chemistry - chemical changes: acids (3.1 - 3.21)

3.1 acids in solution

produce hydrogen ions (H⁺)

3.1 alkalis in solution

produce hydroxide ions (OH^-)

3.2 neutral solution pH

pH 7

3.2 acidic solutions pH

lower than pH 7

3.2 alkaline solutions pH

higher than pH 7

3.3 indicators in acidic & alkaline solutions: litmus, methyl orange, phenolphthalein

3.4 concentration of hydrogen ions effect on pH

higher conc. H⁺ ions in acidic solution = lower pH

3.4 concentration of hydroxide ions effect on pH

higher conc. OH^- ions in alkaline solution = higher pH

3.5 factors of hydrogen ion concentration effect on pH

H⁺ conc. in solution increases by factor of 10 = pH of solution decreases by 1

e.g. HCl (pH 0) is 10⁴ = 10000x more acidic than vinegar (pH 4) & its H⁺ ions conc. is 10000x greater than vinegar

3.6 core practical: investigate change in pH on adding powdered calcium hydroxide or calcium oxide to fixed volume of dilute hydrochloric acid

1. use measuring cylinder to add 50cm³ dilute HCl to beaker

2. estimate & record pH of contents of beaker

   1. put piece of universal indicator paper onto white tile

   2. dip end of glass rod into liquid, tap onto indicator paper

   3. wait 30s, match colour to appropriate pH on pH colour chart

   4. rinse glass rod with water

3. measure 0.3g calcium hydroxide powder onto ‘weighing boat’

4. add calcium hydroxide powder to beaker & stir

5. estimate & record pH of mixture

6. repeat steps 2 & 3 seven times so you add total of 2.4g calcium hydroxide to acid

7. plot graph with pH on y-axis & mass of calcium hydroxide on x-axis

3.7 dilute vs concentrated solutions

dilute: contains small amount of dissolved solute per unit volume

concentrated: contains lots of dissolved solute per unit volume

3.8 weak vs strong acids

weak: molecules do not dissociate completely into ions in solution - low concs. of H⁺ ions

strong: molecules dissociate completely into ions in solution - high concs. of H⁺ ions

3.9 base definition

any substance that reacts with acid to form salt & water only (neutralises acid)

3.10 alkalis definition

soluble bases

3.11 metal + acid (aq)

metal + acid → salt + hydrogen

3.11 metal oxide + acid (aq)

metal oxide + acid → salt + water

3.11 metal hydroxide + acid (aq)

metal hydroxide + acid → salt + water

3.11 metal carbonate + acid (aq)

metal carbonate + acid → salt + water + carbon dioxide

3.12 chemical test for hydrogen

place lit splint near end of test tube containing gas

hydrogen present: squeaky pop heard

3.12 chemical test for carbon dioxide

bubble gas through limewater

carbon dioxide present: limewater turns cloudy

3.13 neutralisation reaction

reaction between acid & base

3.14 acid-alkali neutralisation

reaction in which H⁺ ions from acid react with OH^- ions from alkali to form H₂O

3.15 preparing soluble salts from acid & insoluble reactant

excess reactant added - ensure all acid used up

excess reactant removed - ensure prepared salt is pure (mixture filtered)

solution remaining only salt & water - no excess reactant present

3.16 preparing soluble salts from acid & soluble reactant

titration used - lets you see what volume of reactant needed to react completely with certain volume of other reactant

acid & soluble reactant mixed in correct proportions - reactant added in excess, difficult to separate from products; end with solution containing only water & desired salt

solution remaining only salt & water - exact volume of acid added to alkali

3.17 core practical: investigate preparation of pure, dry, hydrated copper sulfate crystals starting from copper oxide

1. measure 20cm³ of dilute sulfuric acid using measuring cylinder into small conical flask

2. warm acid in water bath at 50°C, measure temp. with thermometer

3. add a little copper oxide powder to acid & stir

4. if all copper oxide reacts & disappears, add more; stop when copper oxide in excess & no longer reacts

5. filter mixture & transfer filtrate to evaporating basin

6. heat evaporating basin by placing over beaker of water heated with Bunsen burner, stop heating when crystals start to form

7. pour solution into watch glass, leave for few days to let all water evaporate

3.18 how to carry out acid-alkali titration to prepare pure, dry salt?

1. measure fixed volume of alkali into conical flask using pipette

2. add few drops of indicator to alkali

3. add acid from burette to alkali in small amount at a time

4. stop when indicator changes colour (end-point)

5. record exact volume of acid needed to neutralise alkali

6. use burette to add correct volume of acid to alkali without indicator

7. evaporate water from solution formed - leaves pure, dry salt

3.19 solubility rules (salts, nitrates, chlorides, sulfates, carbonates, hydroxides)

precipitation reaction

soluble substances in solutions form insoluble precipitate

3.20 named solutions mixed together - predict if precipitate will be formed

check solubility of products

both products soluble - no precipitate forms

e.g. potassium sulfate soluble - doesn’t form precipitate, copper carbonate insoluble - forms precipitate

3.20 named solutions mixed together - name precipitate formed

2 solutions containing soluble salts react together - ions from salts swap

3.21 how to prepare pure, dry sample of insoluble salt?

pure, dry sample of insoluble salt prepared from two soluble salts

1. mix two solutions in beaker

2. filter mixture

3. rinse beaker with a little distilled water & pour through funnel

4. pour a little distilled water over precipitate in funnel

5. remove filter paper containing precipitate & dry in warm oven