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| Question | Answer |
|---|---|
| What does the word electrolysis mean? | breaking down using electricity |
| What happens in the electolysis process? | an electric current is used to break down an ionic compound |
| What is the compound broken down by electrolysis called | the electrolyte |
| What is needed to set up an electrical circuit for electrolysis? | two electrodes are connected to a power source and dipped into the electrolyte with a gap between them |
| What is the electrode called that is connected to the positive terminal of the power supply? | anode |
| What is the electrode called that is connected to the negative terminal of the power supply? | cathode |
| What are electrodes made of? | unreactive substances such as graphite or platinum |
| Why are electrodes made of unreactive substances? | so that they do not react with the electrolyte or the products of electrolysis |
| How do the ions of the electrolyte move during electrolysis? | positive ions move to the cathode and negative ions move to the anode |
| What happens to the ions when they reach the electrodes? | the ions lose their charges at the electrodes and become elements |
| Write an equation to show how zinc chloride is broken down by electrolysis. | ZnCl2(l) → Zn(l/s) + Cl2(g) |
| What kind of compound is zinc chloride? | zinc chloride is an ionic compound |
| Can zinc chloride conduct electricity? |
• zinc chloride cannot conduct electricity when solid because the ions cannot move around • zinc chloride can conduct electricity when liquid because the ions can move freely |
| Write the ions in zinc chloride using symbols. | Zn2+ and Cl- |
| How do the ions of the zinc chloride move during electrolysis? | Zn2+ ions move to the cathode and Cl- ions move to the anode |
| What happens to the the ions of the zinc chloride during electrolysis? |
• Zn2+ ions become Zn metal and coat the cathode • Cl- ions combine to form chlorine gas Cl2 |
| What state do ionic compounds need to be in for electrolysis? | molten (liquid) or dissolved - in both cases the ions can move |
| What complication arises when electrolysing ionic compounds in solution? | water can also form ions and react with the electrolyte or the products of electrolysis |
| Which metals when in aqueous solution are deposited at an electrode during electrolysis? | only the metals below hydrogen in the reactivity series |
| Write an equation to show what happens when copper bromide in solution is broken down by electrolysis. | CuBr2(aq) → Cu(s) + Br2(g) |
| What happens when an aqueous solution of copper bromide is electolysed? | copper is deposited at the cathode and bromine gas is released at the anode |
| Can electrolysis break down a covalent compound? | only if it reacts with water to form ions, otherwise not |
| Question | Answer |
|---|---|
| Where do the mobile ions move during electrolysis? | the mobile ions move towards the electrodes |
| What determines which electrode a mobile ion moves towards? | the charge of the ion |
| Which electrode does a positive ion move towards? | the negative electrode known as the cathode |
| Which electrode does a negative ion move towards? | the positive electrode known as the anode |
| What happens to the ions when they reach the electrodes? | they either gain or lose electrons |
| What happens to a negative ion at the anode? | it loses electrons (it is oxidised) to become a neutral atom |
| What happens to a positive ion at the cathode? | it gains electrons (it is reduced) to become a neutral atom |
| What ions are present in molten PbBr2? | lead ions Pb2+ and bromide ions Br1- |
| What happens to the Pb2+ ions in the electrolysis of molten PbBr2? |
• they move towards the cathode (negative electrode) • at the cathode, each Pb2+ ion gains two electrons • Pb2+ becomes neutral lead atoms Pb(s) • the lead ions have been reduced |
| What happens to the Br1- ions in the electrolysis of molten PbBr2? |
• they move towards the anode (positive electrode) • at the anode, each Br1- ion loses an electron • Br1- ions become neutral bromine atoms Br(s) that combine to form Br2 • the bromide ions have been oxidised |
| Write the half equation that takes place at the cathode? | Pb2+(aq) + 2e- → Pb(s) |
| What type of reaction takes place at the cathode? | a reduction reaction |
| Write the half equation that takes place at the anode? | 2Br1-(aq) → Br2(g) + 2e- |
| What type of reaction takes place at the anode? | an oxidation reaction |
| What makes electrolysis more complex in water? | the presence of the water because the water also ionises |
| Write an equation for the ionisation of water. | H2O(l) → H+(aq) + OH1-(aq) |
| What determines which element will be produced at an electrode if there are two possibilities? | the less reactive will usually be formed |
| Which is less reactive out of hydrogen and potassium? | hydrogen |
| What ions are attracted to the negative electrode (cathode) in a solution of a potassium compound? | both H+ and K+ ions |
| Which element is produced at the cathode in a solution of a potassium compound? | hydrogen gas |
| What happens to the potassium ions in a solution of a potassium compound? | the potassium ions remain in solution as potassium ions |
| Write the half equation for what happens at the cathode? | 2H+(aq) + 2e- → H2(g) |
| What happens at the anode in the electrolysis of aqueous solutions? | hydroxide ions, OH-(aq) are reduced |
| Write the half equation for what happens at the anode? | 4OH-(aq) → 2H2O(l) + O2(g) + 4e- |
| What will the visible evidence of the discharge of OH-(aq) at the anode? | oxygen gas bubbles can be seen |
| When are hydroxide ions not discharged at the anode? | when there is a reasonable concentration of a halide at the anode, the halogen is formed |
