12 — Redox Flashcards
Redox definition
A redox reaction is a reaction in which oxidation and reduction occur simultaneously.
State examples of non-redox reaction
Precipitation
Acid-base reaction
Redox in terms of oxidation state
X in compound is oxidised as x increases in oxidation state from _ in _ to _ in _. Y in compound is reduced as y decreases in oxidation state from _ in _ to _ in _. Since oxidation and reduction occurs simultaneously, this is a redox reaction.
Redox in terms of gain and loss of oxygen
X in compound is oxidised as x gains oxygen to form x compound. Y in compound is reduced as y loses oxygen to form y compound. Since oxidation and reduction occurs simultaneously, this is a redox reaction.
Redox in terms of gain and loss of hydrogen
X in compound is oxidised as x loses hydrogen to form x/compound. Y in compound is reduced as y gains hydrogen to form y compound. Since oxidation and reduction occurs simultaneously, this is a redox reaction.
Redox in terms of gain and loss of electrons
X in compound is oxidised as x lose electrons to form x compound. Y in compound is reduced as y gains electrons to form y/(compound). Since oxidation and reduction occurs simultaneously, this is a redox reaction.
Oxidation state definition
Oxidation state is the charge an atom or an element would have if it existed as an ion in a compound.
Rules of oxidation state of element
1.oxidation state of uncombined element =0
2. Oxidation state of simple ion (ion only made up of 1 element eg cation is only Na) = charge of element ion in periodic table
3. Oxidation state of H with non-metals is +1
Oxidation state of H with metals is -1
Oxidation state of H in Hydrogen gas = 0
4. Oxidation state of O in most compounds = -2
Oxidation states of O in peroxide (2O atoms) = -1
Oxidation state of O in oxygen gas = 0
5. For a compound w no net charge, oxidation state of atoms present in compound add up to 0
6. For an ion w a net charge, oxidation state of atoms present in ion add up to charge of ion
Non-redox reactions explanation
Oxidation state of x remains at _ in _ and _. Since none of the oxidation states of elements have changed, this is a non-redox reaction.
Oxidising agents
Causes the oxidation of another substance while itself is being reduced.
- acidified potassium manganate (VII) KMnO4
- potassium dichromate (VI), O2, H2O2
Reducing agents
Causes the reduction of another substance while itself is being oxidised
- potassium iodide, KI
- C, H2, reactive metals
Test for oxidising agent
Add a few drops of colourless potassium iodide to solution of unknown substance. If the unknown substance is an oxidising agent, colourless KI (aq) will turn yellow-brown. Colour change is due to I^- in KI being oxidised to form iodine (I2) solution by oxidising agent.
Test for iodine: starch solution added will turn dark blue in presence of iodine
Test for reducing agent
Adding a few drops of acidified potassium manganate (VII) to unknown solution. If unknown substance is a reducing agent, purple acidified potassium manganate (VII) turns colourless/decolourise. Colour change is due to Mn^7+ in KMnO4 reduced to form Mn^2+ by the oxidising agent.
Suggest 2 reasons why the reaction in stage 2 requires less energy [2]
Stage 2 requires less energy as less thermal energy is needed to maintain the lower temperature of 370dgC compared to 800dgC for stage 1. [1]
Stage 2 is an exothermic reaction thus less thermal energy is taken in for bond breaking compared to stage 1 which is an endothermic reaction and more thermal energy is taken in for bond breaking. [1]
Suggest reasons why the enthalpy change of combustion of octane is more negative than the enthalpy change of combustion of butane. [2]
More energy is released for bond forming in the combustion of octane due to 32 carbon-oxygen double bonds and 36 oxygen-hydrogen bonds, compared to 16 carbon-oxygen double bonds and 20 oxygen-hydrogen bonds in the combustion of butane. [2]
