Acids, bases and salt preparations

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What are the general rules for predicting the solubility of ionic compounds in water?

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1

What are the general rules for predicting the solubility of ionic compounds in water?

Salt

Solubility

Exceptions

sodium (Na+), potassium (K+) ammonium (NH4+)

soluble

none

nitrates (NO3-)

soluble

none

chlorides (Cl-)

soluble

silver chloride (AgCl)

lead (II) chloride (PbCl2)

sulfates (SO42-)

soluble

barium sulfate (BaSO4) calcium sulfate (CaSO4)

lead (II) sulfate (PbSO4)

carbonates (CO32-)

insoluble

sodium carbonate (Na2CO3)

potassium carbonate (K2CO3) ammonium carbonate ((NH4)2CO3)

hydroxides (OH-)

insoluble

sodium hydroxide (NaOH)

potassium hydroxide (KOH)

calcium hydroxide (Ca(OH)2) (calcium hydroxide is slightly soluble)

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2

What are the roles of acids and bases in terms of proton transfer?

Proton transfer
  • The earlier definition of an acid and a base can be extended

  • In terms of proton transfer, we can further define each substance in how they interact with protons

Acids
  • Acids are proton donors as they ionize in solution producing protons, H+ ions

  • These H+ ions make the aqueous solution acidic

Bases (Alkalis)
  • Bases (alkalis) are proton acceptors as they ionize in solution producing OH- ions which can accept protons

  • These OH- ions make the aqueous solution alkaline

 

Transfer of protons in reaction between acid & base, IGCSE & GCSE Chemistry revision notesDiagram showing the role of acids and bases in the transfer of protons

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3

What are the reactions of hydrochloric acid, sulfuric acid and nitric acid with metals, bases and metal carbonate?

Acid reactions summary

         alkali      +      acid      →      water      +      salt

         base      +      acid      →      water      +      salt

         carbonate      +      acid      →      water      +      salt      +   carbon dioxide

         metal   +   acid   →   salt   +   hydrogen

Acid + Alkali   and   Acid + Base

A base is a substance that can neutralise an acid, forming a salt and water only.

Alkalis are soluble bases. When they react with acids, a salt and water is formed. The salt formed is often as a colourless solution. Alkalis are a source of hydroxide ions (OH⁻) when in solution.

         alkali      +      acid      →      water      +      salt

         base      +      acid      →      water      +      salt

Examples of acid + alkali reactions:

  •          sodium hydroxide   +   hydrochloric acid   →   sodium chloride   +   water

  •          NaOH (aq)         +         HCl (aq)         →         NaCl (aq)         +         H₂O (l)

  •          potassium hydroxide   +   sulfuric acid   →   potassium sulfate   +   water

  •          2KOH (aq)         +         H₂SO₄ (aq)         →         K₂SO₄ (aq)         +         2H₂O (l)

Example of an acid + base reaction:

         CuO (s)         +         H₂SO₄ (aq)         →         CuSO₄ (aq)         +         H₂O (l)

 

Acid + Carbonate

         carbonate      +      acid      →      water      +      salt      +      carbon dioxide

A carbonate is a compound made up of metal ions and carbonate ions. Examples of metal carbonates are sodium carbonate, copper carbonate and magnesium carbonate.

When carbonates react with acids, bubbling is observed which is the carbon dioxide being produced. If the acid is in excess the carbonate will disappear.

Examples of acid + carbonate reactions:

  •          calcium carbonate   +   hydrochloric acid   →   calcium chloride   +   water   +   carbon dioxide

  •          CaCO₃ (s)         +         2HCl (aq)         →         CaCl₂ (aq)         +         H₂O (l)         +         CO₂ (g)

  •          potassium carbonate   +   hydrochloric acid   →   potassium chloride   +   water   +   carbon dioxide

  •          K₂CO₃ (aq)         +         2HCl (aq)         →         2KCl (aq)         +         H₂O (l)         +         CO₂ (g)

 

Acid + Metal

         metal   +   acid   →   salt   +   hydrogen

Metals will react with an acid if the metal is above hydrogen in the reactivity series.

