SC10 Flashcards
SC10a
1) State the meaning of the term ‘electrolyte’
2) Outline what happens during electrolysis
3) Explain the movement of the ions during electrolysis.
1) An electrolyte is a liquid or solution that can conduct electricity.
2) When an electric current is passed through a molten ionic compound the compound decomposes or breaks down. The process also occurs for aqueous solutions of ionic compounds. The ions must be able to move and can only do so in the molten state or when dissolved in a solution, usually aqueous.
When the cell is turned on and an electric current is passed through an electrolyte the ions in the solution start to move towards the electrodes. During electrolysis the electrons move from the power supply towards the cathode. Electrons flow from the anode to the cathode.
3) The negatively charged electrode in electrolysis is called the cathode. Positively charged ions move towards the cathode. The positively charged electrode in electrolysis is called the anode. Negatively charged ions move towards the anode. The gas that is fired at each electrode depends on the solubility of the atoms. The least soluble atom is formed at the electrode.
SC10a
1) How do you write half equations for the reactions at the electrodes.
2) Explain the meaning of oxidation and reduction in terms of the movement of electrons.
3) State the electrodes at which oxidation and reduction occur.
4) Explain what is meant by term ‘inert’ when referring to electrodes
1) A half equation is used to represent the reaction that happens at an electrode during electrolysis. It shows what happens when ions gain or lose electrons. In half equations:
- electrons are shown as e-
- the numbers of atoms of each element must be the same on both sides
- the total charge on each side must be the same (usually zero)
Positively charged ions gain electrons at the cathode. These are half equations for some reactions at the cathode:
Na+ + e- → Na
Pb2+ + 2e- → Pb
Negatively charged ions lose electrons at the anode. These are half equations for some reactions at the anode:
2Cl- → Cl2 + 2e-
2O2- → O2 + 4e-
2) Oxidation is the loss of electrons, and reduction is the gain of electrons.
3) Reduction happens at the negative cathode because this is where positive ions gain electrons. Oxidation happens at the positive anode because this is where negative ions lose electrons.
4) Inert electrodes are electrodes that do not react with the electrolyte.
SC10b
State the products formed from the electrolysis of the following:
1) Copper chloride solution
2) Sodium chloride solution
3) Sodium sulfate solution
1) Copper chloride solution: When copper chloride solution is electrolyzed using copper electrodes, copper metal is deposited at the negative electrode (cathode) while chlorine gas is produced at the positive electrode (anode) and bubbles of green chlorine gas will be visible.
2) Sodium chloride solution: When sodium chloride solution is electrolyzed using inert electrodes (such as platinum or graphite), hydrogen gas is produced at the negative electrode (cathode) and bubbles of hydrogen gas will be seen, while chlorine gas is produced at the positive electrode (anode) and bubbles of green chlorine gas will be visible.
3) Sodium sulfate solution: When sodium sulfate solution is electrolyzed using inert electrodes, oxygen gas and water is produced at the positive electrode (anode) while hydrogen gas is produced at the negative electrode (cathode). This is because sulfate ions are not easily oxidized and water is reduced instead.
SC10b
State the products formed from the electrolysis of the following:
1) Acidified water
2) Molten lead bromide
1) Acidified water: When acidified water (containing sulfuric acid or hydrochloric acid) is electrolyzed using inert electrodes, oxygen gas is evolved at the positive electrode (anode) while hydrogen gas is evolved at the negative electrode (cathode).
2) Molten lead bromide: When molten lead bromide is electrolyzed using inert electrodes, lead metal is deposited at the negative electrode (cathode) while bromine gas is produce at the positive electrode (anode).
SC10b
Explain how the products are formed from the electrolysis of the following:
1) Copper chloride solution
2) Sodium chloride solution
3) Sodium sulfate solution
1) Copper chloride solution:
When copper chloride solution is electrolyzed, copper chloride (CuCl2) dissociates into copper ions (Cu2+) and chloride ions (Cl-). At the cathode (negative electrode), copper ions are reduced and deposited as solid copper (Cu). The half equation is:
Cu2+ + 2e- → Cu
At the anode (positive electrode), chloride ions (Cl-) are oxidized and release chlorine gas (Cl2). The half equation is:
2Cl- → Cl2 + 2e-
2) Sodium chloride solution:
When sodium chloride solution is electrolyzed, sodium chloride (NaCl) dissociates into sodium ions (Na+) and chloride ions (Cl-). At the cathode (negative electrode), water molecules (H2O) are reduced and produce hydrogen gas (H2). The half equation is:
2H2O + 2e- → H2 + 2OH-
At the anode (positive electrode), chloride ions (Cl-) are oxidized and release chlorine gas (Cl2). The half equation is:
2Cl- → Cl2 + 2e-
3) Sodium sulfate solution:
When sodium sulfate solution is electrolyzed, sodium sulfate (Na2SO4) dissociates into sodium ions (Na+) and sulfate ions (SO4 2-). At the cathode (negative electrode), water molecules (H2O) are reduced and produce hydrogen gas (H2). The half equation is the same as in the case of sodium chloride solution:
2H2O + 2e- → H2 + 2OH-
SC10b
Explain how the products are formed from the electrolysis of the following:
1) Acidified water
2) Molten lead bromide
1)Acidified water:
When acidified water (containing sulfuric acid or hydrochloric acid) is electrolyzed, water molecules (H2O) dissociate into hydrogen ions (H+) and hydroxide ions (OH-). At the cathode (negative electrode), hydrogen ions are reduced and produce hydrogen gas (H2). The half equation is the same as in the previous cases:
2H+ + 2e- → H2
At the anode (positive electrode), water molecules (H2O) are oxidized and produce oxygen gas (O2) and hydrogen ions (H+). The half equation is:
2H2O → O2 + 4H+ + 4e-
2) Molten lead bromide:
When molten lead bromide (PbBr2) is electrolyzed, it dissociates into lead ions (Pb2+) and bromide ions (Br-). At the cathode (negative electrode), lead ions are reduced and deposited as solid lead (Pb). The half equation is:
Pb2+ + 2e- → Pb
At the anode (positive electrode), bromide ions (Br-) are oxidized and release bromine gas (Br2). The half equation is:
2Br- → Br2 + 2e-
SC10b
1) How can you know the products formed from the electrolysis of a molten, binary, ionic compound.
