AQA GCSE Chemistry - Reactions of Acids PDF

AQA​ ​GCSE​ ​Chemistry Topic​ ​4:​ ​Chemical​ ​changes ​ ​Reactions​ ​of​ ​acids Notes (Content​ ​in​ ​bold​ ​is​ ​for​ ​Higher​ ​Tier​ ​only) www.pmt.education Reactions​ ​of​ ​acids​ ​with​ ​metals acids​ ​react​ ​with​ ​some​ ​metals​ ​to​ ​produce​ ​a​ ​salt​ ​and​ ​hydrogen: ○ acid​ ​+​ ​metal​...

AQA​ ​GCSE​ ​Chemistry Topic​ ​4:​ ​Chemical​ ​changes ​ ​Reactions​ ​of​ ​acids Notes (Content​ ​in​ ​bold​ ​is​ ​for​ ​Higher​ ​Tier​ ​only) www.pmt.education Reactions​ ​of​ ​acids​ ​with​ ​metals acids​ ​react​ ​with​ ​some​ ​metals​ ​to​ ​produce​ ​a​ ​salt​ ​and​ ​hydrogen: ○ acid​ ​+​ ​metal​ ​->​ ​salt​ ​+​ ​hydrogen These​ ​are​ ​redox​ ​reactions​ ​–​ ​this​ ​means​ ​that​ ​one​ ​substance​ ​is​ ​reduced​ ​and another​ ​substance​ ​is​ ​oxidised you​ ​should​ ​be​ ​able​ ​to​ ​identify​ ​which​ ​substances​ ​are​ ​which​ ​by​ ​looking​ ​at electrons​ ​gained​ ​and​ ​lost​ ​(following​ ​OIL​ ​RIG) o e.g.​ ​2HCl​ ​+​ ​Mg​ ​->​ ​MgCl​2​​ ​+​ ​H​2 o magnesium:​ ​Mg​ ​->​ ​Mg​2+​,​ ​so​ ​ionic​ ​equation​ ​is​ ​Mg​ ​->​ ​Mg​2+​​ ​+​ ​2e​-​,​ ​Mg​ ​has lost​ ​electrons​ ​so​ ​Mg​ ​has​ ​been​ ​oxidised o hydrogen:​ ​2H​+​​ ​->​ ​H2​​ ,​ ​so​ ​ionic​ ​equation​ ​is​ ​2H​+​​ ​+​ ​2e​-​​ ​->​ ​H2​​ ,​ ​H​ ​has​ ​gained electrons,​ ​so​ ​H​ ​has​ ​been​ ​reduced o because​ ​magnesium​ ​has​ ​been​ ​oxidised​ ​and​ ​hydrogen​ ​has​ ​been​ ​reduced in​ ​the​ ​same​ ​reaction,​ ​this​ ​is​ ​a​ ​redox​ ​reaction Neutralisation​ ​of​ ​acids​ ​and​ ​salt​ ​production Acids​ ​are​ ​neutralised​​ ​by​ ​alkalis​ ​(e.g​ ​soluble​ ​metal​ ​hydroxides)​ ​and​ ​bases​ ​(e.g insoluble​ ​metal​ ​hydroxides​ ​and​ ​metal​ ​oxides)​ ​to​ ​produce​ ​salts​ ​and​ ​water ○ acid​ ​+​ ​alkali​ ​->​ ​salt​ ​+​ ​water ○ acid​ ​+​ ​base​ ​->​ ​salt​ ​+​ ​water acids​ ​are​ ​neutralised​​ ​by​ ​metal​ ​carbonates​ ​to​ ​produce​ ​salts,​ ​water​ ​and​ ​carbon dioxide ○ acid​ ​+​ ​metal​ ​carbonate​ ​->​ ​salt​ ​+​ ​water​ ​+​ ​carbon​ ​dioxide The​ ​salt​ ​produced… o In​ ​alkali​ ​and​ ​base​ ​reactions​ ​depends​ ​on​ ​the​ ​acid​ ​used… ▪ Hydrochloric​ ​acid​ ​(HCl)​ ​produces​ ​chlorides​ ​(XCl) ▪ Nitric​ ​acid​ ​(HNO​3​)​ ​ ​produces​ ​nitrates​ ​(XNO​3​) ▪ Sulfuric​ ​acid​ ​(H​2​SO​4​)​ ​produces​ ​sulfates​ ​(XSO​4​) o It​ ​also​ ​depends​ ​on​ ​the​ ​positive​ ​ions​ ​in​ ​the​ ​base,​ ​alkali​ ​or​ ​carbonate​ ​i.e. the​ ​metal​ ​(which​ ​is​ ​the​ ​X​ ​in​ ​the​ ​salts​ ​above). o remember​:​ ​the​ ​charges​ ​on​ ​the​ ​positive​ ​ion​ ​from​ ​the base/alkali/carbonate​ ​and​ ​the​ ​negative​ ​ion​ ​from​ ​the​ ​acid​ ​must​ ​add​ ​up​ ​to zero. e.g.​ ​if​ ​you​ ​have​ ​sodium​ ​hydroxide​ ​and​ ​sulfuric​ ​acid,​ ​you​ ​have​ ​Na​+​​ ​ions​ ​and SO​4​2-​​ ​ions,​ ​so​ ​you​ ​need​ ​2x​ ​Na​+​​ ​ions,​ ​giving​ ​you​ ​the​ ​salt:​ ​Na​2​SO​4 the​ ​charges​ ​on​ ​the​ ​ions​ ​from​ ​acids​ ​are:​ ​Cl​-​,​ ​NO​3​-​​ ​and​ ​SO​4​2- Soluble​ ​salts They​ ​can​ ​be​ ​made​ ​from​ ​acids​ ​by​ ​reacting​ ​them​ ​with​ ​solid​ ​insoluble​ ​substances,​ ​such​ ​as metals,​ ​metal​ ​oxides,​ ​hydroxides​ ​or​ ​carbonates: 1) Add​ ​the​ ​chosen​ ​solid​ ​insoluble​ ​substance​ ​to​ ​the​ ​acid​ ​then​ ​the​ ​solid​ ​will​ ​dissolve. 2) You​ ​know​ ​the​ ​acid​ ​has​ ​been​ ​neutralised​ ​when​ ​excess​ ​solid​ ​sinks​ ​to​ ​the​ ​bottom, so​ ​keep​ ​adding​ ​until​ ​this​ ​happens www.pmt.education 3) Filter​ ​out​ ​excess​ ​solid​ ​leaving​ ​the​ ​salt​ ​solution,​ ​then​ ​evaporate​ ​some​ ​water,​ ​then leave​ ​the​ ​rest​ ​to​ ​evaporate​ ​slowly. This​ ​is​ ​called​ ​crystallisation. The​ ​pH​ ​scale​ ​and​ ​neutralisation Acids​ ​produce​ ​H​+​​ ​ions​ ​in​ ​aqueous​ ​solutions Alkalis​ p ​ roduce​ ​OH​-​​ ​ions​ ​in​ ​aqueous​ ​solutions The​ ​pH​ s​ cale​ ​(0​ ​to​ ​14)​ ​measures​ ​the​ ​acidity​ ​or​ ​alkalinity​ ​of​ ​a​ ​solution,​ ​and​ ​can​ ​be measured​ ​using​ ​universal​ ​indicator​ ​of​ ​a​ ​pH​ ​probe o pH​ ​7​ ​is​ ​neutral o pH​ ​​ ​7​ ​is​ ​alkaline H​+​(aq)​ ​+​ ​OH​-​(aq)​ ​->​ ​H2​​ O(l)​ ​is​ ​the​ ​ionic​ ​equation​ ​of​ ​any​ ​neutralisation​ ​reaction Titrations​ ​(chemistry​ ​only) The​ ​volumes​ ​of​ ​acid​ ​and​ ​alkali​ ​solutions​ ​that​ ​react​ ​with​ ​each​ ​other​ ​can​ ​be​ ​measured​ ​by titration​ ​using​ ​a​ ​suitable​ ​indicator. How​ ​to​ ​carry​ ​out​ ​a​ ​titration: 1. Wash​ ​burette​ ​using​ ​dilute​ ​hydrochloric​ ​acid​ ​and​ ​then​ ​water 2. Fill​ ​burette​ ​to​ ​100cm​3​​ ​with​ ​acid​ ​with​ ​the​ ​meniscus’​ ​base​ ​on​ ​the​ ​100cm​3​​ ​line 3. Use 25cm​3 pipette to add 25cm​3 of alkali