IGCSE CHEMISTRY SECTION 4 LESSON 1 Content The iGCSE Chemistry course Section 1 Principles of Chemistry Section 2 Chemistry of the Elements Section 3 Organic Chemistry Section 4 Physical Chemistry Section 5 Chemistry in Society Content Section 4 Physical Chemistry a) Acids, alkalis and salts b) Energetics c) Rates of reaction d) Equilibria Lesson 1 a) Acids, alkalis and salts 4.1 describe the use of the indicators litmus, phenolphthalein and methyl orange to distinguish between acidic and alkaline solutions 4.2 understand how the pH scale, from 0–14, can be used to classify solutions as strongly acidic, weakly acidic, neutral, weakly alkaline or strongly alkaline 4.3 describe the use of universal indicator to measure the approximate pH value of a solution 4.4 define acids as sources of hydrogen ions, H+, and alkalis as sources of hydroxide ions, OH¯ 4.5 predict the products of reactions between dilute hydrochloric, nitric and sulfuric acids; and metals, metal oxides and metal carbonates (excluding the reactions between nitric acid and metals) 4.6 understand the general rules for predicting the solubility of salts in water: i all common sodium, potassium and ammonium salts are soluble ii all nitrates are soluble iii common chlorides are soluble, except silver chloride iv common sulfates are soluble, except those of barium and calcium v common carbonates are insoluble, except those of sodium, potassium and ammonium 4.7 describe experiments to prepare soluble salts from acids 4.8 describe experiments to prepare insoluble salts using precipitation reactions 4.9 describe experiments to carry out acid-alkali titrations. pH scale and indicators The pH scale is a measure of how acidic or alkaline a solution is. pH scale and indicators The pH scale is a measure of how acidic or alkaline a solution is. neutral ACIDIC Very Slightly 1 2 3 4 5 6 ALKALINE Slightly Very 7 8 9 10 11 12 13 14 pH scale and indicators The pH scale is a measure of how acidic or alkaline a solution is. neutral ACIDIC Very Slightly 1 2 3 4 5 6 ALKALINE Slightly Very 7 8 9 10 11 12 13 14 A substance forms an aqueous solution when it dissolves in water. Water itself is neutral. When substances dissolve in water, they dissociate into individual ions. pH scale and indicators Water, H2O H+(aq) OH-(aq) hydrogen ion hydroxide ion acid alkali pH scale and indicators Indicators are special dyes that change colour according to whether they are in acidic, alkaline or neutral solutions. pH scale and indicators Indicators are special dyes that change colour according to whether they are in acidic, alkaline or neutral solutions. Three common indicators are: Litmus Phenolphthalein Methyl Orange pH scale and indicators Litmus 3 4 5 5.0 6 ALKALINE neutral Very 1 2 ACIDIC Slightly Slightly Very 7 8 9 10 11 12 13 14 8.0 pH scale and indicators Methyl Orange 3 4 5 6 4.4 6.2 neutral Very 1 2 ACIDIC Slightly ALKALINE Slightly Very 7 8 9 10 11 12 13 14 pH scale and indicators Phenolphthalein 3 4 5 6 ALKALINE neutral Very 1 2 ACIDIC Slightly Slightly Very 7 8 9 10 11 12 13 14 8.3 10 pH scale and indicators Universal indicator is a mixture of dyes, and shows a complete colour range across the pH scale. Common acids and alkalis ACIDS Name Hydrochloric acid Sulphuric acid Nitric acid Formula HCl H2SO4 HNO3 Common acids and alkalis ACIDS Name Hydrochloric acid Sulphuric acid Nitric acid Formula HCl H2SO4 HNO3 ALKALIS Name Sodium hydroxide Formula NaOH Potassium hydroxide Calcium hydroxide KOH Ca(OH)2 Salt formation When acids and alkalis react together, salts are formed. The general equation is: Acid + Base Salt + Water Salt formation When acids and alkalis react together, salts are formed. The general equation is: Acid + Base Salt + Water This is known as a neutralisation reaction because the products are neutral. Salt formation Salts consist of two parts – a metal part, and the non-metal ion from the acid. Acid