Chapter 9: Redox reactions Flashcards
What is oxidation and reduction?
Oxygen is the loss of hydrogen or the gain of oxygen
Reduction is the gain of hydrogen or the loss of oxygen
Also OIL RIG
Oxidation - loss of electrons
Reduction - gain of electrons
What does [H] and [O] mean in an reaction equation?
[O] indicates oxygen added in an oxidation reaction
[H] indicates hydrogen added in a reduction reaction
Why must oxidation and reduction always occur together?
If something loses electrons, some else must gain them
If something is oxidised, something else must be reduced
They are called redox reaction, indicating that both reduction and oxidation occur
When do you wrote the signs first (-2) and when do you write it after (2-)?
Oxidation number is written with the sign first, e.g. -2, but a charge is written with the number first e.g. 2-.
What are the general rules for working out oxidation numbers?
- Treat the compound as totally ionic (if the compound is ionic then the charges on the ions are the oxidation numbers)
- The most electronegative atom in a molecule is assigned a negative oxidation number according to how many electrons it needs to gain to have a full outer shell.
- Assign oxidation numbers accordingly to give the overall charge on the molecule/ion - the sum of the oxidation numbers, taking into account signs and coefficients, is equal to the overall charge on the molecule/ion
- The oxidation number of atoms in an element is zero - so the oxidation number of oxygen in O2 is 0
- The elements in groups 1 and 2 virtually always have the group number as their oxidation number
- The maximum possible oxidation number for an element will be its group number for elements in group 1, and 2 and the group number -10 for elements in groups 13-17. It is not possible to lose more electrons than there are in the outer shell - the maximum possible oxidation number for a group 16 element is +6 because there are six electrons in the outer shell
What is the oxidation number of O and H?
O: -2
H: +1
Learn these (not the numbers but just their general names)
What is oxidation and reduction in terms of oxidation numbers?
Reduction - decrease in oxidation number
Oxidation - increase in oxidation number
For a reaction to be a redox reaction, what must it involve in terms of oxidation number?
It must involve a change in oxidation number
What is a oxidising or reducing agent?
An oxidising agent is a substance that oxidises something else.
- Oxidising agents (oxidants) oxidise other species and, in the process, are themselves reduced. An oxidising agent takes electrons away from something.
A reducing agent is a substance that reduces something else.
- Reducing agents (reductants) reduce other species and, in the process, are themselves oxidised. A reducing agent gives electrons to something.
How to quickly identified whether the half equation is oxidation or reduction?
If the e- is on the right side = oxidation
If the e- is on the left side = reduction
In half equations, what must balance?
in terms of number of atoms on both sides and the total charge on both sides
electrons lost in the oxidation reaction is equal to the number of electrons gained in the reduction reaction
What is the procedure that should be followed for balancing equations in acidic solution?
- Balance all atoms except H and O
- Add H2O to the side deficient in O to balance O
- Add H+ to the side deficient in H to balance H
- Add e- to the side deficient in negative charge to balance charge
What must you do before combining half equations to produce an overall redox equation?
Balance the electrons then cancel them out
Then subtract any duplications on left and right hand side
Overall redox equations never contain electrons - only half equations have electrons
How are metals in activity series arranged?
In order of how easily they are oxidised to form positive ions
Why are metals high up in the activity series and why are they strong reducing agents?
Metals form positive ions more easily and so are more reactive.
They lose electrons more readily and stronger reducing agents
They are strong reducing agents because they lose electrons easily for others to gain them
What do oxidising and reducing agents do?
Oxidising → takes electrons from the other reactant, usually a non-metal, fluorine is the most powerful oxidising element (powerful attraction)
Reducing → gives electrons to other reactants, usually metal, lithium is the most powerful reducing element
How is the strength of reducing and oxidising agents determined?
depending on their relative tendencies to lose or gain electrons
What determines whether there will be an reaction or not?
The reducing agent but be strong enough to reduce the other element
What is the biochemical oxygen demand aka biological oxygen demand?
BOD: is used as a measure of the quality of water. It is a measure of the amount of oxygen used by microorganisms to oxidise the organic matter in the water.
Any organic pollutants in the river water will be decomposed (oxidised) by microorganisms (aerobic bacteria) in the water and this process uses up dissolved oxygen.
The higher the BOD, the more organic waste there is in water.
What might increase the BOD?
Sewage is released into a river or lake
Tree leaves falling, animal manure, dead plants and animals, effluent from water treatment plants also contain organic matter
What does good quality river water have a BOD of?
Less than 1ppm
Water is generally regarded as unpolluted if it has a BOD lower than 5 ppm.
What is the basic principle in measuring BOD?
The basic principle in measuring BOD is to compare the initial amount of dissolved oxygen in a sample of water with the amount present when the sample has been incubated for 5 days at 20 °C.
What do voltaic cells provide us with?
a way of harnessing redox reactions to generate electricity
this is the basis of cells (batteries)
What happens when a piece of zinc is put into a solution of copper (II) sulfate?
an exothermic reaction occurs and the zinc becomes coated with copper and the blue colour of the copper sulfate solution fades
What happens when zinc metal is added to a solution containing Cu2+ ions? (in terms of electrons)
electrons are transferred from the zinc to the Cu2+ - the Cu2+ is reduced and the zinc is oxidised
What is this
a voltaic cell
What is happening in the left-hand beaker?
- zinc atoms are oxidised to Zn2+ ions
- electrons that are lost flow around the circuit to the other beaker where they are gained by Cu2+ ions to form copper
If the reaction was allowed to keep going, what would we see?
- zinc electrode getting smaller (as the zinc goes into solution as Zn2+)
- Cu eletrode getting larger (as it is coated with copper)
- the colour of the solution in the right-hand beaker becoming paler (as copper ions are converted to copper atoms)
How does the reaction differ between zinc and copper (II) ions together and when it is separated in a voltaic cell?
Zinc and copper (II) ions
* exothermic
* chemical energy (internal energy) converted to heat
In voltaic cell
* converted to electrical energy