into a conical flask, drawing alkali into the pipette​ ​using​ ​a​ ​pipette​ ​filler 4. Add a few drops of a suitable indicator to the conical flask (eg: phenolphthalein which​ ​is​ ​pink​ ​when​ ​alkaline​ ​and​ ​colourless​ ​when​ ​acidic) 5. Add​ ​acid​ ​from​ ​burette​ ​to​ ​alkali​ ​until​ ​end-point​ ​is​ ​reached​ ​(as​ ​shown​ ​by​ ​indicator) 6. The titre (volume of acid needed to exactly neutralise the acid) is the difference between​ ​the​ ​first​ ​(100cm​3​)​ ​and​ ​second​ ​readings​ ​on​ ​the​ ​burette 7. Repeat​ ​the​ ​experiment​ ​to​ ​gain​ ​more​ ​precise​ ​results Titration​ ​calculations 1dm​3​​ ​=​ ​1000cm​3 One​ m ​ ole​ ​of​ ​a​ ​substance​ ​in​ ​grams​ ​the​ ​same​ ​as​ ​its​ ​relative​ ​atomic​ ​mass​ ​in grams. www.pmt.education Working​ ​out​ ​concentrations: E.g​ ​25​ ​cm​3​ of​ ​dilute​ ​hydrochloric​ ​acid​ ​is​ ​neutralised​ ​by​ ​20​ ​cm​3​ of​ ​0.5​ ​mol/dm​3​sodium hydroxide.​ ​What​ ​is​ ​the​ ​concentration​ ​of​ ​the​ ​hydrochloric​ ​acid? 1. Convert​ ​volumes​ ​into​ ​dm​3​. 25/1000=0.025dm​3​​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​20/1000=0.02dm​3 2. Work​ ​out​ ​the​ ​moles​ ​of​ ​NaOH moles​ ​=​ ​volume​ ​x​ ​concentration So,​ ​0.02​ ​x​ ​0.5​ ​=​ ​0.01 3. Work​ ​out​ ​mole​ ​ratio​ ​from​ ​equation HCl​ ​+​ ​NaOH​ ​->​ ​H2​​ O​ ​+​ ​NaCl 1:1​ ​ratio,​ ​so​ ​moles​ ​of​ ​NaOH​ ​=​ ​moles​ ​of​ ​HCl,​ ​so​ ​moles​ ​of​ ​HCl=0.01 4. Work​ ​out​ ​concentration conc​ ​=​ ​moles​ ​/​ ​vol​ ​=​ ​0.01​ ​/​ ​0.025​ ​=​ ​0.4​ ​mol​ ​dm​-3 Strong​ ​and​ ​weak​ ​acids Strong​ ​acid​ ​=​ ​completely​ ​ionised​ ​in​ ​aqueous​ ​solution o e.g.​ ​hydrochloric,​ ​nitric​ ​and​ ​sulfuric​ ​acids Weak​ ​acid​ ​=​ ​partially​ ​ionised​ ​in​ ​aqueous​ ​solution o Ethanoic,​ ​citric​ ​and​ ​carbonic​ ​acids Stronger​ ​an​ ​acid,​ ​lower​ ​the​ ​pH​ ​(for​ ​a​ ​given​ ​conc.​ ​of​ ​aq.​ ​solutions) ​ As​ ​the​ ​pH​ ​decreases​ ​by​ ​one​ ​unit,​ ​the​ ​H​+​ conc.​ ​of​ ​the​ ​solution​ ​increases​ ​by​ ​a factor​ ​of​ ​10. Strong​ ​and​ ​weak​ ​is​ ​NOT​ ​the​ ​same​ ​as​ ​concentrated​ ​and​ ​dilute​ ​–​ ​the​ ​latter​ ​refers to​ ​the​ ​amount​ ​of​ ​substance​ ​in​ ​a​ ​given​ ​volume​ ​,​ ​whereas​ ​the​ ​former​ ​refers​ ​to the​ ​H+​​ ​ ​ion​ ​conc​ ​in​ ​aq.​ ​solutions www.pmt.education