What are reversible reactions?

Doc Brown's Chemistry KS4 science

GCSE/IGCSE/O level Revision Notes

PART A REVERSIBLE REACTIONS

All my GCSE/IGCSE/US grade 8-10 Chemistry Revision notes

Sub-index for this page

(a) Explaining what a reversible reaction is

(b) The thermal decomposition of ammonium chloride

(c) The thermal decomposition of hydrated copper(II) sulphate

(d) The hydrolysis reaction of bismuth chloride with water

(e) The formation and decomposition of ammonia

(f) The formation and hydrolysis of an ester

(g) The thermal decomposition of limestone (calcium carbonate)

Keywords: What is a reversible reaction? This page introduces you to the idea of a reversible reaction. Examples of the reversible reactions described include the thermal decomposition of ammonium chloride, hydrated copper sulfate, reaction of bismuth chloride with water, formation of ammonia and the thermal decomposition of limestone. These revision notes on reversible reactions, should prove useful for the new AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) & (5-1) science courses.

Hopefully you will soon understand what we mean by a reversible reaction and how changing conditions can change the direction of reversible reaction AND changing directions changes whether energy is absorbed or given out.

So, now we can move to take a look at reversible reactions in the context of what we call chemical equilibrium and look again at some examples of reversible chemical reactions..

• A reversible reaction is a chemical change in which the products can be converted back to the original reactants under suitable conditions.
  • In a reversible reaction, changing the reaction conditions e.g. concentration, pressure or temperature will change the net direction the reaction goes i.e. more to the right (forward) or more to left (backward).
  • It also means a reversible reaction does not go to completion in either direction and all components, original reactants or ensuing products,
  • and ALL co-exist in the reaction mixture (see notes on chemical equilibrium).
• This means the reaction can go in either direction i.e.
  • A + B ==> C + D    or written as    C + D ==> A + B
• A reversible reaction is shown by the sign ,
  • a half-arrow to the right (direction of forward reaction),
  • and a half-arrow to the left (direction of backward reaction).
  • It is really important you understand that the terms right & left AND forward & backward are used in the context of how the equation is presented.
• Most reactions are not reversible (irreversible) and have the usual complete arrow only pointing to the right.

Five examples 1a to 1e of reversible reactions are described below:

Part A contd. (b) The thermal decomposition of ammonium chloride

• On heating strongly above 340^oC, the white solid ammonium chloride, thermally decomposes into a mixture of two colourless gases ammonia and hydrogen chloride.
• On cooling the reaction is reversed and solid ammonium chloride reforms.
  • This is an example of sublimation but here it involves both physical and chemical changes.
  • When a substance sublimes it changes directly from a solid into a gas without melting and on cooling reforms the solid without condensing to form a liquid.
  • But, more importantly in this section, it is an example of a reversible reaction.

  • Ammonium chloride + heat ammonia + hydrogen chloride

  • NH₄Cl_(s) NH_3(g) + HCl_(g)

  • Heat moves the reaction to the right (referred to as the forward reaction) and cooling moves the reaction to the left (referred to as the backward reaction).

  • So, the thermal decomposition of ammonium chloride into ammonia and hydrogen chloride is the forward reaction, and the formation of ammonium chloride from ammonia and hydrogen chloride is the backward reaction.

  • The terms forward (L to R) and backward (R to L) must be used in the context of the direction reversible reaction equation is written.

  • This is a simple Pyrex test tube experiment for the school laboratory, the solid ammonium chloride changes to a colourless gas (endothermic) but the crystals reform higher up on the cooler surface of the test tube (exothermic).

  • By putting damp neutral litmus paper at the top of the tube and half-way down you might see if it change blue from the alkaline ammonia and red from the acidic hydrogen chloride.

  • The molecules diffuse at different rates so you might some separation to allow the observation of both litmus changes - ammonia (NH₃ M_r = 17) diffuses faster than hydrogen chloride (HCl M_r = 36.5) and should get to the top of the tube first.