When metals react with acids, bubbling is observed which is the hydrogen being produced. If the acid is in excess the metal will disappear.

Examples of acid + metal reactions:

  •          magnesium   +   sulfuric acid   →   magnesium sulfate   +   hydrogen

  •          Mg (s)         +         H₂SO₄ (aq)         →         MgSO₄ (aq)         +         H₂ (g)

  •          aluminium   +   hydrochloric acid   →   aluminium chloride   +   hydrogen

  •          2Al (s)         +         6HCl (aq)         →         2AlCl₃ (aq)         +         3H₂ (g)

  •          copper   +   hydrochloric acid   →   no reaction (since copper is below hydrogen in the reactivity series)

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4

What are bases and alkalis?

A base is a substance that neutralises an acid by combining with the hydrogen ions in them to produce water.

A base usually means a metal oxide, a metal hydroxide or ammonia.

Alkalis are bases which are soluble in water.

 

Some metal oxides are soluble in water and react with it to form solutions of metal hydroxides. For example:

Na₂O (s)         +         H₂O (l)         →         2NaOH (aq)

Apart from this and other group 1 oxides (such as potassium oxide) most other metal oxides are not soluble in water.

One exception is calcium oxide which does dissolve slightly in water to form calcium hydroxide (known as limewater):

CaO (s)         +         H₂O (l)         →         Ca(OH)₂ (aq)

 

Ammonia is another base. Ammonia reacts with water to form ammonium ions and hydroxide ions:

NH₃ (aq)         +         H₂O (l)         ⇋         NH₄⁺ (aq)         +         OH⁻ (aq)

 

All the solutions produced here contain hydroxide ions (OH⁻) so they are all alkalis.

What makes a base act like a base?
  • Bases are substances which can neutralise an acid, forming a salt and water

  • The term base and alkali are not the same

  • A base which is water-soluble is referred to as an alkali

    • So, all alkalis are bases, but not all bases are alkalis

  • Alkalis have pH values of above 7

  • In basic (alkaline) conditions red litmus paper turns blue

  • Bases are usually oxides, hydroxides or carbonates of metals

  • The presence of the OH- ions is what makes the aqueous solution an alkali

  • One unusual base is ammonia solution

    • When ammonia reacts with water it produces hydroxide ions

Some Common Alkalis and the Ions They Contain

Equilibria Table 1_Common Alkalis, downloadable AS & A Level Chemistry revision notes

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5

How do you prepare a pure, dry sample of a soluble salt, starting from an insoluble reactant?

Step

Explanation

Heat acid (H2SO4) in a beaker

Speeds up the rate of reaction

Add base (CuO) until in excess (no more copper oxide dissolves) and stir with glass rod

Neutralises all the acid

Filter the mixture using filter paper and funnel

Removes any excess copper oxide

Gently heat the filtered solution (CuSO4)

To evaporate some of the water

until crystals form on a glass rod

Shows a hot saturated solution formed

Allow the solution to cool so that hydrated crystals form

Copper sulfate less soluble in cold solution

Remove the crystals by filtration

Removes crystals

Dry by leaving in a warm place

Evaporates the water

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6

How do you prepare a pure, dry sample of a soluble salt, starting from an acid and alkali?

Aim:

To prepare a sample of a dry salt starting from an acid and an alkali

Diagram:

Using-titration-to-prepare-a-salt1, IGCSE & GCSE Chemistry revision notesDiagram showing the apparatus needed to prepare a salt by titration

 Method:

  • Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange)

  • Add the acid into the burette and note the starting volume

  • Add the acid very slowly from the burette to the conical flask until the indicator changes to appropriate colour

  • Note and record the final volume of acid in burette and calculate the volume of acid added (starting volume of acid - final volume of acid)

  • Add this same volume of acid into the same volume of alkali without the indicator

  • Heat to partially evaporate, leaving a saturated solution

  • Leave to crystallise decant excess solution and allow crystals to dry

Results:

A dry sample of a salt is obtained

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7

How do you prepare a pure, dry sample of an insoluble salt, starting from two soluble reactants?

