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20 Electrochemistry
Electrochemistry
| Question | Answer |
|---|---|
| List the electrochemical series of cations from most to least reactive. | K+, Na+, Ca2+, Mg2+, Al3+, Zn2+, Fe2+, Sn2+, Pb2+, H+, Cu2+, Ag+ |
| K is the highest in the series. What does this mean for its ease of ionization, reducing ability and ease of discharge. | - Ionizes most easily ( most reactive) - Strongest reducing agent, giving away its electrons - reducing the other reactant. - Forms the most stable ions (when it has reacted to form compounds) and hardest to displace/discharge from its compound. |
| Ag is the lowest in the series. What does this mean for its ease of ionization, reducing ability and ease of discharge. | - Ionizes least easily - Weakest reducing agent in series - Forms the least stable ions (when in compounds) and therefore easy to be displaced/discharged from the compound. |
| Why will a metal be able to displace another metal that is a below it in the electrochemical series? | The higher metal is a stronger reducing agent and gives its electrons to the ions of the lower metal. The higher metal therefore ionizes and the ions of the lower metal are converted to atoms (are discharged). |
| Will a reaction occur if a strip of magnesium ribbon is placed in copper(II) sulfate solution? | Yes, since Mg is higher than Cu in the electrochemical series. |
| Will a reaction occur if some iron filings are placed in a solution of zinc nitrate? | No, since Fe is below Zn in the electrochemical series. |
| T or F. Only metals which are above hydrogen react with acids since H has to be displaced from the acid in the reaction to produce hydrogen gas (H2). | T |
| Will a reaction occur if Zn is placed in HCl(aq)? | Yes, since Zn is higher than H in the electrochemical series. |
| Will a reaction occur if some Cu turnings are placed in H2SO4(aq)? | No, since Cu is below H in the electrochemical series. |
| List the electrochemical series of anions from most to least reactive. | F-. SO4^2-, NO3^-, Cl-, Br-, I-, OH- (memory tip: Food-Should-Not-Contain-Bacteria-In-Oven) |
| F- is at the top of the list of anions. What does this mean about its ease of ionization, oxidising ability and ease of discharge. | - It ionizes the most easily (is most reactive) - Is the strongest oxidising agent ( removing electrons from the other reactant) - Forms the most stable compound and is most difficult to displace/discharge from compound. |
| Why are metals reducing agents and non-metals oxidising agents? | Metals lose electrons when they ionize and these are used to reduce the other reactant. Non-metals gain electrons when they ionize and these are used to oxidize the other reactant. |
| T or F. Non-metals which are higher in the electrochemical series are able to displace non-metals lower in the series from their compounds. | T |
| Why is a non-metal which is higher in the electrochemical series better able to displace a non-metal lower in the series ? | The higher non-metal is a stronger oxidizing agent, therefore, it removes electrons from the ions of the lower non-metal. The higher non-metal ionizes and the ions of the lower non-metal are discharged forming atoms. |
| Will a reaction occur if Cl2(g) is bubbled into NaI (aq)? | Yes, since Cl is higher than I in the electrochemical series of anions. |
| Will a reaction occur if I2(g) is added to NaCl(aq) ? | No, since I is lower than Cl in the electrochemical series of anions. |
| Will a reaction occur if Br2 (g) is added to KI(aq) ? | Yes, since Br is higher than I in the electrochemical series of anions. |
| What are conductors? | Substances which allow electricity to pass through. They can be solids, liquids or solutions. E.g. graphite, molten ionic compounds, solutions of ionic compounds, aqueous acids and alkalis. |
| What are non-conductors? | Substances which do not allow an electric current to pass through. They can be solids, liquids, solutions of gases. E.g. non-metals (except graphite – an allotrope of carbon), plastics, covalent substances and solid ionic compounds. |
| What is an electrolyte? | A compound that forms ions when molten or in aqueous solution. |
| Distinguish between metallic and electrolytic conduction in terms of electron/ion movement and chemical properties. | Metallic - is carried by movement of electrons - no change in chemical properties of conductor Electrolytic - is carried by movement of ions - decomposes the electrolyte as a result of the chemical reaction |
| Distinguish between metallic and electrolytic conduction in terms of matter transfer and temperature. | Metallic - does not involve the transfer of matter - conduction decreases with increase in temp. Electrolytic - involved the transfer of matter as ions - conduction increases with increase in temp. |
| What is a strong electrolyte? | A substance which is fully ionized when dissolved in water. E.g strong acids, strong alkalis and soluble ionic compounds. They produce high concentrations of ions in solution. |
| What is a weak electrolyte? | A substance which is partially ionized when dissolved in water. E.g. weak acids and weak bases. These have low concentrations of ions in solution. |
| What is a non-electrolyte? | A substance which remains as molecules when in the liquid state or dissolved in water, It does not ionize. E.g. kerosene, gasoline, molten covalent compounds, ethanol, glucose etc. |
| T or F. Pure water is an extremely weak electrolyte which undergoes spontaneous ionization to produce H+ and OH- ions. | T |
| Define electrolysis. | The chemical change (decomposition) which occurs when an electric current is passed through an electrolyte. |
