redox + electrochem Flashcards
what is oxidation
- gain in oxygen
- loss of hydrogen
- loss of electrons
- increase in oxidation state
what is reduction
- loss of oxygen
- gain in hydrogen
- gain in electrons
- decrease in oxidation state
what is oxidation state
the charge an atom or element would have if it existed as an ion in a compound
how to determine oxidation state
- atoms of the same element have an oxidation state of zero
- group 1 and 2 elements in their compounds have oxidation states of +1 and +2 respectively
- hydrogen has an oxidation state of -1 when it is bonded with metals
- oxygen has an oxidation state of -1 in peroxides
how to structure which substance is oxidised and reduced questions
(compound) is oxidised as the oxidation state of (element) increases from _ in (compound) to _ in (product)
(compound) is reduced as the oxidation state of (element) decreases from _ in (compound) to _ in (product)
what is an oxidising agent
oxidise other substances and reduce themselves
potassium iodide is used to test for oxidising agents. it will turn from colourless to brown if positive
what is a reducing agent
reduce other substances and oxidise themselves
acidified potassium manganate(VII) can be used to test for reducing agents. solution will turn from purple to colourless if positive
what is electrolysis
the conduction of electricity through an electrolyte which leads to chemical changes
what is an electrolytic cell
device that converts electrical energy into chemical energy
why must electrolytes be in molten or aqueous state?
solids have a giant ionic lattice structure where strong electrostatic forces of attraction between oppositely charged ions hold them in fixed positions, hence there are no mobile ions to act as mobile charge carriers to conduct electricity
components of electrolytic cells
external circuit: electrons flow from negative terminal to positive terminal
electrolyte: movement of ions inside completes the circuit
negative electrode (cathode): attracts cations which gain electrons and are reduced
positive electrode (anode): attracts anions which lose electrons and become oxidised
advantages and disadvantages of graphite as an electrode
advantages: inert (will not take part in electrolysis reaction), high melting point, will not melt during electrolysis of molten binary ionic compounds
disadvantages: reacts with oxygen gas under high temperatures to produce carbon dioxide, hence anode will shrink over time and need to be replaced
advantages and disadvantages of platinum as an electrode
advantages: inert, does not take part in the electrolysis reaction
disadvantages: lower melting point than graphite, might melt when used in the electrolysis of molten binary ionic compounds (→ mainly used in the electrolysis of aqueous electrolytes)
why are inert electrodes used?
prevent the contamination of products during electrolysis
when will there be a selective discharge of ions from electrolytes?
aqueous solutions because the water that the solute is dissolved in also contains its own hydrogen and hydroxide ions due to the reversible ionisation of water molecules
cation electrochemical series in order of increasing ease of discharge of cation
potassium > sodium > calcium > magnesium > zinc > iron > lead > hydrogen > copper > silver
why is it that less reactive ions discharge easier?
the less reactive the metal, the less stable it is and the easier it is for it to be converted back to a metal
anion electrochemical series in order of increasing ease of discharge of anion
sulfate > nitrate > chloride > bromide > iodide > hydroxide
what happens in a concentrated aqueous solution
the ease of discharge of cl- br-, and i- anions increases and this overrides the electrochemical series (hence these anions will be selectively discharged over oh- ions)
how to predict the products of electrolysis
- identify ions present in electrolyte
- determine the anion discharged at the anode
- determine the cation discharged at the cathode
- write the overall equation for the reaction
- identify the anion and cation left behind to determine the products in the electrolyte
half equations for water at anode
(alkaline electrolytes) 4OH- (aq) → O2 (g) + 2H2O (l) + 4e-
(acidic and neutral electrolytes) 2H2O (l) → O2 (g) + 4H+ (aq) + 4e-
half equations for water at cathode
(acidic electrolytes) 2H+ (aq) + 2e- → H2 (g)
(alkaline and neutral electrolytes) 2H2O (l) + 2e- → H2 (g) + 2OH-
overall equation for discharge of H+ and OH- ions from water
2H2O (l) → 2H2 (g) + O2 (g)
where is electrolysis used in industries
metal purification
- place impure metal at anode, pure metal will be deposited at cathode
- impurities fall to the bottom to form anode slime
- anode decreases in mass and cathode increases in mass
electroplating
- place object to be plated at the cathode, and plating metal at the anode
- cathode is coated with a layer of the metal and increases in mass
- electrolyte is an aqueous solution of the salt containing the plating metal
