iGCSE chemistry Section 4 lesson 1

The iGCSE Chemistry course 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

Section 4 Physical Chemistry Acids, alkalis and salts Energetics Content Section 4 Physical Chemistry Acids, alkalis and salts Energetics Rates of reaction 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. Lesson 1 a) Acids, alkalis and salts

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. ACIDIC neutral ALKALINE Very Slightly Slightly Very 1 2 3 4 5 6 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. ACIDIC neutral ALKALINE Very Slightly Slightly Very 1 2 3 4 5 6 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 ACIDIC neutral ALKALINE Very Slightly Slightly Very 1 2 3 4 5 6 7 8 9 10 11 12 13 14 5.0 8.0

pH scale and indicators Methyl Orange ACIDIC neutral ALKALINE Very Slightly Slightly Very 1 2 3 4 5 6 7 8 9 10 11 12 13 14 4.4 6.2

pH scale and indicators Phenolphthalein ACIDIC neutral ALKALINE Very Slightly Slightly Very 1 2 3 4 5 6 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 Name Formula Hydrochloric acid HCl Sulphuric acid H2SO4 Nitric acid HNO3

Common acids and alkalis Name Formula Hydrochloric acid HCl Sulphuric acid H2SO4 Nitric acid HNO3 ALKALIS Name Formula Sodium hydroxide NaOH Potassium hydroxide KOH Calcium hydroxide Ca(OH)2

Acid + Base  Salt + Water Salt formation When acids and alkalis react together, salts are formed. The general equation is: Acid + Base  Salt + Water

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 Formula Metal ion Non-metal ion Copper sulphate CuSO4 Cu2+ SO42- Sodium chloride NaCl Na+ Cl- Potassium nitrate KNO3 K+ NO3- Calcium sulphate CaSO4 Ca2+

Acid + Metal  Salt + Hydrogen Reactions of salts Acids + Metals Acid + Metal  Salt + Hydrogen

Reactions of salts Mg + 2HCl  MgCl2 + H2 Acids + Metals Acid + Metal  Salt + Hydrogen Magnesium + Hydrochloric  Magnesium + Hydrogen Acid chloride Mg + 2HCl  MgCl2 + H2

Have you got that? Are you really sure? Let’s try a few examples. 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  Iron chloride + Hydrogen Lead + Sulphuric Acid  Lead sulphate + Hydrogen

Acid + Metal oxide  Salt + Water Reactions of salts Acids + Metal oxides Acid + Metal oxide  Salt + Water

Reactions of salts CuO + H2SO4  CuSO4+ H2O 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  Iron + Sulphuric Acid  oxide Lead + Hydrochloric Acid  oxide

Reactions of salts Acids + Metal oxide Magnesium + Hydrochloric Acid  Magnesium + Water oxide chloride Iron + Sulphuric Acid  Iron sulphate + Water oxide Lead + Hydrochloric Acid  Lead chloride + Water oxide

Acid + Metal  Salt + Carbon + Water Reactions of salts Acids + Metal carbonate Acid + Metal  Salt + Carbon + Water carbonate dioxide

Reactions of salts CuCO3 + 2HCl  CuCl2 + CO2 + H2O Acids + Metal carbonate Acid + Metal  Salt + Carbon + Water carbonate dioxide Copper + Hydrochloric  Copper + Carbon + Water Carbonate acid chloride dioxide CuCO3 + 2HCl  CuCl2 + CO2 + H2O

Guess what? That’s right, no more examples! 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.

Solute (the solid) + Solvent (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!

All ammonium, potassium and sodium salts are soluble in water. 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 Most carbonates are insoluble in water (except sodium, potassium and ammonium) 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

Preparing insoluble salts using precipitation reactions Preparing soluble salts from acids 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) Acid + Alkali  Salt + Water Filtrate Steam HCl(aq) + NaOH(aq)  NaCl(aq) + Water(l) Water bath Bunsen burner

Preparing insoluble salts using precipitation reactions Eg. the preparation of lead iodide

Preparing insoluble salts using precipitation reactions Eg. the preparation of lead iodide Yellow precipitate of lead iodide Lead nitrate Potassium 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 + Potassium  Lead + Potassium nitrate iodide iodide nitrate Pb(NO3)2(aq) + 2KI(aq)  PbI(s) + 2KNO3(aq) Lead iodide

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 meniscus 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.

Acid – Alkali Titrations Stage 1 meniscus 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.

Acid – Alkali Titrations Stage 2 Two or three drops of an acid-alkali 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 insoluble salts using precipitation reactions Preparing soluble salts from acids Carrying out acid-alkali titrations

Preparing insoluble salts using precipitation reactions Preparing soluble salts from acids Carrying out acid-alkali titrations

Preparing insoluble salts using precipitation reactions Preparing soluble salts from acids 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