Electrolysis of Aqueous NaCl & Dilute Sulfuric Acid

• Brine is a concentrated solution of aqueous sodium chloride

  • It can be electrolysed using inert electrodes made from platinum or carbon/graphite
• When aqueous sodium chloride is electrolysed, it produces bubbles of gas at both electrodes as chlorine and hydrogen are produced, leaving behind sodium hydroxide solution
• The products of electrolysis of aqueous sodium chloride all have important industrial uses:

  • Chlorine is used to make bleach
  • Hydrogen is used to make margarine
  • Sodium hydroxide is used to make soap and detergents
• Product at the Negative Electrode (aqueous sodium chloride):

  • The H+ ions are discharged at the cathode as they are less reactive than sodium ions
  • The H +ions gain electrons to form hydrogen gas
• Product at the Positive Electrode (aqueous sodium chloride):

  • The Cl- ions are discharged at the anode
  • They lose electrons and chlorine gas forms
  • The Na+ and OH- ions remain behind and form the NaOH solution
• Product of electrolysis of aqueous sodium

  A) Hydrogen gas

  B) chlorine gas

  C) sodium chloride

  D) brine

  E) sodium

  F) hydroxide

  
• Dilute sulfuric acid can be electrolysed using inert electrodes made from platinum or carbon/graphite

  • Bubbles of gas are seen at both electrodes
• Product at the Negative Electrode (dilute sulfuric acid)

  • H+ ions are attracted to the cathode, gain electrons and form hydrogen gas
• Product at the Positive Electrode

  • OH- ions are attracted to the anode, lose electrons and form oxygen gas and water
• If the gas produced at the anode relights a glowing splint dipped into a sample of the gas then the gas is oxygen

  
• If the gas produced at the anode bleaches damp litmus paper then the gas is chlorine

  A) blue

  B) red

  C) white

  D) chlorine

  
• If the gas produced at the cathode burns with a 'pop' when a sample is lit with a lighted splint then the gas is hydrogen

  
• Water is a poor conductor of electricity

  • very small number of water molecules ionize to form H+ and OH- ions
  • water can conduct if it contains salt impurities
• Electrolysis of water with some sulfuric acid (H2SO4)

  A) double the gas

  B) hydrogen

  C) carbon

  D) cathode

  E) oxygen

  F) platinum

  G) anode

  
• Electrolysis of water with some sulphuric acid (in a circuit)

  
• A solution of sodium chloride in water contains the following ions:

  • H+ (aq) and OH-(aq) from the water
  • Na+ (aq) and Cl-(aq) from the sodium chloride
• Both positive ions migrate to the negative electrode and both negative ions move to the positive electrode

  • cations → cathode
  • anions → anode
  • at each electrode, one or both of the ions may be discharged

  
• Metals (cations), if produced, are discharged at the cathode electrode
• Hydrogen is produced at the cathode electrode only and is produced in preference to metals unless unreactive metals such as Cu and Ni are present
• Other non-metals (anions) are produced at the anode electrode
• Reactive metals tend not to be formed at the cathode during electrolysis of aqueous solutions
• • Investigating electrolysis of aqeuous solutions

  A) products

  B) oxygen

  C) chlorine

  D) bromine

  E) iodine

  F) positive electrode

  G) anode

  H) solution

  I) negative electrode

  J) cathode

  K) tested

  L) products

  M) hydrogen

  N) copper

  O) lead

  P) silver

  
• Flowchart

  A) non-metal

  B) metal

  C) oxygen

  D) dilute

  E) oxygen

  F) concentrated

  G) halogen

  H) lower

  I) metal

  J) higher

  K) H2

  
• Flowchart

  • Anode
  • If a dilute halide or OH- is present, oxygen is formed
  • If a concentrated halide is present, halogen is formed e.g. bromide → bromine, iodide → iodine
  • Cathode - metal must be less reactive than H to give the metal otherwise H2 is produced
  • metal or H+ formed
• Reactivity Series Mnemonic

  • Lovely - Li
  • Katie - K
  • Never - Na
  • Could - Ca
  • Manage - Mg
  • A - Al
  • Zebra - Zn
  • For - Fe
  • She - Sn
  • Punishes - Pb
  • Him - H
  • Cruelly - Cu
  • Ag
  • Au
  • P+
• Ag, Au and P+ are less reactive
• which electrode does reduction take place?

  Cathode
• which electrode does oxidation take place?

  Anode
• What is the half equation for chloride?

  2Cl- = Cl2 + 2e-
• Where does the hydrogen come from in the reaction of sodium chloride (NaCl)?

  Water
• What are the solutions left behind after the electrolysis of concentrated sodium chloride?

  sodium hydroxide
• What are the solutions left behind after the electrolysis of dilute sodium chloride?

  remaining solution of sodium chloride and the water is decomposed
• Ions in the concentrated NaCl are:

  • water - H+ and OH-
  • NaCl - Na+ and Cl-
• NaCl at the anode:

  • 2Cl- → Cl2 + 2e-
  • Cl- oxidised to Cl2
• NaCl at the cathode:

  • 2H+ → H2
  • H+ reduced H2
• Electrolysis of concentrated aqueous NaCl

  • leaves a high concentration of OH- around the cathode
  • sodium ions are drawn through the membrane where OH- react to form NaOH
• Electrolysis of concentrated aqueous NaCl

  A) chlorine

  B) hydrogen

  C) water

  D) sodium hydroxide

  E) solution

  F) nickel cathode

  G) membrane for separation

  H) titanium anode

  I) salt

  J) solution

  K) concentrated salt

  
• Electrolysis is an important industrial process as it produces 3 important products which are used to make other products:

  • H2 - makes HCl and CH4, fuel for rocket shops and some vehicles, NH3 for fertilizers
  • Cl2 - cleaning pools, kills bacteria, makes HCl, makes bleachh
  • NaOH - paper, soap
• Hydrochloric acid is a strong acid and completely ionizes

  • HCl → H+ + + Cl-
• Ions mitagate to the oppositely charged electrode and turn into molecules in the reaction of hydrochloric acid

  • cathode: 2H+ + 2e- → H2
  • anode: 2Cl- + 2e- → Cl2