KS4 Reversible Reactions

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    Boardworks Ltd 2001

    Reversible Reactions and

    Equilibrium

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    Mg + 2HCl MgCl 2 + H 2

    Irreversible Reactions

    Most Chemical reactions are consideredirreversible in that products are notreadily changed back into reactants.

    When wood burns it ispretty difficult to un-burn itback into wood again!

    When magnesium reactswith acid it is not easy tounreact it and get back themagnesium

    Woodreactingwithoxygen

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    Although most chemical reactions are difficultto reverse it is possible to find reactionsranging from irreversible to the fully reversible.

    There are also some veryimportant industrialreactions, like the HaberProcess, that are reversible.

    Indeed many of thebiochemical reactions thattake place in living thingsare reversible.

    Many biochemical reactionsare reversible

    Irreversible Reactions

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    Dynamic Equilibrium A reversible reaction is where products can,under appropriate conditions, turn back intoreactants.

    There will be a range of conditions over which

    both the forward and backward reaction will takeplace and this can lead to a state of balance withboth reactants and products present inunchanging amounts. This is called a dynamic

    equilibrium.

    A B A B

    These combine These decompose

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    Equilibrium because of the unchanging amounts .

    Dynamic because reaction is still occurring. It is rather like the situation where a man is walking the

    wrong way along a moving pavement or escalator. Neitherhave stopped but the man could remain in the same placefor ever!

    The symbol is used to mean dynamic equilibrium

    Moving pavement

    Walking man Overall the

    man stays inthe same place!

    Dynamic Equilibrium

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

    We may have mostly reactants with just a littleproduct or vice versa.

    There are 2 factors that we can change thatinfluence the position of an equilibrium: Temperature

    Concentration (or pressure in gas reactions)

    Finding the conditions that gives the most productis really important in industrial chemical reactions.

    Dynamic Equilibrium

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    Gets hot going forward (exothermic)

    Gets cold going backward (endothermic)

    3H 2 + N 2 2NH 3

    Which direction is endothermic.

    Which direction do reactions move when heated?

    Will heating give more or less NH3 in the equilbriummixture?

    The reaction of nitrogen and hydrogen to form ammonia(NH 3) is exothermic. How will temperature affect thecomposition of the equilibrium mixture?

    backward

    backward

    less

    Activity

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    Pressure This applies to gas reactions.

    Here the rule depends upon the number of gasmolecules on each side of the equation

    Get less gas molecules in forward direction

    Get more gas molecules in backward direction

    2NO 2(g) N2O 4 (g)

    The higher the pressure the more the reaction

    moves in the direction with less gas molecules.

    Increasing the pressure will give more N2O 4

    Decreasing pressure gives more NO 2 at equilibrium. .

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    Look at the reaction of nitrogen and hydrogen toform ammonia

    Get less gas molecules in forward direction

    Get more gas molecules in backward direction

    3H 2(g) + N 2 (g) 2NH 3 (g)

    Increasing the pressure will give more NH 3

    Decreasing the pressure give less NH 3 at equilibrium. .

    Which direction produces less gas molecules.

    Which direction do reactions move when compressed?

    Will high pressure give more or less NH3 in the equilbriummixture?

    forward

    The side that has less gas molecules

    more

    Activity

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    Concentration This applies to reactions in solution.

    BiCl 3(aq) + H 2O (l) BiOCl(s) + 2HCl(aq)

    Eg. Bismuth chloride reacts with water to give a whiteprecipitate of bismuth oxychloride.

    Increasing the concentration of a substance tips theequilibrium in the direction that uses up (decreases) theconcentration of the substance added.

    Adding water will produce more BiOCl solid (to useup the H 2O).

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

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    Chlorine gas reacts with iodine chloride ( abrown liquid) converting it to iodine trichloride (a

    yellow solid). ICl(l) + Cl 2(g) ICl 3(s)

    Brown pale green yellow

    What effect will adding more chlorine have

    upon the colour of the mixture in the U-tube?

    If the U-tube is turned on its side heavychlorine gas pours out of the tube. Whichway will this tip the equilibrium?

    Produce more ICl3 and somore yellow solid

    Produce less ICl and somore brown liquid

    Activity

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    1. Is the forward reaction exothermic orendothermic?

    2. Will heating the mixture give an equilibriummixture with more or less ammonia?

    3. Are there more gas molecules of reactant orproduct?

    4. Will raising the pressure give an equilibriummixture with more or less ammonia?

    3H 2(g) + N 2 (g) 2NH 3 (g) H=-92kJ/mol

    exothermic

    less

    reactant

    more

    Activity

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    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    100

    0 100 200 300 400 500 600 700

    pressure (atm)

    % c

    o n v e r s

    i o n

    at a temperature of 400C

    1. What does the graph show about the effect of pressureon the Haber process?

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

    3H 2(g) + N 2 (g) 2NH 3 (g) H=-92kJ/mol

    Hint: costs?

    Increases %conversion

    ActivityThe Haber Compromise

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    The Haber Compromise

    1. The aim of the chemical industry is not to makechemicals. It is to make money!

    2. If we use low temperatures it takes ages to reachequilibrium. Its better to get a 40% yield in 2minutes than an 80% yield in 2 hours!

    3. If we use very high pressures the cost of theequipment used increases drastically and there

    are also safety issues. Better 90% conversion at200atm than 95% conversion at 600 atm.4. Unchanged reactants can always be recycled.

    3H 2(g) + N 2 (g) 2NH 3 (g) H=-92kJ/mol

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    Which of these is true about a dynamicequilibrium?

    A. All the product molecules are used upB. All the reactants molecules are used up.C. The reaction has stopped both in the

    forward and backward directions

    D. The composition of the reaction mixtureremains the same.

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    Which of these is a reversible process?

    A. Reacting acid with alkali

    B. Heating hydrated (blue) copper sulphate.C. Burning coalD. Dissolving magnesium in acid

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    Which of these is true about the effect ofpressure on the reaction below?

    A.Increased pressure gives more N 2O 4

    B.Increased pressure does not affect theequilibrium.

    C.Increased pressure makes N 2O 4 decompose

    D.Increased pressure slows down the reaction

    2NO 2(g) N2O 4 (g)

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    Which of these is true about the effect ofi nc reased temperature on the reaction?

    A.gives more N2O

    4

    B.does not affect the equilibrium.C.slows down the reactions.D.Achieves equilibrium more quickly

    2NO 2(g) N2O 4 (g) H=-58kJ/mol

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    Which of these is NOT true about the HaberProcess?

    A. An iron catalyst is usedB. Heat is used to increase the ammonia

    present in the equilibrium mixture.C. High pressure is used to increase the

    ammonia present in the equilibrium mixtureD. Unreacted starting materials are recycled.

    3H 2(g) + N 2 (g) 2NH 3 (g) H=-92kJ/mol