Acids, bases and salts

solubility rules

soluble

• all ammonium salts and salts of group 1 metals

• all nitrates

• all sulfates (except BaSO₄, CaSO₄, PbSO₄, AgSO₄ - sparingly soluble)

• all chlorides (except AgCl, PbCl₂)

insoluble

• all carbonates (except (NH₄)₂CO and carbonates of group 1 metals)

• all oxides (except oxides of group 1 metals and CaO - slightly soluble)

• all hydroxides (except hydroxides of group 1 metals and Ca(OH)₂ - slightly soluble)

pH

• pH of a solution is determined by the relative concentration of H⁺ and OH^- ions

• pH scale → 1 to 14

  • 0 to 2 - strongly acidic

  • 3 to 6 - weakly acidic

  • 7 - neutral

  • 8 to 10 - weakly alkaline

  • 11 to 14 - strongly alkaline

• neutral solution (pH = 7): concentration of H⁺ = concentration of OH^-

• acidic solution (pH < 7): concentration of H⁺ > concentration of OH^-

  • the lower the pH value, the more acidic the solution, the higher concentration of H⁺ ions

• alkaline solution (pH > 7): concentration of H⁺ < concentration of OH^-\

  • the higher the pH value, the more alkaline the solution, the higher the concentration of OH^- ions

indicators

• an indicator is an organic compound that changes colour based on the pH of a solution

• universal indicator: provides a wide range of colours corresponding to different pH values

  • red - strongly acidic

  • orange/yellow - weakly acidic

  • green - neutral

  • blue - weakly alkaline

  • violet - stringly alkaline

pH meter

• pH of a solution can be measured more accurately using a pH meter, which consists of a pH electrode connected to a meter

• the pH electrode is dipped into the solution, and the pH value is displayed on the meter

• can be used for data logging, allowing continuous recording of rapid pH changes

pH and agriculture

• pH of soils affects plant growth and determines which plants can thrive in it

• many plants do not grow well in soils that are too acidic or too alkaline

• soil tends to become acidic due to acid rain and overuse of chemical fertilisers (e.g. ammonium sulfate)

• to neutralise acidic soil, slaked lime (calcium hydroxide) or quicklime (calcium oxide) is added, a process known as “liming the soil”, to maintain optimal pH for plant growth

acids

• an acid is a substance which ionises in water or aqueous solutions to give hydrogen ions

strength

• strength of an acid depends on the extent of ionisation of the acid molecules in water/aqueous solution

strong acids

• a strong acid is an acid that completely ionises in water to give hydrogen ions

• e.g. hydrochloric acid, sulfuric acid, nitric acid

• all acid molecules form ions

weak acid

• a weak acid is an acid that partially ionises in water to give hydrogen ions

• e.g. ethanoic acid, carbonic acid

• most acids remain unchanged, few ions are formed

concentration

• concentration of a solution is a measure of how much the solute has dissolved in 1dm³ of the solution

• unit: mol/dm³

basicity

• basicity of an acid is the maximum number of hydrogen ions produced by one molecule of the acid when the acid molecule ionises in water

• e.g.

  • monobasic acids: hydrochloric acid (HCl), nitric acid (HNO₃), ethanoic acid (CH₃COOH)

  • dibasic acids: sulfuric acid (H₂SO₄), carbonic acid (H₂CO₃)

  • tribasic acids: phosphoric acid (H₃PO₄)

physical properties

• sour taste

• turns blue litmus paper red

• has a ph less than 7

• can conduct electricity when dissolved in water

  • in the presence of water, acid molecules are able to ionise to form ions. these ions can move freely in water to conduct electricity

reactions

• acid + reactive metal → salt + hydrogen

  • metals that are not reactive: copper, silver, gold, platinum

  • lead is a reactive metal but appears not to react with HCl(aq) or HSO(aq). this is because the initial reaction of lead with these dilute acids form a layer of PbCl₂ or PbSO, which coats the metal and prevents further reaction between the metal and the acid

• acid + carbonate → salt + water + carbon dioxide

  • carbon dioxide gas reacts with limewater (aqueous calcium hydroxide) to give calcium carbonate (CaCO₃), an insoluble salt which appears as a white precipitate

• acid + base → salt + water

  • neutralisation

bases

• a base is a substance that reacts with an acid to form salt and water only

• usually metal oxides or metal hydroxides (except aqueous ammonia)

alkali

• an alkali is a substance that ionises in water to produce hydroxide ions

• aqueous solutions of sodium hydroxide, potassium hydroxide and calcium hydroxide are strong alkalis as these soluble bases completely ionise in water to give hydroxide ions

