Chapter 4 - Electrochemistry Flashcards
What is electrolysis?
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
What is an electrode?
Electrode is a rod of metal or graphite through which an electric current flows into or out of an electrolyte.
What is an electrolyte?
Electrolyte is the ionic compound in a molten or aqueous solution that conducts the electricity.
What is an anode?
Anode is the positive electrode of an electrolysis cell.
What are anions?
Anions are negatively charged ions which are attracted to the anode.
What is a cathode?
Cathode is the negative electrode of an electrolysis cell.
What is a cation?
Cation is a positively charged ion which is attracted to the cathode.
What forms at the cathode?
Metals and hydrogen form positively charged ions so either a metal or hydrogen gas is formed at the cathode.
What forms at the anode?
Nonmetals form negatively charged ions, so nonmetals (except hydrogen) are formed at the anode.
How do electrons flow through an electrolysis circuit?
The power supply provides the cathode with a supply of electrons, causing it to become negatively charged. Positive ions (cations) in the electrolyte more towards the cathode where they gain electrons (reduction) and negative ions (anions) in the electrolyte more towards the anode where they lose electrons (oxidation). The electrons move from the anode back to the power supply so in a complete circuit electrons are the charge carriers in the external circuit and ions are the charge carriers in the electrolyte.
Describe the electrolysis of molten(II) lead bromide
Add lead(II) bromide into a beaker and heat it so it will turn molten, allowing ions to be free to move and conduct an electrical change. Add two graphite rods as the electrodes and connect this to a power pack/battery. Turn on the power pack/battery and allow electrolysis to take place. Negative bromide ions more to the positive electrode (anode) and each loses one electron to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off. Positive lead ions more to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode.
Describe the electrolysis of concentrated aqueous sodium chloride
Brine is a concentrated solution of aqueous sodium chloride. It can be electrolysed using inert electrodes made from platinum or carbon/graphite. When electrolysed, it produces bubbles of gas at both electrodes as chlorine and hydrogen are produced, leaving behind sodium hydroxide solution. Chlorine is used to make bleach, hydrogen is used to make margarine and sodium hydroxide is used to make soap and detergents. Product at the negative electrode (cathode): the H+ ions are discharged at the cathode as they are less reactive than sodium ions. The H+ ions gain electrons (reduction) to form hydrogen gas. Product at the positive electrode (anode): the Cl- ions are discharged at the anode. They lose electrons (oxidation) and chlorine gas forms. The Na+ and OH- ions remain behind and form the NaOH solution.
Describe the electrolysis of dilute sulphuric acid
Dilute sulphuric 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 (cathode): H+ ions are attracted to the cathode, they gain electrons (reduction) to form hydrogen gas. Product at the positive electrode (anode): OH- ions are attracted to the anode, they lose electrons (oxidation) and form oxygen gas and water.
How do you test oxygen, chlorine and hydrogen gases?
If the gas produces at the anode relights a glowing splint dipped into a sample of gas, then it is oxygen. If the gas produced at the anode bleaches damp litmus paper, then the gas is chlorine. If the gas produces at the cathode burns with a ‘pop’ when a sample is lit with a lighted split, then the gas is hydrogen.
What are aqueous solutions?
Aqueous solutions will always have water present. In the electrolysis of aqueous solutions, the water molecules dissociate producing H+ and OH- ions. H2O ⇌ H+ + OH–
What forms at the anode?
Negatively charged OH- ions and nonmetal ions are attracted to the positive electrode (anode). If halide ions are present, then the halide ion is discharged at the anode, loses an electron, and forms a halogen. If there are no halide ions present, then the OH- is discharged at the anode, loses and electron and form oxygen gas.
What is the difference between a concentrated and a dilute halide during electrolysis?
If a concentrated halide is being electrolysed, the halogen forms at the anode but if a dilute halide solution is being electrolysed, oxygen is formed.
What forms at the cathode?
Positively charged H+ ions and metal ions are attracted to the negative electrode (cathode) and either hydrogen gas or a metal will be produced. If the metal ion is above hydrogen in the reactivity series, then hydrogen is produced, and bubbling will be seen at the cathode. If the metal ion is less reactive then hydrogen, that metal will be produced.
Describe the electrolysis of aqueous copper(II) sulphate using carbon or graphite electrodes
Copper ions are discharged at the cathode which gain electrons and are reduced to form copper metal. OH- ions lose electrons more readily than SO4^2- thus OH- lose electrons and are oxidised to form oxygen gas.
Describe what happens to the electrodes during the electrolysis of copper(II) sulphate using copper electrodes
The cathode increases in mass while the anode decreases in mass. This occurs as atoms are oxidised at the anode and reduced at the cathode.
What redox reactions occur at each electrode?
At the anode, negatively charged ions lose electrons and are thus oxidised. At the cathode, the positively charged ions gain electrons and are thus reduced.
What are ionic half equations?
Ionic half equations show the oxidation and reduction of the ions involved. Firstly, balance the equations and then add the electrons to the most positive side.
What is electroplating?
Electroplating is a process where the surface of one metal is coater with a layer of a different metal. The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode/the metal you want to electroplate the cathode with. The cathode is the object to be electroplated and the anode is made from the pure metal you want to coat your object with.
Why is electroplating done?
Electroplating is done to make metals more resistant to corrosion or damage and improve their appearance.
Electroplating of tin
What are hydrogen-oxygen fuel cells?
Hydrogen-oxygen fuel cells use hydrogen and oxygen to produce electricity with water as the only chemical product.
What are the advantages of hydrogen-oxygen fuel cells?
Do not produce any pollution, the only product is water.
They release more energy per kilogram than either petrol or diesel.
No power is lost in transmission as there are no moving parts.
Quieter so less noise pollution compared to a petrol engine.
What are the disadvantages of hydrogen-oxygen fuel cells?
Materials used in producing fuel cells are very expensive.
Hydrogen is more difficult and expensive to store compared to petrol as it is very flammable and easily explodes when under pressure.
Fuel cells are affected by low temperatures and become less efficient.
There are only a small number of hydrogen filling stations across the country.
Hydrogen is often obtained by methods that involve the combustion of fossil fuels, therefore releasing carbon dioxide and other pollutants into the atmosphere.