2) Explain how the electrolysis of copper sulfate solution using copper electrodes can be used to purify copper.
3) How can you ensure that the metal and non metal are deposited at the electrodes?
1) The metal is formed at the cathode (negative electrode) because that is where the positive metal ions are attracted. The metal is produced at the cathode if it is less reactive than hydrogen. Hydrogen is produced at the cathode if the metal is more reactive than hydrogen.
The non-metal element is formed at the anode (positive electrode) where the negative non-metal ions are attracted. Oxygen is produced (from hydroxide ions), unless halide ions (group 7 ions, eg, chloride, bromide or iodide ions) are present. In that case, the negatively charged halide ions lose electrons and form the corresponding halogen (chlorine, bromine or iodine).
2) Copper can be purified by the electrolysis of copper sulfate solution using copper electrodes. The copper atoms in the anode lose electrons to become copper ions. These dissolve in the solution and migrate to the cathode, where they are deposited as pure copper. Impurities from the anode do not form ions and collect below the anode as a ‘sludge’.
3) The electrolysis on the molten metal with inert electrodes will deposit the metal and non metal at the electrodes, instead of the metal dissolved in a solution where oxygen or hydrogen gas could form at the electrodes.
SC10a - Core Practical
1) What is the aim of the electrolysis core practical?
2) What is the method for the electrolysis?
1) Investigate the electrolysis of copper sulfate solution with inert electrodes and copper electrodes
2) Using copper electrodes
A. Select two clean pieces of copper foil. Label one ‘anode’ and the other ‘cathode’. Measure and record the masses of the two electrodes.
B. Set up an electrolysis circuit.
C. Turn on the power and adjust the variable resistor to give a current of about 0.2A. Record the current and adjust the variable resistor to keep it constant. Leave the power on for 20 minutes.
D. Turn off the power and remove the electrodes from the beaker. Gently wash the electrodes with distilled water then dip them into propanone. Lift the electrodes out and gently shake off the propanone. Allow the remainder of the propanone to evaporate.
E. Measure and record the masses of the dry electrodes.
F. Repeat the experiment using currents of 0.3 A, 0.4A and 0.5A.
Using graphite electrodes
G. Set up the circuit using graphite rods as electrodes.
H. Turn on the power and observe what happens at each electrode.
SC10a - Core Practical
1) What are the independent, dependent and control variables for the electrolysis core practical?
2) Explain the safety precautions for the electrolysis core practical
1) Independent variable: current applied to the electrolysis cell
Dependent variables: The masses of the dry electrodes
Control variables: the type and concentration of electrolyte used, the distance between the electrodes, the size and shape of the electrodes, and the temperature and pressure of the environment.
2) - Copper(II) chloride solution - irritant – inform teacher immediately if its gets into eyes or is swallowed / wash hands after use
- Oxygen gas - oxidising
- Hydrogen gas - extremely flammable
- Chlorine gas - toxic - don’t run the experiment for too long / turn off current as soon as observations have been made OR work near an open window
- mains electricity – risk of electrocution – use low voltage / do not use equipment with exposed wires
SC10a - Core Practical
1) Explain why electrolysis would not occur if the electrodes were touching each other.
2) Explain a way to check if the electrolysis circuit is working properly.
3) Compare the results you would see if the electrolysis experiment was done with different voltage
1) Electricity will flow directly through one electrode to the other. No electricity will flow through the solution.
2) Insert a bulb or ammeter into circuit.
3) There would be the same observations and products at each electrode because the same reactant is used. However, there will be a different rate (speed) of reaction. The lower voltage will have a lower speed of reaction.
SC10a - Core Practical
1) Explain the conditions that ions must be in, in order for electrolysis to occur.
2) Why would the pH of the solution change during electrolysis?
3) How would you explain the formation of products in electrolysis for a 4 mark exam question?
1) Ions cannot flow or move to electrodes in a solid. It must be dissolved before the ions can move to the electrodes.
2) The pH of the solution rises and becomes alkaline because of OH– ions in excess as the non-metal ions are removed from the solution.
3) - hydrogen (H) and sodium (Na+) ions attracted to cathode, hydroxide (OH) ions and sulfate (SO42) ions attracted to anode
- because the ions are attracted to the oppositely charged electrode
- 2 hydrogen ions/2 H+ gain 2 e to form
hydrogen molecule/H2
- 4 hydroxide ions/4 OH- lose 4 e to form oxygen molecule/O2
- State which electrodes all of the ions are attracted to
- State that this is because ions are attracted to the oppositely charged electrode
- State which ions and how many of the ions form at the anode, and how many electrons they lose (eg. 2 hydrogen ions (2 H+ ions) lose 2 electrons form a hydrogen molecule, H2)
- State which ions and how many of the ions form at the cathode, and how many electrons they lose (eg. 4 hydroxide ions (4 OH- ions) form an oxygen molecule, O2)
SC10a
State what electrolysis means
Electrolysis is using electricity to break down ionic compounds.
SC10
What are the electrolysis rules that state what are formed at the cathode and anode?
At the cathode, copper, silver or gold is present. Anything else makes hydrogen. At the anode, chlorine, bromine or iodine is made, anything else makes oxygen.