| Electrolysis takes place in an apparatus known as the electrolytic cell. State the 3 main parts of an electrolytic cell. | 1) a d.c. power supply (battery) - which provides the electric current 2) two electrodes - connected via wires to the power supply 3) the electrolyte - a solution with mobile ions or a molten ionic compound |
| What is the function of the electrodes in an electrolytic cell? | Electrodes take the current into and out of the electrolyte. They may be active or inert. |
| What is the anode? | The positive electrode which is connected to the positive terminal of the battery. |
| What is the cathode? | The negative electrode which is connected to the negative terminal of the battery. |
| What happens at the anode during electrolysis? | Anions(-ve ions) are attracted to the anode(+ve electrode). OXIDATION occurs at the anode. (memory tip : AnOX) |
| What happens at the cathode during electrolysis? | Cations (+ve ions) are attracted to the cathode (-ve electrode). REDUCTION occurs at the cathode. (memory tip: redCAT) |
| T or F. Molten electrolytes contain only two different ions - one cation and one anion. | T |
| What can be expected to happen at the anode during the electrolysis of molten lead (II) bromide using inert graphite electrodes? | Br- ions lose electrons (become oxidized) to form Br atoms which bond with each other to form Br2(g). |
| What can be expected to happen at the cathode during the electrolysis of molten lead (II) bromide using inert graphite electrodes? | Pb2+ gains electrons and becomes Pb (l) which drips off the cathode. |
| T or F. Aqueous solutions contain at least two different cations and anions. H+ and OH- are always present in addition to the other two ions. | T |
| In the electrolysis of dilute sulphuric acid using inert electrodes, what can be observed at each electrode? NB. You must think about all the ions present and the preferential discharge leading to the reaction which occurs at each electrode. | Anode : Effervescence seen as O2(g) is evolved Cathode: Effervescence seen as H2(g) is evolved |
| In the electrolysis of dilute sodium chloride solution using inert electrodes, what can be observed at each electrode? | Anode: Effervescence seen as O2(g) evolved Cathode: Effervescence seen as H2(g) evolved. |
| In the electrolysis of concentrated sodium chloride solution (brine) using inert electrodes, what can be observed at each electrode? | Anode: Effervescence observed with a yellow-green gas as (Cl2(g)) is evolved Cathode: Effervescence observed as H2(g) is evolved |
| In the electrolysis of copper(II) sulfate solution using inert electrodes, what can be observed at each electrode? | Anode: Effervescence as O2(g) is evolved Cathode: Pink deposits of copper Cu(s) build up around the cathode and the cathode increases in size. |
| In the electrolysis of copper (II) sulphate solution using active copper electrodes, what can be observed at each electrode? | Anode: The anode itself becomes ionizes to form Cu2+. Cathode: Pink deposits (Cu(s)) build up around the cathode and it increases in size. |
| What is quantitative electrolysis? | That the mass of a substance produced at an electrode during electrolysis is directly proportional to the quantity of electricity passing through the electrolytic cell. |
| What did Michael Faraday propose concerning quantitative electrolysis? | That the mass of a substance produced at an electrode during electrolysis is directly proportional to the quantity of electricity passing through the electrolytic cell. |
| Define the Faraday constant. | This is the size of of the electrical charge on one mole of electrons and has a value of 96 500 Cmol-1 |
| State the two factors which affect the quantity of electricity (measured in Coulombs) flowing through an electrolyte. | 1) The rate of flow of electrical charge (the current) measured in amperes(A) 2) The length of time that current flows for measured in seconds (s). |
| Give the formula for calculation quantity of electricity (electrical charge). | Q = I x t Q: quantity of electrical charge (unit: Coulombs) I : current(unit: Amperes) t : time (unit: seconds) |
| List 4 industrial applications of electrolysis. | 1) Extracting metals from their ores. 2) Purifying metals (electrorefining) 3) Plating one metal with another (electroplating) 4) Coating a metal with its oxide (anodising) |
| Describe the application of electrolysis in the extraction of metals. | Electrolysis is a powerful method of reduction used to extract Al and metals above from their ores. During electrolysis of the molten ores, the metal cations move towards the cathode where they are discharged. |
| Describe the application of electrolysis in the purification (electrorefining) of metals (below H in the series - purification of Cu). | Impure metal acts as the anode. The cathode is a thin sample of pure metal. The electrolyte is an aqueous solution which contains ions of the metal being purified. The anode, being active, ionizes and ions of the metal anode enter the electrolyte. |
| Describe the application of electrolysis in the electroplating (deposition of a layer on one metal on top of another) of objects. NB. Only metals below H in the series can be used for plating e.g. gold plating, silver plating etc | The object to be electroplated is made the cathode ( it is connected to the negative terminal of the battery). The anode is a pure sample of the metal which is used for plating. |
| Describe the application of electrolysis in the anodising process (artificially thickening the aluminium oxide layer on the surface of aluminium objects). | The aluminium article to be anodized is made the anode. The electrolyte is usually a dilute sulphuric acid solution. The active anode ionizes forming Al3+. OH- reacts with the Al3+ forming a layer of aluminium oxide on the aluminium object. |
Created by:
Ms O'Farrell
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