Unit 3 Acids, Bases and Metals
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pH • pH is a scale of acidity. • It can be measured using: • pH paper • Universal Indicator solution • A pH meter.
pH • pH is a continuous scale of acidity which runs from below 0 to above 14. • Acids have a pH of less than 7 • Alkalis have a pH of more than 7. • Pure water, and other neutral solutions have a pH equal to 7.
Oxides • Non-metal oxides which dissolve in water, give acid solutions. • Metal oxides and hydroxides, which dissolve in water, give alkaline solutions. • Ammonia dissolves in water to produce an alkali. • Acids and alkalis are in common use at home or in the laboratory.
Ions • Acids and alkalis both contain ions. • Acids contain the H+(aq) ion • Alkalis contain the OH-(aq) ion • In water the concentration of ions is very low.
H+ and OH- ions • Water, and other neutral solutions, contain equal numbers of H+ and OH- ions. • Acidic solutions contain more H+ ions than OH- ions . • Alkalis contain more OH- ions than H+ ions.
Dilution • When an acid is diluted its acidity decreases and its pH increases. • When an alkali is diluted its alkalinity decreases and its pH decreases.
When an acid (or alkali) is diluted then the number of H+ (or OH- ) ions per cm3 of solution decrease and so the acidity (or alkalinity) decrease.
Equilibrium • There is an equilibrium in water: • H2O(l) H+(aq) + OH-(aq) • This means that the concentrations of reactants and products are always the same (but not equal).
Concentration • The concentration of a solution is measured in moles per litre (mol l-1) • A 1 mol l-1 solution means 1 mole is divided in each litre of solution.
To connect the concentration of a solution to the number of moles and concentration use the triangle opposite. c = concentration (m/l) n = number of moles v = volume (l) n c v
Strong and Weak Acids • A strong acid is one which completely dissociates in water: HCl(aq) H+(aq) + Cl-(aq) • A weak acid is one which partially dissociates in water: CH3CO2H(aq)H+(aq) + CH3CO2-(aq)
Strong Acids • Hydrochloric acid, nitric acid and sulphuric acid are strong acids. HCl(aq) H+(aq) + Cl-(aq) HNO3(aq) H+(aq) + NO3-(aq) H2SO4(aq) 2H+(aq) + SO42-(aq)
Weak Acids • Ethanoic acid, is a weak acid. CH3CO2H(aq)H+(aq) + CH3CO2-(aq)
Strong and Weak Bases • A strong base is one which completely dissociates in solution: NaOH(aq) Na+(aq) + OH-(aq) • A weak base is one which partially dissociates in solution: NH4OH(aq)NH4+(aq) + OH-(aq)
Strong Bases • Solutions of metal hydroxides are strong bases NaOH(aq) Na+(aq) + OH-(aq) KOH(aq) K+(aq) + OH-(aq)
Weak Bases • A solution of ammonia is a weak base. NH3(g) + H2O(l) NH4OH(aq) NH4OH(aq) NH4+(aq) + OH-(aq)
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Neutralisation • Neutralisation is the reaction of an acid with a base. • Metal oxides, hydroxides and carbonates are all examples of bases. • Bases which dissolve in water are called alkalis.
During a neutralisation reaction then the pH of the acid involved moves up nearer to 7. During a neutralisation reaction then the pH of the alkali involved moves down nearer to 7.
In the reaction of an acid and an alkali the hydrogen ions and hydroxide ions form water. H+ + OH- H2O HCl + NaOH NaCl + H2O
In the reaction of an acid and a metal oxide the hydrogen ions and oxide ions form water. 2H+ + O2- H2O H2SO4 + CuO CuSO4 + H2O
In the reaction of an acid and a metal carbonate the hydrogen ions and carbonate ions form carbon dioxide and water. 2H+ + CO32- CO2 + H2O 2HNO3 + CaCO3 Ca(NO3)2 + CO2 + H2O
Examples of neutralisation involve adding lime to soil or water to reduce its acidity treating acid indigestion with magnesium hydroxide the reaction of H+ (aq) to form water.
Acids and metals • Acids react with some metals to release hydrogen. The hydrogen ions in the the acid form hydrogen molecules. • The test for hydrogen is that it burns with a “pop”.
Acid Rain • Sulphur dioxide is produced by the burning of fossil fuels. • Nitrogen dioxide is produced by the sparking of air in car engines. • Both these gases dissolve in water in the atmosphere to produce acid rain.
Acid rain has damaging effects on buildings made from carbonate rock, structures made of iron and steel, soils and plant and animal life.
Acids and carbonates • An acid reacts with a metal carbonate to release carbon dioxide. Thus acid rain will dissolve rocks or buildings which contain carbonates. • The hydrogen ions from the acid react with the carbonate ions, to form carbon dioxide and water. 2H+ + CO32- H2O + CO2
Remember Moles? • To connect gram formula mass, mass in grams and number of moles use the triangle opposite • gfm = mass of 1 mole • n = number of moles • m = mass of substance m n gfm
Remember solutions? • To connect volume, concentration and the number of moles in a solution use the triangle opposite. • c = concentration (m/l) • n = number of moles • v = volume (l) n c v
Work out unknown concentrations and volumes from the results of volumetric titrations. You use the equation VH MH NH = VOH MOH NOH V = volume M = molarity NH = number of H+ ions in acid NOH =number of OH- ions in alkali H = acid OH = alkali Working out about neutralisations
Salts • A salt is the compound formed when the hydrogen ion of an acid is replaced by a metal ion (or an ammonium ion). • Salts are formed by the reactions of acids with bases or metals.
Making salts • There are three main methods of making salts. • Which method to use depends upon solubilities. • Those solubilities can be found in the Data Booklet.
Titration • Titration is experiment where alkali is measured out using a pipette. • Indicator is added. • Acid is added from a burette, until the indicator changes colour. • The water can then be evaporated to get the salt.
Neutralisation and Evaporation. • An easy way to prepare salts is to react an acid with an insoluble metal oxide or metal carbonate. • The excess can be removed from the reaction mixture by filtration. • The solution is now evaporated to separate the salt.
Precipitation • Precipitation is the reaction in which two solutions react to form an insoluble salt. • The salt can then be filtered out and dried.
How to make a salt. Is salt soluble? No Yes Is base soluble? Titration No Yes Precipitation Neutralisation and Evaporation
Ionic equations • Normally when we write equations we do so like this: HNO3 + NaOH NaNO3 + H2O
We can change to write the equations using ions. H+ + NO3- + Na+ + OH- Na+ + NO3- + H2O
If we look closely we can see that some ions appear unchanged on both sides. We call these spectator ions. H+ + NO3- + Na+ + OH- Na+ + NO3-+ H2O
We can rewrite the equation, without the spectator ions. This shows the ions that participate in the reaction. H+ +OH- H2O
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