    • It is a rule that the smaller the molecular mass of a gas molecules the greater the average speed of the molecules, so the faster the molecules spread out - diffuse.

    • For the experiment to show this see gases - diffusion notes.

• Note:

  • Reversing the reaction conditions reverses the direction of chemical change, typical of a reversible reaction.
  • Thermal decomposition means using 'heat' to 'break down' a molecule into smaller ones.
  • The decomposition of ammonium chloride into ammonia and hydrogen chloride  is endothermic (heat absorbed or heat taken in from the surroundings - the hot test tube).
  • The formation of ammonium chloride from ammonia and hydrogen chloride is exothermic (heat released or heat given out to the surroundings - the cooler parts of the test tube).
  • This means if the direction of chemical change is reversed, the energy change is also reversed.
  • Ammonium fluoride (decomposes >180^oC), ammonium bromide (decomposes >450^oC) and ammonium iodide (decomposes >550^oC), with a similar formula, all sublime in a similar physical-chemical way when heated, so the equations will be similar i.e. just swap F, Br or I for the Cl, in other words ...
    • (NH₄X_(s) NH_3(g) + HX_(g)    (where X represent F, Br or I)
    • Similarly, ammonium sulphate also sublimes when heated above 235^oC and thermally decomposes into ammonia gas and sulphuric acid vapour.
      • (NH₄)₂SO_4(s) NH_3(g) + H₂SO_4(g)
  • For more on sublimation, see the States of Matter webpage.

Part A contd. (c) The thermal decomposition of hydrated copper(II) sulphate

• On heating the blue solid, hydrated copper(II) sulphate, steam is given off and the white solid of anhydrous copper(II) sulphate is formed.
• When the white solid is cooled and water added, blue hydrated copper(II) sulphate is reformed i.e. a reversible reaction.

  • blue hydrated copper(II) sulphate + heat white anhydrous copper(II) sulphate + water

  • CuSO₄.5H₂O_(s) CuSO_4(s) + 5H₂O_(g)

  • The dehydration decomposition to give the white solid is the forward endothermic reaction and the 're-hydration' to reform the blue crystals is the backward exothermic reaction.

  • This is also a simple Pyrex test tube experiment for the school laboratory, the blue solid changes to a white solid (endothermic) and water vapour condenses out higher up on the cooler surface of the test tube (exothermic). After allowing to cool (to avoid cracking the test tube!) adding a few drops of water restores the blue colour of the original crystals.

  • A nice simple class experiment.

• Note:
  • The 5H₂O in the formula of hydrated copper(II) sulphate is called the water of crystallisation and forms part of the crystal structure when copper(II) sulphate solution is evaporated and crystals form.
  • This crystal structure is broken down on heating and the water is given off.
  • The thermal decomposition is endothermic as heat is absorbed to drive off the water.
  • The reverse reaction is exothermic i.e. on adding water to white anhydrous copper(II) sulphate the mixture heats up as the blue crystals reform.
  • The reverse reaction is used as a simple chemical test for water i.e. white anhydrous copper(II) sulphate turns blue.

Part A contd. (d) The hydrolysis reaction of bismuth chloride with water

• When bismuth chloride (BiCl₃) is added to water, it dissolves and then reacts with water to form a white precipitate of bismuth oxychloride (BiOCl) and a colourless solution of hydrochloric acid (HCl).

  • A hydrolysis reaction is when a substance chemically reacts with water to give two or more products.

  • forward reaction

  • bismuth chloride + water ==> bismuth oxychloride + hydrochloric acid

  • BiCl_3(aq) + H₂O_(l) ==> BiOCl_(s) + 2HCl_(aq)

  • A reaction in which a molecule reacts with water to give at least two products is called a hydrolysis reaction.

• If hydrochloric acid is added to the mixture, the bismuth oxychloride dissolves to reform the bismuth chloride solution.

