to a state of balance with reactants and products present in unchanging amounts.

This is called a dynamic equilibrium.

B

A

Dynamic equilibrium

A

B

these decompose

these combine

dynamic equilibrium1
Dynamic equilibrium

What is special about the forward and backward reactions during dynamic equilibrium?

dynamic equilibrium2
It is rather like the situation where a man is walking the wrong way along a moving pavement or escalator.

Neither have stopped but the man could remain in the same place for ever!

The symbol is used to mean dynamic equilibrium.

Dynamic equilibrium

Equilibrium – becauseof the unchanging amounts

Dynamic – because the reaction is still occurring

The man stays in the same place!

Dynamic equilibrium can only take place in a closed system, otherwise products would be able to escape.

changing the equilibrium
In reversible reactions, equilibrium means balance but this balance does not have to be at the half-way point.

We may have mainly reactants with just a little product, or vice versa.

There are two factors that we can change that influence the position of an equilibrium:

Temperature

Concentration (or pressure in gas reactions)

Changing the equilibrium

Adding a catalyst speeds up the time it takes to reach equilibrium, but does not change the position of equilibrium.

  • Finding the conditions that gives the most product is really important in industrial chemical reactions.
contents2
Contents

Reversible reactions

Comparing reactions

Dynamic equilibrium

Changing conditions

The Haber process

Summary activities

temperature and equilibrium
All reactions are exothermic (give out heat) in one direction and endothermic (take in heat) in the other. E.g. nitrogen dioxide (NO2) joins to form dinitrogen tetroxide (N2O4) exothermically. Temperature and equilibrium

Gets cold going backward (endothermic)

2NO2 N2O4

Gets hot going forward (exothermic)

The hotter a reaction is, the more likely it is to go in the endothermic direction.

  • Heating will give more NO2 in the equilibrium mixture
  • Cooling will give more N2O4in the equilibrium mixture..
opposing changes in temperature
Opposing changes in temperature

The reaction of nitrogen and hydrogen to form ammonia (NH3) is exothermic. How will temperature affect the composition of the equilibrium mixture?

Gets cold going backward (endothermic)

3H2 + N2 2NH3

Gets hot going forward (exothermic)

Which direction is endothermic?

In which direction do reactions move whenheated?

Will heating give more or less NH3 in theequilibrium mixture?

backward

backward

less

pressure and equilibrium
This applies to gas reactions.

Here the rule depends upon the number of gas molecules on each side of the equation

Pressure and equilibrium

Get more gas molecules in backward direction

2NO2(g) N2O4 (g)

Get fewer gas molecules in forward direction

The higher the pressure, the more the reaction moves in the direction with fewer gas molecules.

  • Increasing the pressure will give more N2O4
  • Decreasing pressure gives more NO2at equilibrium.
opposing changes in pressure
Look at the reaction of nitrogen and hydrogen to form ammonia. Opposing changes in pressure

Get more gas molecules in backward direction

3H2(g) + N2 (g)  2NH3 (g)

Get fewer gas molecules in forward direction

Which direction produces less gas molecules.

In which direction do reactions move when compressed?

Will high pressure give more or less NH3 in the equilbrium mixture?

forward

The side that has fewer gas molecules

  • Increasing the pressure will give more NH3
  • Decreasing the pressure give less NH3 at equilibrium..

more

concentration and equilibrium
This applies to reactions in solution. Concentration and equilibrium

Increasing the concentration of a substance changes the equilibrium in the direction that uses up (decreases) the concentration of the substance added.

  • E.g. Bismuth chloride reacts with water to give a white precipitate of bismuth oxychloride.

BiCl3 (aq) + H2O (l)  BiOCl (s)+ 2HCl (aq)

Adding water will produce more BiOCl solid (to use up the H2O).

Adding acid (HCl) will result in less BiOCl solid to use up the HCl.

opposing changes in concentration
Chlorine gas reacts with iodine chloride (a brown liquid) converting it to iodine trichloride (a yellow solid). Opposing changes in concentration

ICl (l) + Cl2 (g)  ICl3 (s)

brown pale green yellow

What effect will adding more chlorine have on the colour of the mixture in the U-tube?

If the U-tube is turned on its side, chlorine gas pours out of the tube.

In which way will this change the equilibrium?

Produce more ICl3 and so more yellow solid.

Produce less ICl and so more brown liquid.

contents3
Contents

Reversible reactions

Comparing reactions

Dynamic equilibrium

Changing conditions

The Haber process

Summary activities

making ammonia
Is the forward reaction exothermic or endothermic?

Will heating the mixture give an equilibrium mixture with more or less ammonia?

Are there more gas molecules of reactant or product?

Will raising the pressure give an equilibrium mixture with more or less ammonia?

Making ammonia

3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol

exothermic

less

reactant

more

the haber process and temperature
What does the graph show about the effect of temperature on the Haber process?

Suggest why a temperature of 400°C is chosen when a lower temperature gives an equilibrium mixture with greater % conversion to ammonia.

The Haber process and temperature

Reduces % conversion

Hint: reaction rates?

3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol

the haber process and pressure
What does the graph show about the effect of pressure on the Haber process?

Suggest why a pressure of 200 atm is chosen when a higher pressure gives an equilibrium mixture with greater % conversion to ammonia.

The Haber process and pressure

Increases % conversion

Hint: costs?

3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol

the haber compromise
The Haber compromise

3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol

  • The aim of the chemical industry is not to make chemicals. It is to make money!
  • If we use low temperatures it takes ages to reach equilibrium. It’s better to get a 40% yield in 2 minutes than an 80% yield in 2 hours!
  • If we use very high pressures the cost of the equipment used increases drastically and there are also safety issues. Better 90% conversion at 200 atm than 95% conversion at 600 atm.
  • Unchanged reactants can always be recycled.
  • An iron catalyst is used to speed up the reaction, but it speeds up the reaction in both directions.
contents4
Contents

Reversible reactions

Comparing reactions

Dynamic equilibrium

Changing conditions

The Haber process

Summary activities

glossary
Glossary
  • closed system – A system in which reactants and products cannot be added or removed once the reaction has begun.
  • dynamic –An equilibrium in which the forward and backward reactions take place at the same rate, so no overall change takes place.
  • Haber process – The industrial-scale process for making ammonia from nitrogen and hydrogen.
  • irreversible – A reaction that is impossible or very difficult to reverse.
  • reversible – A reaction in which the product(s) can be turned back into the reactants.