Ions in solution Salts formed Hydrochloric acid H+ Cl- chlorides Sulphuric acid 2H+ SO42- sulphates Nitric acid H+ NO3- nitrates Salt formation Examples of salts Salt Copper sulphate Sodium chloride Potassium nitrate Calcium sulphate Formula Metal ion Non-metal ion CuSO4 Cu2+ SO42- NaCl Na+ Cl- KNO3 K+ NO3- CaSO4 Ca2+ SO42- Reactions of salts Acids + Metals Acid + Metal Salt + Hydrogen Reactions of salts Acids + Metals Acid + Metal Salt + Hydrogen Magnesium + Hydrochloric Magnesium + Hydrogen Acid chloride Mg + 2HCl MgCl2 + H2 Reactions of salts Acids + Metals Have you got that? Are you really sure? Let’s try a few examples. Reactions of salts Acids + Metals Magnesium + Sulphuric Acid Iron + Hydrochloric Acid Lead + Sulphuric Acid Reactions of salts Acids + Metals Magnesium + Sulphuric Acid Magnesium sulphate + Hydrogen Iron + Hydrochloric Acid Lead + Iron chloride + Hydrogen Sulphuric Acid Lead sulphate + Hydrogen Reactions of salts Acids + Metal oxides Acid + Metal oxide Salt + Water Reactions of salts Acids + Metal oxides Acid + Metal oxide Salt + Water Copper + Sulphuric Copper + Water oxide Acid sulphate CuO + H2SO4 CuSO4+ H2O Reactions of salts Acids + Metal oxides Have you got that? Are you really, really sure? Let’s try a few more examples. Reactions of salts Acids + Metal oxide Magnesium + Hydrochloric Acid oxide Iron + Sulphuric Acid oxide Lead + oxide Hydrochloric Acid Reactions of salts Acids + Metal oxide Magnesium + Hydrochloric Acid Magnesium + Water oxide chloride Iron + Sulphuric Acid oxide Lead + oxide Iron sulphate + Water Hydrochloric Acid Lead chloride + Water Reactions of salts Acids + Metal carbonate Acid + Metal Salt + Carbon + Water carbonate dioxide Reactions of salts Acids + Metal carbonate Acid + Metal Salt + Carbon + Water carbonate dioxide Copper + Hydrochloric Copper + Carbon + Water Carbonate acid chloride dioxide CuCO3 + 2HCl CuCl2 + CO2 + H2O Reactions of salts Acids + Metal carbonate Guess what? That’s right, no more examples! Solubility of salts If a substance is soluble, then this means that it will dissolve in a solvent. Solubility of salts If a substance is soluble, then this means that it will dissolve in a solvent. The most common solvent you will come across is WATER. Solubility of salts If a substance is soluble, then this means that it will dissolve in a solvent. The most common solvent you will come across is WATER. Solute (the solid) + Solvent (water) Solution (aqueous) Solubility of salts There’s no easy way around this – you’ve just got to learn the relative solubility of salts! Solubility of salts All ammonium, potassium and sodium salts are soluble in water. Solubility of salts All ammonium, potassium and sodium salts are soluble in water. All nitrates are soluble in water Solubility of salts All ammonium, potassium and sodium salts are soluble in water. Most chlorides are soluble in water (except lead and silver). PbCl2 is soluble in hot water. All nitrates are soluble in water Solubility of salts All ammonium, potassium and sodium salts are soluble in water. Most sulphates are soluble in water (except barium, calcium and lead) Most chlorides are soluble in water (except lead and silver). PbCl2 is soluble in hot water. All nitrates are soluble in water Solubility of salts All ammonium, potassium and sodium salts are soluble in water. Most carbonates are insoluble in water (except sodium, potassium and ammonium) Most sulphates are soluble in water (except barium, calcium and lead) Most chlorides are soluble in water (except lead and silver). PbCl2 is soluble in hot water. All nitrates are soluble in water Preparing soluble salts from acids Preparing insoluble salts using precipitation reactions Carrying out acid-alkali titrations Preparing soluble salts from acids Eg. the preparation of sodium chloride by neutralization Preparing soluble salts from acids Eg. the preparation of sodium chloride by