  • backward reaction

  • bismuth oxychloride + hydrochloric acid ==> bismuth chloride + water

  • BiOCl_(s) + 2HCl_(aq) ==> BiCl_3(aq) + H₂O_(l)

• Therefore the reaction is reversible and what is formed depends on the relative amounts of hydrochloric acid and water, and so the reaction equation should be written as:

  • BiCl_3(aq) + H₂O_(l) BiOCl_(s) + 2HCl_(aq)

• Bismuth chloride is a poisonous chemical - teacher demonstration only at GCSE level.

Part A contd. (e) The formation and decomposition of ammonia

• The synthesis of ammonia from nitrogen and hydrogen is a reversible reaction and by changing the temperature and pressure of the reacting gases you can make the reaction go one way more than another,

• e.g. high pressure makes more ammonia (forward reaction) and higher temperature causes ammonia to decompose into hydrogen and nitrogen (backward reaction).

  • nitrogen + hydrogen ammonia

  • N_2(g) + 3H_2(g) 2NH_3(g)

  • The forward reaction forms the basis of ammonia manufacture by the Haber Process (full details in section C)

  • If the pressure is reduced, or the temperature raised, the reaction tends to reverse to give a greater proportion of nitrogen and hydrogen.

  • If the pressure is increased, or the temperature lowered, the reaction tends to give a greater proportion of ammonia.

  • The rules governing these reversible reaction situations are discussed and explained with examples in Part B.

  • This is the very important Haber synthesis reaction for the industrial production of ammonia for fertilisers.

Part A contd. (f) The formation and hydrolysis of an ester

• Ethyl ethanoate, an ester,  is formed by the reaction of ethanoic acid with ethanol and is another example of a reversible reaction. e.g.
• ethanoic acid + ethanol ethyl ethanoate + water
• + + H₂O
• If the ester is warmed with water or any dilute acid (faster), it changes back into the original acid and alcohol.
  • This reverse reaction is called hydrolysis (backward reaction, as written in the symbol equation) ...
    • ethyl ethanoate + water ==> ethanoic acid + ethanol
  • whereas esterification (forward reaction, as written in the symbol equation) is
    • ethanoic acid + ethanol ==> ethyl ethanoate + water
  • The term hydrolysis is a reaction in which a molecule reacts with water to give at least two products.
  • Esterification simply means an ester forming reaction.
  • An ester is particular kind of organic molecules formed from an organic acid and alcohol.
  • For more details of making the ester see Esters, chemistry and uses including perfumes

• If limestone is strongly heated above 900oC it decomposes into calcium oxide (lime) and carbon dioxide

  • calcium carbonate ==> calcium oxide + carbon dioxide

    • CaCO₃ ==> CaO + CO₂

• It the lime (calcium oxide) is cooled, it recombines with any carbon dioxide present.

  • calcium oxide + carbon dioxide ==> calcium carbonate

    • CaO + CO₂ ==> CaCO₃

• Therefore this reversible reaction can be expressed as ...

  • CaCO_3(s) CaO_(s) + CO_2(g)

• So, again, by changing the reaction conditions, in this case changing the temperature, the reaction can be made to either way, i.e. a class reversible reaction.

• This is an important industrial process for making lime for the building industry and agriculture.

Index: A Reversible Reactions (this page)  *  B Reversible reactions and Equilibrium

 C The Haber Synthesis of ammonia  *  D(a) The Uses of ammonia-nitric acid-fertilisers

 D(b) Fertilisers-environmental problems  *  E The nitrogen cycle 

QUIZ Combined QUIZ on rates of reaction, reversible reactions and equilibrium - Le Chatelier's Rules

Foundation tier (easier) multiple choice QUIZ on ammonia, nitric acid and fertilisers etc.

Higher tier (harder) multiple choice QUIZ on ammonia, nitric acid and fertilisers etc.

Advanced A Level Notes - Equilibrium (use indexes)

Advanced A Level Chemistry Notes p-block nitrogen & ammonia

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