Redox and Electrolysis Quiz

Redox and Electrolysis

• Redox is short for reduction-oxidation.
• An electrolytic cell converts electrical energy to chemical energy.
• Electrolysis is used to extract reactive metals from ores, electroplate metals, and refine metals like copper.
• Electrolysis involves breaking down compounds.
• Redox reactions involve simultaneous oxidation and reduction.
• Oxidation is the gain of oxygen or loss of electrons.
• Reduction is the loss of oxygen or gain of electrons.
• Roman numerals indicate oxidation numbers (e.g., copper(II)).
• Not all redox reactions involve oxygen gain/loss. Reactions like the displacement of copper by magnesium involve electron transfer.
• Oxidation involves loss of electrons.
• Reduction involves gain of electrons.
• Oxidation state: a number assigned to show the number of electrons gained or lost in ions; positive oxidation state indicates oxidation, and negative indicates reduction.).
• Oxidation causes an increase in the oxidation state.
• Reduction causes a decrease in the oxidation state.

Working with Oxidation States

• Elements have an oxidation state of 0.
• The sum of oxidation states in a compound is 0.
• The oxidation number of a monatomic ion is the same as its charge.
• Example: In CuO, oxygen has an oxidation state of -2 and copper has an oxidation state of +2.

Harder Work Example

• Manganese(VII), ion (MnO4^−) has an oxidation state of +7 for the manganese ion.
• Calculating the oxidation states requires understanding the charges on the other elements involved.

During a Reaction

• Oxidation causes an increase in oxidation state.
• Reduction causes a decrease in oxidation state.
• Example: MnO4^− → Mn^2+ (oxidation state of +7 changes to +2, meaning reduction)

Writing Redox Half-Equations

• Identify the oxidised and reduced substances.
• Write a simple equation showing the change in them.
• Balance the equations for the atoms.
• Balance the charge equation by using electrons.

Reduction and Oxidising Agents

• An oxidizing agent oxidises another substance during a redox reaction and itself is reduced.
• A reducing agent reduces another substance during a redox reaction and itself is oxidised.
• An example: in the reaction copper oxide + carbon → copper + carbon dioxide, copper oxide is the oxidising agent, and carbon is the reducing agent.

Electrolysis

• Electrolysis is the breakdown of an ionic compound by passing electricity through it (molten or in aqueous solution).
• Electrolysis occurs in an electrolytic cell.
• An electrolytic cell has a battery, two electrodes, and an electrolyte.
• The cathode is the negative electrode.
• The anode is the positive electrode.
• The electrolyte is the substance being broken down (usually an ionic compound), it must be molten or in aqueous solution for the ions to move.

Electrodes

• Electrodes are often made of graphite or platinum (inert).

Main Stages of Electrolysis

• The electrolyte is melted or dissolved in water to make an aqueous solution.
• Electrical current is passed through the electrolyte.
• Cations move to the cathode (negative electrode).
• Anions move to the anode (positive electrode).
• Reactions involving ions at the electrodes will occur.

Reactions at Electrodes (Redox)

• Electrons flow from the anode to the cathode.
• At the cathode, cations gain electrons (reduction).
• At the anode, anions lose electrons (oxidation).

Electrolysis of Molten Electrolytes

• Products produced depend on the electrolyte.
• Examples include producing lead from molten lead(II) bromide and chlorine from molten sodium chloride.

Electrolysis of Aqueous Electrolytes

• Electrolysis in aqueous solutions is more complex because water itself can be decomposed.
• The ions that are released are more easily oxidised or reduced than the water.
• This means the products are sometimes different to what you'd expect from solely focusing on the ions of the original substance.

Predicting Products

• Products formed at the cathode depend on the reactivity series.
• Products formed at the anode depend on the electrochemical series.
• Any halide(Cl, Br, I) will be preferentially oxidised over water.
• If no halide ions are present, the hydroxide ion is then oxidised to form oxygen.

Copper Refining and Purification

• Impure copper serves as the anode, pure copper is used at the cathode.
• Ions of copper leave the anode and deposit as pure copper metal at the cathode.

Application of Electrolysis (Electroplating)

• Coating one metal with another (e.g., nickel, gold, chromium).
• Electrolyte: solution of metal ion to provide the metal for plating.
• Object to be coated is the cathode.

Copper Electrolysis

• Copper electrodes can be used to refine copper. Products depend on the electrode used.
• Effect of dilution of solution will affect the products (e.g., aqueous sodium chloride).

Electrolysis Products

• The electrolysis of concentrated aqueous sodium chloride produces hydrogen, chlorine, and sodium hydroxide.
• The specific products produced at each electrode.