neutralization Dilute sodium hydroxide + indicator solution Dilute hydrochloric acid Preparing soluble salts from acids Eg. the preparation of sodium chloride by neutralization Filtrate Decolourising charcoal Evaporating basin Preparing soluble salts from acids Eg. the preparation of sodium chloride by neutralization Crystals of sodium chloride forming) Filtrate Acid + Alkali Salt + Water Steam Water bath Bunsen burner HCl(aq) + NaOH(aq) NaCl(aq) + Water(l) Preparing insoluble salts using precipitation reactions Eg. the preparation of lead iodide Preparing insoluble salts using precipitation reactions Eg. the preparation of lead iodide Lead nitrate Potassium iodide Yellow precipitate of lead iodide Preparing insoluble salts using precipitation reactions Eg. the preparation of lead iodide Filter to separate the precipitate. Wash with distilled water and dry to get the pure product. Lead iodide Preparing insoluble salts using precipitation reactions Eg. the preparation of lead iodide Filter to separate the precipitate. Wash with distilled water and dry to get the pure product. Salt + Salt Insoluble + Soluble salt salt Lead iodide Lead + Potassium Lead + Potassium nitrate iodide iodide nitrate Pb(NO3)2(aq) + 2KI(aq) PbI(s) + 2KNO3(aq) Acid – Alkali Titrations A titration is a very accurate way of adding an acid to an alkali to get a salt. Acid – Alkali Titrations To carry out an acid-alkali titration we need the right bits of kit. Conical flask Acid – Alkali Titrations To carry out an acid-alkali titration we need the right bits of kit. Pipette Acid – Alkali Titrations To carry out an acid-alkali titration we need the right bits of kit. Burette Acid – Alkali Titrations Stage 1 Dilute sodium hydroxide (NaOH) solution is sucked up into a pipette using a pipette filler. The pipette contains exactly 25.0cm3 of solution when the bottom of the meniscus is level with the pipette mark. The sodium hydroxide solution is then released in to the conical flask. meniscus Acid – Alkali Titrations Stage 1 Dilute sodium hydroxide (NaOH) solution is sucked up into a pipette using a pipette filler. The pipette contains exactly 25.0cm3 of solution when the bottom of the meniscus is level with the pipette mark. The sodium hydroxide solution is then released in to the conical flask. meniscus Acid – Alkali Titrations Stage 2 Two or three drops of an acidalkali indicator such as litmus solution are added to the sodium hydroxide solution using a teat pipette. The alkali turns the litmus blue. Acid – Alkali Titrations Stage 3 The burette is filled with dilute hydrochloric acid to the zero reading. (Again, look for the meniscus) Acid – Alkali Titrations Stage 4 The conical flask is placed on a white tile beneath the burette. Acid is added from the burette until the colour of the solution turns from blue to red. The point at which the colour changes is called the end-point. At this stage the conical flask only contains salt and water. Acid – Alkali Titrations Stage 4 The conical flask is placed on a white tile beneath the burette. Acid is added from the burette until the colour of the solution turns from blue to red. The point at which the colour changes is called the end-point. At this stage the conical flask only contains salt and water. At the end-point the volume of acid added can be measured by reading off the volume used in the burette – this is the titre Preparing soluble salts from acids Preparing insoluble salts using precipitation reactions Carrying out acid-alkali titrations Preparing soluble salts from acids Preparing insoluble salts using precipitation reactions Carrying out acid-alkali titrations Preparing soluble salts from acids Preparing insoluble salts using precipitation reactions Carrying out acid-alkali titrations End of Section 4 Lesson 1 In this lesson we have covered: pH scale and indicators Common acids and alkalis Salt formation and reactions Preparing salts
