An Equilibrium Using Copper(II) And Ammonia | Experiment

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An equilibrium using copper(II) and ammonia

In association with Nuffield Foundation

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Try this class practical to explore an equilibrium involving copper(II) ions

In this experiment, students add ammonia to a solution of copper(II) sulfate, observe the colour changes taking place, and then reverse the reaction by the addition of sulfuric acid.

The experiment is best carried out by students working individually. It takes about 15–20 minutes.

Plan a lesson on this topic

Try this practical as part of a complete lesson plan on transition metal complexes and ligand exchange from our Assessment for Learning collection.

Equipment

Apparatus

• Eye protection
• Test tubes, x3
• Test tube rack
• Test tube holder
• Dropping pipettes, x2

Chemicals

• Copper(II) sulfate solution, 1.0 M (HARMFUL), about 3 cm3 (see note 3 below)
• Ammonia solution, 1.0 M, about 10 cm3
• Dilute sulfuric acid, 1.0 M (IRRITANT), about 10 cm3

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Copper(II) sulfate solution, CuSO4(aq), (HARMFUL) – see CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031. The copper(II) sulfate solution is most conveniently supplied in a bottle fitted with teat pipette.
• Ammonia solution, NH3(aq) – see CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006.
• Dilute sulfuric acid, H2SO4(aq) (IRRITANT) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.

Procedure

1. While wearing eye protection, put 10 drops of copper(II) sulfate solution into each of two test tubes.
2. Add ammonia solution drop-by-drop to the first test tube. Shake the tube gently from side to side after adding each drop. What happens as you add a few drops of the solution?
3. Add more drops of ammonia solution. What happens? Continue until you have a clear blue solution.
4. Divide the solution from step 3 into two test tubes. Add dilute sulfuric acid drop-by-drop to one of the solutions from step 3. Shake the tube gently from side to side after adding each drop. Do you get back to where you started – compare the three test tubes?
5. Can you repeat the whole process by adding ammonia again to the acidified solution?

Teaching notes

If this experiment is being carried out with pre-A-level students, the reactions occurring can simply be explained by reference to the addition of an alkali (containing hydroxide ions) being added to a solution of a copper compound, producing copper(II) hydroxide initially and later a complex compound of ammonia. The reversal of the process is easy to explain since sulfuric acid is capable of neutralising the alkaline ammonia and causing the reaction to reverse back to the start:

CuSO4(aq) (pale blue solution) + 2NH3(aq) + 2H2O(l) → Cu(OH)2(s) + (NH4)2SO4(aq) (pale blue precipitate)

Cu(OH)2(s) (pale blue precipitate) + ammonia → complex copper compound (dark blue solution)

A rather more advanced treatment in terms of complexes and ligand exchange would involve the following explanation:

1. Ammonia is a weak base and forms a few ammonium and hydroxide ions in solution:NH3(g) + H2O(l) ⇌ NH4+(aq) + OH–(aq)
2. The hexa-aqua-copper(II) ions react with hydroxide ions to form a precipitate. This involves deprotonation of two of the water ligand molecules:[Cu(H2O)6]2+(aq)(pale blue) + 2OH–(aq) → [Cu(H2O)4(OH)2](s)(pale blue precipitate) + 2H2O(l)
3. The copper(II) hydroxide precipitate reacts with ammonia molecules to form tetra-amine-di-aqua-copper(II) ions This involves ligand exchange:[Cu(H2O)4(OH)2](s)(pale blue precipitate) + 4NH3(aq) ⇌ [Cu(NH3)4(H2O)2)]2+(aq)(dark blue solution) + 2OH–(aq) + 2H2O(l)
4. Thus the overall reaction, combining 2 with 3, gives:[Cu(H2O)6]2+(aq) + 4NH3(aq) ⇌ [Cu(NH3)4(H2O)2)]2+(aq) + 4H2O(l)
5. Addition of dilute sulfuric acid introduces H+ ions, which react with NH3 molecules to form NH4+ ions, and this draws the equilibrium in 4 back to the left-hand side, regenerating the hexa-aqua-copper(II) ions in the process.

Additional information

This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.

