# AQA A2 Chemistry Unit 4

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• Created on: 10-06-13 14:01

## Rates of Reaction

Rate of reaction- change of concentration of any of the reactants or products per unit time.

Rate Expression-  X+Y --> Z  , [x][Y] proportional to rate, this means that if the concentration                     of  X or Y was doubled so would the rate. If the rate equation was [X][Y]^2 then                         doubling the concentration of Y would quadruple the rate. If doubling a  species                     has no affect on the rate that species is said to be zero order.

Determining rate equation- find initial rate at t=0 & compare initial rates & initial concs

Rate Constant- rate expression can be written rate = K[X][Y], where K is the rate constant.

Effect of temperature on K- K increases with temperature.

Order of reaction- the sum of the orders of all of the species, e.g. if the equation was                                             rate = K[X][Y]^2 then the order of the reaction would be 3.

Rate determining step-  In a multi step reaction, steps nearly always follow after each other so products from 1 step are starting materials of the next. So the rate of the slowest step (rate determining step) may govern the rate of the whole process.

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## Chemical equilibrium

Equilibrium constant Kc - for equation A+B <=> C+D , Kc = [C] [D]      when the reaction                                                                                                [A] [B]       is at  equilibrium.

Calculating Kc -  At start:         A          +         B         <=>          C           +           D                                                         0.1mol            0.1mol                        0.1mol                 0.1                       At Equilibrium:           0.03mol          0.03mol                     0.07mol               0.07mol            Then divide moles by volume of system to find the concentrations and put into Kc equation.

Effect of changes conditions on Kc: temperature- For exothermic reaction increasing temp                                           decreases Kc and move equilibrium to the left, for endothermic reaction                                         increasing temp increases Kc and move equilibrium to the right.          Concentration- Kc stays the same as it is only affected by temperature. But the equilibrium                                would move.

If Kc > 1 equilibrium is to the right, if Kc < 1 equilibrium is to the left.

Catalysts and Kc- Have no effect on position equilibrium or Kc but the rate would change as                                     both forward and backward reactions are affected equally.

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## Acids & Bases / pH Scale

Acid- proton donator

Base- proton acceptor

water can form both an acid or a base as it can form H+ ions and OH- ions as well as H3O+ ions when a proton is donated.

Monoprotic- One mole of acid will release one proton so one mole of acid produced onle mole                    of H+ ions. H+ concentration is the same as the acid concentration.

Diprotic- Each mole of acid produces two protons when it dissociates and so one mole of acid           produces two moles of H+ ions. H+ concentration is twice teh concentration of the acid.

Ionic product of water Kw- it is equal to Kc x [H2O]  and it is a constant (1.0x10^-14)                                                     Kw = [H+][OH-] and in pure water Kw = [H+]^2 as each water molecule                                       splits into one H+ ion and one OH- ion.

pH- defined as -log[H+]     ([H+] = 10^-pH)                                                                                       pH can change with temperature

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## pH calculations with strong acid and alkaline solu

Finding [H+] from pH- pH = -log[H+] ,  let pH = 2.00                                                                                                                        -2 = log[H+]  ,                                                                                                                                                 take antilogs of both sides.

Finding [OH-] from pH-  find [H+] (same as above),                                                                                                                          we know [H+][OH-] = 1.0 x 10 ^-14  , let [H+] = 1.0x10^-10                                                                    1.0 x10^-14 = 1.0x10^-10 X [OH-]

Finding pH of strong acid solutions-strong acids dissociate completely so [H+] =                                                                   concentration of acid.                                                                                                                                   let concentration = 0.2 ,                                                                                                                               pH = -log[H+] = -log[0.2]

Finding pH of alkaline solutions- calculate [OH-] (same as concentration of solution)                                                                             use 1.0x10^-14 = [H+][OH-] to find [H+]                                                                                                     put [H+] into pH equation.

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## pH calculations with weak acids and bases

Weak acids and bases dont fully dissociate into ions,                                                                                                 e.g. CH3COOH <=> H+  +  CH3OO-                                                       Before dissociation:             1000                0               0                                                                  At Equilibrium:              996                 4               4

Dissociation of weak acids:   HA <=> H+  +   A-  therefore Kc =  [H+] [A-]                                                                                                                                     [HA]                                                                                 Kc is sometimes called Ka for wak acids.

calculating pH of weak acids (this is sometimes called pKa instead of pH)-                                                                      [H+] = [A-] therefore Ka = [H+]^2                                                                                                                                    [HA]                                                                                                  use values of Ka and [HA] given to find [H+].                                                                              put value of [H+] into pH equation.