  • calcium hydroxide is only slightly soluble in water, but the dissolved portion fully ionises into Ca²⁺ and OH^- ions. thus, aqueous calcium hydroxide is still a strong alkali

• aqueous ammonia is a weak alkali as ammonia partially ionises in water to give hydroxide ions

  • when ammonia gas dissolves in water, only a small fraction of ammonia molecules react with water to form ammonium ions and hydroxide ions, while most ammonia remain as unreacted molecules in solutions

physical properties

• has a bitter taste

• feels slippery and soapy

• turn red litmus paper blue

• has a ph of greater than 7

• can conduct electricity when dissolved in water

reactions

• acid + base → salt + water

• base + ammonium salt → salt + water + ammonia

  • ammonia is very soluble in water, hence no effervescence is observed and warming is required to decrease the solubility of ammonia, allowing the gas to be released

precipitation reactions - important in the identification of metal ions

• precipitation is a chemical reaction that produces an insoluble solid, known as a precipitate, when two aqueous solutions are mixed

• metal salt solution + alkali → insoluble metal hydroxide (precipitate) + salt

oxides

• oxides are formed when elements combine with oxygen

• 4 types: basic, amphoteric, neutral, acidic

acidic oxides

• most oxides of non-metals are acidic oxides

• usually dissolve and react with water to produce acids

• react with alkalis to produce salts and water (neutralisation)

  • acidic oxide + alkali → salt + water

neutral oxides

• show neither basic or acidic properties

• does not dissolve in water

• does not react with acids or alkalis

basic oxides

• oxides from most metals are basic oxides

• insoluble in water soluble basic oxides (e.g. group 1 metal oxides and slightly soluble calcium oxide) dissolce and react with water to produce alkalis

• react with acids to produce salt and water (neutralisation)

  • basic oxide + acid → salt + water

amphoteric oxides

• show both basic and acidic properties (react with both acids and alkalis to form salts and water)

• e.g. ZAP - ZnO, Al₂O₃, PbO

• insoluble in water

• behave like basic oxides - react with acids

  • amphoteric oxide + acid → salt + water

• behave like acidic oxides - react with alkali

  • amphoteric oxide + axid → salt + water

salts

• a salt is a compound formed when the hydrogen ions of an acid are partially or completely replaced by a metal ion or an ammonium ion

salt preparation

• depends on

  • solubility of salt in water

  • solubility of reactants in water

precipitation

• method

  • mix the 2 aqueous solutions together

  • filter the mixture to obtain the insoluble salt as residue

  • wash the residue with distilled water

  • dry the residue between pieces of filter paper

reaction of acid with excess insoluble salt

• involves the reaction of an acid with excess moderately reactive metal, insoluble base or insoluble carbonate

• method

  • add excess metal (only moderate metals such as Mg, Zn, Fe) / insoluble base / insoluble carbonate to the acid

    • acid can be heated to speed up the reaction

    • excess solid reactant is to ensure that all the acid reacts so that the salt obtained will not be contaminated with the acid

  • filter the mixture to obtain the salt solution as the filtrate

  • heat the salt solution until it is saturated

    • the filtrate is not heated to dryness to obtain the crystals upon heating as hydrated salt may lose its water of crystallisation and become anhydrous (some salts may also decompose)

  • allow the saturated solution to cool for crystals to form (crystals will form as solubility of salt decreases as temperature decreases)

  • filter the mixture to obtain the crystals as the residue

  • dry the crystals between pieces of filter paper

titration

• involves reaction of acid with alkali or carbonate solution

• both reactants are soluble - if either reactant is in excess, it will contaminate to salt produced

  • hence the amount of each reactant used must be exact

• method

  • using a pipette, place a fixed volume (25.0cm³) of the alkali / carbonate solution into a conical flask

  • add a few drops of a suitable indicator to the solution in the flask

  • add acid from a burette to the flask until the indicator just changes colour

  • record the volume of acid used

  • repeat the experiment without using the indicator, adding the recorded volume of acid to the conical flask to obtain a solution of the salt

  • heat the solution to dryness (for NaCl) / until it becomes saturated

  • allow the saturated solution to cool for crystals to form

  • filter the mixture to obtain the crystals as the residue

  • dry the crystals between pieces of filter paper