AQA AS Chemistry Unit 2 Notes

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  • Created on: 10-03-13 01:28
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4. Redox
oxidation is the process of electron loss: reduction is the process of electron gain:
Zn Zn2+ + 2e- Cl2 + 2e- 2Cl-
It involves an increase in oxidation number It involves a decrease in oxidation number
Rules for assigning oxidation numbers
eg . Zn, Cl2, O2, Ar all have oxidation numbers of zero
1. All uncombined elements have an oxidation number of zero
2. The oxidation numbers of the elements in a compound add In NaCl Na= +1 Cl= -1
up to zero Sum = +1 -1 = 0
3. The oxidation number of a monoatomic ion is equal to the
e.g. Zn2+ = +2 Cl- = -1
ionic charge
4. In a polyatomic ion (CO32-) the sum of the individual e.g. in CO32- C = +4 and O = -2
oxidation numbers of the elements adds up to the charge
sum = +4 + (3 x -2) = -2
on the ion
5. Several elements have invariable oxidation numbers in their
common compounds.
Group 1 metals = +1
Group 2 metals = +2
Al = +3 We use these rules to
H = +1 (except in metal hydrides where it is ­1 eg NaH) identify the oxidation
numbers of elements that
F = -1 have variable oxidation
Cl, Br, I = ­1 except in compounds with oxygen and fluorine numbers.
O = -2 except in peroxides (H2O2 ) where it is ­1 and in compounds with fluorine.
Note the oxidation number of Cl
What is the oxidation number of Fe in FeCl3
in CaCl2 = -1 and not -2 because
Using rule 5, Cl has an O.N. of ­1 there are two Cl's
Using rule 2, the O.N. of the elements must add up to 0 Always work out the oxidation for
one atom of the element
Fe must have an O.N. of +3
in order to cancel out 3 x ­1 = -3 of the Cl's
Redox equations and half equations
Br2 (aq) + 2I- (aq) I2 (aq) + 2 Br- (aq)
Br2 (aq) + 2e- + 2 Br- (aq) 2I- (aq) I2 (aq) + 2 e-
Br has reduced as it has gained electrons I has oxidised as it has lost electrons
A reduction half equation only shows the parts An oxidation half equation only shows the reducing agents are
of a chemical equation involved in reduction parts of a chemical equation involved in electron donors
The electrons are on the left oxidation
The electrons are on the right
oxidising agents are
electron acceptors
The oxidising agent is Bromine The reducing agent is the Iodide
water . It is an electron acceptor ion. It is an electron donor
When naming oxidising
and reducing agents
An oxidising agent (or oxidant) is the A reducing agent (or reductant) is the
species that causes another element to species that causes another element always refer to full name
oxidise. It is itself reduced in the reaction reduce. It is itself oxidised in the reaction. of substance and not
just name of element
Copyright N Goalby Bancroft's School

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Balancing Redox equations
Writing half equations
1. Work out oxidation numbers for element being oxidised/ reduced Zn Zn2+ Zn changes from 0 to +2
2. Add electrons equal to the change in oxidation number
For reduction add e's to reactants
Zn Zn2+ + 2e-
For oxidation add e's to products
3.…read more

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Mass spectroscopy Mass spectrum for butane
The peak with the highest
When organic molecules are 43 mass/charge ratio will be normally
passed through a mass due to the original molecule that
spectrometer, it detects both hasn't fragmented (called the
the whole molecule and molecular ion) . As the charge of
fragments of the molecule the ion is +1 the mass/ charge ratio
is equal to Mr.…read more

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Spectra for
Spectra for
O-H absorptions tend ethanoic acid
to be broad
"rogue" absorptions can also occur and are indicators of impurities
The absorption of infra-red radiation by bonds in this type of spectroscopy is the same absorption
that bonds in CO2, methane and water vapour in the atmosphere do that maybe causing global
global warming.…read more

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Definition: Enthalpy change is the amount of heat energy taken in
If an enthalpy change occurs then energy is
system and
transferred between
or given out during any change in a system provided the
surroundings . The system is the chemicals
is constant and the surroundings is everything outside
the chemicals.
In an exothermic change energy is transferred from the
In an exothermic reaction the
system (chemicals) to the surroundings.…read more

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Calculating the enthalpy change of reaction, Hr from experimental data
The heat capacity of water is
General method 4.18 J g-1K-1. In any reaction
where the reactants are
1. Using q= m x cp x T calculate energy change for quantities used dissolved in water we assume
that the heat capacity is the
2. Work out the moles of the reactants used same as pure water.
3.…read more

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Hess's Law
Hess's law is a version of the first law
Hess's law states that total enthalpy change for a reaction is of thermodynamics, which is that
independent of the route by which the chemical change takes place energy is always conserved.
Using Hess's law to determine enthalpy changes of reaction from enthalpy changes of formation.…read more

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Example 6.…read more

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More complicated examples that may occur at AS
Working out Hf of a compound using bond energies and other data This is a more complicated
example of the type in example 8
elements Compound in
The H's can be
standard state
combinations of
H to turn to different data
elements into H to turn to
gaseous atoms Gaseous atoms compound into
gaseous atoms
Can be bond energies
E(Cl-Cl) Cl2 2Cl bond energies of
compound + (H to turn to
Or atomisation energies (if gas if…read more

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Collision theory
Reactions can only occur when collisions take place between particles
having sufficient energy. The energy is usually needed to break
the relevant bonds in one or either of the reactant molecules.…read more



Amazing! Thank you :)

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