  • Insoluble salts can be prepared using a precipitation reaction

  • The solid salt obtained is the precipitate, thus in order to successfully use this method the solid salt being formed must be insoluble in water

  • The preparation of a soluble salt follows this pattern:

soluble salt 1 + soluble salt 2 ⟶  insoluble salt + soluble salt 3

AB + CD ⟶ AD + CB

  • The method involves measuring out a fixed volume of one solution and then adding the second salt solution until it is in a slight excess

    • This ensures the maximum amount of precipitate will be obtained

  • The precipitate is recovered by filtration and then it must be washed with distilled water remove reactants that are contaminating the residue (recovered solid)

    • It is then left to dry

  • This method is a good way to prepare silver and lead(II) salts which are often insoluble; the starting material will usually be the nitrate of silver or lead(II) since all nitrates are soluble

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8

How do you prepare a sample of pure, dry hydrated copper(II) sulfate crystals starting from copper(II) oxide?

Aim:

To prepare a pure, dry sample of hydrated copper(II) sulfate crystals

Materials:

  • 1.0 mol / dm3 dilute sulfuric acid

  • Copper(II) oxide

  • Spatula & glass rod

  • Measuring cylinder & 100 cm3 beaker

  • Bunsen burner

  • Tripod, gauze & heatproof mat

  • Filter funnel & paper, conical flask

  • Evaporating basin and dish.

IGCSE & GCSE Chemistry revision notes

The preparation of copper(II) sulfate by the insoluble base method

Practical Tip:

The base is added in excess to use up all of the acid, which would become dangerously concentrated during the evaporation and crystallisation stages

Method:

  1. Add 50 cm3 dilute acid into a beaker and warm gently using a Bunsen burner

  2. Add the copper(II) oxide slowly to the hot dilute acid and stir until the base is in excess (i.e. until the base stops dissolving and a suspension of the base forms in the acid)

  3. Filter the mixture into an evaporating basin to remove the excess base

  4. Gently heat the solution in a water bath or with an electric heater to evaporate the water and to make the solution saturated

  5. Check the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end

  6. Leave the filtrate in a warm place to dry and crystallise

  7. Decant excess solution and allow the crystals to dry

Results:

Hydrated copper(II) sulfate crystals should be bright blue and regularly shaped

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9

How do you prepare a sample of pure, dry lead(II) sulfate?

Aim:

To prepare a dry sample of lead(II) sulfate

Diagram:

Preparation of lead(II)sulfate 1, downloadable IGCSE & GCSE Chemistry revision notesPreparation of lead(II)sulfate 2, downloadable IGCSE & GCSE Chemistry revision notesPreparation of lead(II)sulfate 3, downloadable IGCSE & GCSE Chemistry revision notesPreparation of lead(II)sulfate 4, downloadable IGCSE & GCSE Chemistry revision notes

The preparation of lead(II)sulfate by precipitation from two soluble salts

Method:

  • Measure out 25 cm3 of 0.5 mol dm3 lead(II)nitrate solution and add it to a small beaker

  • Measure out 25 cm3 of 0.5 mol dm3 of potassium sulfate add it to the beaker and mix together using a stirring rod

  • Filter to remove precipitate from mixture

  • Wash filtrate with distilled water to remove traces of other solutions

  • Leave in an oven to dry

 Soluble salt 1 = lead(II) nitrate        Soluble salt 2 = potassium sulfateEquation for the reaction:

Pb(NO3)2  (aq) + K2SO4 (aq) → PbSO4 (s) + 2KNO3 (aq)

lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate

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