2B- Acids, Bases and Salts

acid definition

a proton (H⁺ ion) donor

strong acid definition

an acid that dissociates completely in aqueous solution

weak acid definition

an acid that partly dissociates in aqueous solution

strong acid dissociation equation arrow

→ (one way)

weak acid dissociation equation arrow

⇌ (reversible)

base definition

a proton (H⁺) acceptor e.g. metal oxide/hydroxide, ammonia

alkali definition

a type of base that dissolves in water forming hydroxide ions OH- (aq)

which alkalis are bases

all of them

which metal hydroxides form alkaline solutions containing hydroxide ions

Group I metal hydroxides, Ca(OH)₂, Mg(OH)₂, and Ba(OH)₂ - all soluble in water

which metal oxides form alkaline solutions containing hydroxide ions

Group I metal oxides, CaO and BaO

hydroxides making alkaline solutions equation (Mg)

Mg(OH)_{2 (s)} + aqua → Mg²⁺_(aq) + 2OH^-_(aq) - when writing equations for the dissolving of a solid, use ‘aqua’ to represent water, if H₂O used the equation wouldn’t balance

oxides making alkaline solutions equation (Ca)

CaO_(s) + H₂O_(l) → Ca²⁺_(aq) + 2OH^-_(aq) - as a chemical reaction is taking place, H₂O is needed to balance the equation

how does ammonia form an alkaline solution with hydroxide ions

ammonia (gas) reacts with water to produce a solution containing hydroxide ions

ammonia making an alkaline solution equation

NH_{3 (g)} + H₂O_(l) → NH₄⁺_(aq) + OH^-_(aq)

salt definition

the ionic compound formed when the H+ ions in an acid are replaced by positive metal or ammonium ion

acid + metal hydroxide →

salt + water

acid + metal oxide →

salt +water

acid + metal carbonate →

salt + water + carbon dioxide

acid + metal →

salt + hydrogen

acid + ammonia →

salt

ions that form soluble compounds

group 1 ions, ammonium NH₄⁺, nitrate NO₃^-, hydrogen carbonate HCO₃^-, halides Cl^-, Br^-, I^-, sulfate SO₄^2-

ions that form soluble compounds exceptions

halides when combined with Ag⁺, Pb²⁺, Hg²⁺, sulfate when combined with Ag⁺, Pb²⁺, Ca²⁺, Sr²⁺, Ba²⁺

ions that form insoluble compounds

oxide O^2-, carbonate CO₃^2-, phosphate PO₄^3-, hydroxide OH^-, sulfide S^2-, dichromate Cr₂O₇^2-

ions that form insoluble compounds exceptions

group I ions, oxide when combined with Ca²⁺, Ba²⁺, carbonate when combined with ammonium, phosphate when combined with ammonium, hydroxide when combined with ammonium, Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺, sulfide when combined with ammonium, dichromate when combined with ammonium, Ca²⁺, Mg²⁺

making a soluble salt from an insoluble base method

1. heat acid solution gently in a beaker

2. add excess of insoluble base to warm acid until no more solid dissolves

3. filter mixture, collect filtrate and transfer to an evaporating basin

4. heat gently to evaporate water- until most of the water has been removed

5. leave to cool and crystallise

ways to make a salt

• making insoluble salt: mixing 2 soluble ionic solutions

• making soluble salt: mixing solid insoluble base + soluble acid solution

• making soluble salt: mixing soluble base solution + soluble acid solution

why are other ways of making salts not used

e.g. metal + acid → salt + hydrogen

may be hazardous

very slow

produce poor yield

expensive

test for halide ions

• add dilute nitric acid to remove carbonate ions- carbonate ions also react with silver ions to form a ppt.

• add silver nitrate solution

• precipitate forms

• add dilute ammonia solution

• if nothing happens add concentrated ammonia solution

halide test results

AgCl: white ppt., soluble in dilute and concentrated ammonia solution

AgBr: cream ppt., not soluble in dilute ammonia solution, soluble in concentrated ammonia solution

AgI: yellow ppt., not soluble in dilute or concentrated ammonia solution

test for carbonate ions

• add dilute nitric acid

• effervescence occur

• bubble gas through limewater- turns cloudy

test for sulfate ions

• add barium nitrate solution

• white ppt. forms

• barium ions reacts with sulfate ions

test for ammonia ions

• add sodium hydroxide solution

• ammonia gas forms- not very volatile- unlikely to see it

• warm test tube in a hot water bath

• hold damp red litmus paper over opening of test tube

• turns blue

test for ammonia ions equation (Cl)

NH₄Cl + NaOH → NH₃ + NaCl +H₂O

order of testing for ions

1. carbonates

2. sulfates

3. halides

possible false positives in ions tests

if carbonate is present in sulfate test- false positive of white ppt. produced, if carbonate or sulfate present in halide test- false positive of white ppt. produced

test for sulfate ions equation

Ba²⁺_(aq) + SO₄^2-_(aq) → BaSO_{4 (s)}

test for carbonate ions equation (Ca)

CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂

test for chloride ions equations

Ag⁺_(aq) + Cl^-_(aq) → AgCl_(s)

steps for making an ionic equation

1. separate out all ionic aqueous substances/ acids into ions

2. identify an spectator ion (ions that are the same either side of the arrow)

3. remove spectator ions to leave ionic equation