© Nuffield Foundation and the Royal Society of Chemistry

Health and safety checked, 2016

• Four out of five

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Level

• 16-18 years

Use

• Practical experiments

Category

• Reactions and synthesis
• Equilibrium

Specification

• Scotland
  • Higher
    • SQA Chemistry
      • 3. Chemistry in society
        • (d) Equilibria
          • For a given reversible reaction, the effect of altering temperature or pressure or of adding/removing reactants/products can be predicted.
• Wales
  • A/AS level
    • WJEC Chemistry
      • Unit 3: PHYSICAL AND INORGANIC CHEMISTRY
        • 3.4 Chemistry of the d-block transition metals
          • (e) idea of ligand exchange and how this can lead to a change in coordination number as exemplified by the reactions of [Cu(H₂O)₆]²⁺ and [Co(H₂O)₆]²⁺ with concentrated HCl
          • (f) colours and formulae of the approximately octahedral complex ions [Cu(H₂O)₆]²⁺, [Cu(NH₃)₄(H₂O)₂]²⁺ and [Co(H₂O)₆]²⁺ and the approximately tetrahedral ions [CuCl₄]²⁻ and [CoCl₄]²⁻
• Northern Ireland
  • A/AS level
    • CCEA Chemistry
      • Unit A2 3: Further Practical Chemistry
        • demonstrate the relative strengths of ligands using hydrated copper(II) ions and hydrochloric acid; and
      • Unit A2 2: Analytical, Transition Metals, Electrochemistry and Organic Nirtrogen Chemistry
        • 5.5 Transition metals
          • 5.5.9 demonstrate understanding of the ligand replacement reactions of hexaaquacopper(II) ions with concentrated hydrochloric acid and ammonia solution, including colours and shapes of the complexes;
• Republic of Ireland
  • Junior Cycle
    • Science
      • Chemical world
        • Building blocks
          • 2. Develop and use models to describe the nature of matter; demonstrate how they provide a simple way to to account for the conservation of mass, changes of state, physical change, chemical change, mixtures, and their separation.
• England
  • GCSE
    • OCR Combined science A: Gateway
      • C5 Monitoring and controlling chemical reactions
        • C5.2 Eqilibria
          • C5.2a recall that some reactions may be reversed by altering the reaction conditions
    • OCR Chemistry A: Gateway
      • C5 Monitoring and controlling chemical reactions
        • C5.3 Eqilibria
          • C5.3a recall that some reactions may be reversed by altering the reaction conditions
    • AQA Chemistry
      • 4.6 The rate and extent of chemical change
        • 4.6.2 Reversible reactions and dynamic equilibruim
          • 4.6.2.1 Reversible reactions
            • In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented: A + B ⇌ C + D.
    • AQA Combined science: Synergy
      • 4.7 Movement and interactions
        • 4.7.4 The rate and extent of chemical change
          • 4.7.4.8 Reversible reactions
            • Recall that some reactions may be reversed by altering the reaction conditions.
    • AQA Combined science: Trilogy
      • 5.6 The rate and extent of chemical change
        • 5.6.2 Reversible reactions and dynamic equilibruim
          • 5.6.2.1 Reversible reactions
            • In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented: A + B ⇌ C + D.
    • Edexcel Chemistry
      • Topic 4 - Extracting metals and equilibria
        • Reversible reactions and equilibria
          • 4.13 Recall that chemical reactions are reversible, the use of the symbol ⇌ in equations and that the direction of some reversible reactions can be altered by changing the reaction conditions
    • Edexcel Combined science
      • Topic 4 - Extracting metals and equilibria
        • Reversible reactions and equilibria
          • 4.13 Recall that chemical reactions are reversible, the use of the symbol ⇌ in equations and that the direction of some reversible reactions can be altered by changing the reaction conditions
    • OCR Chemistry B: 21st century
      • C6 Making useful chemicals
        • C6.3 What factors affect the yield of chemical reactions?
          • C6.3.1 recall that some reactions may be reversed by altering the reaction conditions including: reversible reactions are shown by the symbol ; reversible reactions (in closed systems) do not reach 100% yield
    • OCR Combined science B: 21st Century
      • C6 Making useful chemicals
        • C6.3 What factors affect the yield of chemical reactions?
          • C6.3.1 recall that some reactions may be reversed by altering the reaction conditions including: reversible reactions are shown by the symbol ⇌; reversible reactions (in closed systems) do not reach 100% yield
  • A/AS level
    • AQA Chemistry
      • Physical Chemistry
        • Chemical equilibria, Le Chatelier's principle and Kc
          • Chemical equilibria and Le Chatelier's principle
            • Many chemical reactions are reversible.
      • Inorganic chemistry
        • Transition metals
          • Substitution reactions
            • H₂O, NH₃ and Cl⁻ can act as monodentate ligands.
            • The ligands NH₃ and H₂O are similar in size and are uncharged.
            • Exchange of the ligands NH₃ and H₂O occurs without change of co-ordination number (eg Co²⁺ and Cu²⁺).
          • General properties of transition metals
            • The characteristic properties include: complex formation.
            • Formation of coloured ions.
            • Variable oxidation state.
    • Edexcel Chemistry
      • Topic 10: Equilibrium I
        • 1. know that many reactions are readily reversible and that they can reach a state of dynamic equilibrium in which: the rate of the forward reaction is equal to the rate of the backward reaction; the concentrations of reactants and products remain…
      • Topic 15: Transition Metals
        • Topic 15A: Principles of transition metal chemistry
          • 5. understand that dative (coordinate) bonding is involved in the formation of complex ions
          • 6. know that a complex ion is a central metal ion surrounded by ligands
          • 7. know that transition metals form coloured ions in solution
        • 15B: Reactions of transition metal elements
          • 24. be able to record observations and write suitable equations for the reactions of Cr³⁺(aq), Fe²⁺(aq), Fe³⁺(aq), Co²⁺(aq) and Cu²⁺(aq) with aqueous sodium hydroxide and aqueous ammonia, including in excess
    • OCR Chemistry A
      • Module 2: Foundations in chemistry
        • 2.1 Atoms and reactions
          • 2.1.5 Redox
      • Module 5: Physical chemistry and transition elements
        • 5.3 Transition elements
          • 5.3.1 Transition elements
            • cii) illustration, using at least two transition elements, of: ii) the formation of coloured ions
            • j) reactions, including ionic equations, and the accompanying colour changes of aqueous Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺ and Cr# with aqueous sodium hydroxide and aqueous ammonia, including: precipitation reactions; complex formation with excess aqueous sodium hyd…

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