| Write the half equation for what happens at the anode when a solution contains chloride ions. | 2Cl-(aq) → Cl2(g) + 2e- |
| State the discharge at the anode. | starting with the easiest: halide ion > hydroxide > all other negatively charged ions |
| Question | Answer |
|---|---|
| List some uses of aluminium or its alloys. |
• pans • overhead cables • aeroplanes • cooking foil • drink cans • window and patio door frames • bicycle frames and car bodies |
| How reactive is aluminium compared to magnesium, zinc and iron? | aluminium is more reactive than zinc and iron but less reactive than magnesium |
| Can carbon be used to extract aluminium? | no, as aluminium is more reactive |
| How is aluminium extracted? | using electrolysis |
| What is the compound from which aluminium is extracted? | Al2O3 |
| What is the name of the ore containing Al2O3? | bauxite |
| How is bauxite mined? | from open cast mining meaning the ore is on the surface |
| What is the first step in removing aluminium from bauxite? | removing Al2O3 from the ore |
| Why must Al2O3 be removed from bauxite before aluminium is extracted? | because the ore contains other impurities, in particular, a lot of iron (III) oxide |
| What is the colour of the waste water from the separation process? | a rusty brown colour |
| Why is the water from the separation process a rusty brown colour? | because it contains iron (III) oxide |
| What is done with the water from the separation process? | in large lagoons |
| How is aluminium extracted from aluminium oxide? | electrolysis |
| Is the electrolysis of aluminium energy efficient? | no, the electrolysis of aluminium requires a lot of energy an aluminium plant uses the same amount of energy as a small town |
| Describe the process of purifying aluminium oxide. |
• the aluminium oxide is mixed with molten cryolite • the molten mixture is ionised at 850 °C • the large amount of energy added to the molten mixture keeps the mixture molten |
| What is the melting point of aluminium? | 2050 °C |
| Why is the alumnium oxide added to cryolite? | because it reduces the melting point to 850 °C saving some energy |
| Write the word equation for the electrolysis of aluminium oxide? | aluminium oxide → aluminium + oxygen |
| Write the symbol equation for the electrolysis of aluminium oxide? | 2Al2O3 → 4Al + 3O2 |
| At which electrode does aluminium form? | at the cathode (negative electrode) |
| At which electrode does oxygen form? | at the anode (positive electrode) |
| What happens at the cathode (negative electrode)? |
• each Al3+ ion gains three electrons • the ions turn into aluminium atoms • the Al3+ ions are reduced • the molten aluminium collects at the bottom and is siphoned off |
| What happens at the anode (positive electrode)? |
• each O2- ion loses electrons • the ions turn into oxygem atoms • the oxygen atoms form O2 forming bubbles around the electrode • the O2- ions are oxidised |
| Write the half equation for what happens at the cathode. | Al3+(l) + 3e- → Al(l) |
| Write the half equation for what happens at the anode. | 2O2-(l) → O2(g) + 4e- |
| What happens to the oxygen at the anode? | it reacts with the carbon electrode |
| Write the reaction that happens at the anode/ | C(s) + O2(g) → CO2(g) |
| Why do the anode electrodes need to be replaced periodically? | because the electrodes wear away as they react with the oxygen released |
| Question | Answer |
|---|---|
| What is the anode? | the positive electrode |
| What is the cathode? | the negative electrode |
| What is brine? | sodium chloride solution |
| What products result from the electrolysis of brine? |
• chlorine gas is produced at the anode • hydrogen gas is produced at the cathode • sodium hydroxide solution is produced |
| Write an equation for the electrolyis of brine. | sodium chloride solution → hydrogen gas + chlorine gas + sodium chloride solution |
| What happens at the anode when brine is electrolysed? |
• the anode is the positive electrode • the negative chloride ions Cl-1are attracted to the positive electrode • the Cl-1 ions each lose one electron at the anode • the chloride ions are oxidised • the chlorine atoms bond together to form pairs • the pairs are given off as chlorine gas Cl2 |
| Write the equation for the reaction at the anode. | Cl-(aq) → Cl2(g) + 2e- |
| What happens at the cathode when brine is electrolysed? |
• the cathode is the negative electrode • the positive hydrogen ions H+are attracted to the negative electrode • the H+ ions each gain one electron at the cathode • the hydrogen ions are reduced • the hydrogen atoms bond together to form pairs • the pairs are given off as hydrogen gas H2 • the positive sodium ions are also attracted to the cathode • the hydrogen ions are discharged and the sodium ions stay in solution |
| Where do the hydrogen ions come from in brine? | the hydrogen ions are formed when water breaks down |
| Write the formation of hydrogen ions. | H2O(l) → H+(aq) + OH-(aq) |
| Write the equation for the reaction at the cathode. | 2H+(aq) + 2e- → H2(g) |
| Which ions are attracted to the cathode? | the hydrogen ions and the sodium ions |
| Which ions are discharged at the cathode? | only the hydrogen ions |
| What happens to the sodium ions attracted to the cathode? | they remain in solution |
| Why do sodium ions remain in solution while hydrogen ions are discharged? | the less reactive element is discharged |
| What will an acid/base indicator show around the cathode? | an alkaline solution |
| What happens to the sodium ions? | the sodium and hydroxide ions remain in solution forming an alkaline sodium hydroxide solution |
| How is the sodium hydroxide solution formed? |
• NaCl forms Na+ ions and Cl- ions • water H2O forms H+ and OH- ions • the Cl- and H+ ions are removed during electrolysis • the Na+ and OH- ions remain in solution |
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