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## Titration curves & Indicators

Titration curves- used to find the concentration of an alkali by using a known concentration of                               acid.                                                                                                                                       measure out some acid of known concentration using a pipette and putting it in                           a flask with an appropriate indicator. Titrate the alkali into the acid and record                               the  amount of alkali need to neutralise the acid.

Indicators:methy orange - red to yellow from low pH to high pH                                                                   phenolphthalein - colourless to pink from low pH to high pH

for strong acid/strong alkali - can use either - rapid pH change over the range for both indicators     for strong acid/weak alkali - only methy orange will do, the pH changes rapidly across the                                                     range  for methyl orange.                                                                 for weak acid/strong alkali - use phenolphthalein                                                                         for weak acid/weak alkali - no sharp pH change - so neither will work

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## Acidic & Basic Buffers

Buffers - solutions that resist change in pH when small amounts of acid or alkali are added.                       A buffer doesn't stop the pH from changing completely only it does makes the pH                         change slightlyso they only work for small amounts of acid or alkali.

Acidic Buffers- made from a weak acid and one of its salts.                                                                              Adding an alkali: alkali reacts with HA so initially move the equilibrium to the                                                     right (HA + OH- --> H2O + A-). This gets rid of the extra OH-                                                       so pH remains almost the same.                                                                        Adding an acid: initially the equilibrium move to the left as the added H+ ions                                                     react with the A- ions to form undissociated HA, however the                                                       suppy of A- soon runs out and the solution is still acidic. By                                                       adding more of an soluble salt that fully dissociates and so                                                         there are more A- ions that can be used up.

Basic Buffers- made from a weak base anad the salt of that base.                                                                   e.g. a buffer solution of ammonia and ammonium chloride                                                           The ammonia removes H+ ions (NH3 + H+ --> NH4+)                                                                 The ammonium removes OH- ions (NH4+ + OH- --> NH3 + H2O)

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## Buffer Calculations : calculation pH

Ka = [H+][A-]                               pH = -log[H+]                                                                                [HA]

E.g. Calculate the pH of the buffer formed when 500cm^3 of 0.400 moldm^-3 NaOH is added to 500cm^3  1.00 moldm^-3 HA. Ka = 6.25 x 10^-5 .

Moles of HA: mol = concentration x volume                                                                                                         = 1.00 x 500/1000 = 0.500 mol                                                                        Moles of NaOH: mol = 0.400 x 500/1000 =0.200 mol

Equation: HA + NaOH --> H2O + NaA                                                                                         initial:    0.5        0.2         0         0                                                                                           final:      0.3         0                    0.2

Final volume: 1000cm^3                                                                                                 Final Concentrations: [HA] = 0.300 moldm^-3                                                                                                            [A-] = 0.200 moldm^-3                                                                          Then put these two values and the value given for Ka into the Ka equation at the top to find the       H+ concentration and then put this value into the pH equation.

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## Polymerisation

Addition Polymers: made from monomers which contain a carbon-carbon double bond (normally                                an alkene). Addition polymers are not biodegradable as they have a strong                                carbon backbone.

Condensation Polymers: made from monomers which contain OH , H or Cl groups. when the                                             two molecules are condensed a water molecule is lost. they can be                                             broken by hydrolysis.

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## Organic Synthesis & Analysis

Relationships between Chemical Functional Groups:

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## Organic Synthesis & Analysis

• Adding CN- will increase the carbon chain by one carbon
• Alcohols can be converted to alkenes by passing their vapors over aluminium oxide or by acid-catalysed elimination reactions.
• Aromatic reactions:  Acyl-subed benzene <-- Benzene --> Nitrobenzene --> Phenylamine
• Chemical Reactions:                                                                                                                - If the compond is acidic it suggests a carboxylic acid, if the compound is basic it                    suggests an amine.                                                                                                          - If it dissolves it suggects the compond has polar groups.

Functional Group                             Test                                                 Result                Alkene  -C=C-                   shake w/ bromine water                       Red-Brown colour dissapears   haloalkane R-X             1) add NaOH & heat, 2) Acidify                    precipitate of AgX formed                                                w/ HNO3, 3) add AgNO3.                                                                   Alcohol R-OH                    add acidifyed K2Cr2O7                         orange--> green w/ primary &                                                                                                          secondary alcohols           Aldehyde  R-CHO             add Fehlings/Tollens solution                red ppt formed or silver mirror   Acyl chloride  R-COCl                     add AgNO3                               vigorous reaction, white ppt

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## Mass Spectrometry / Infa-red spectroscopy

Mass spectrometry:

vaporisation --> Ionisation --> Acceleration --> Deflection --> Detection

some compounds fragment to form different species : M+• → X+ + Y•

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Infa-red Spectroscopy:

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## nmr spectroscopy

nmr gives information about carbon-13 and H atoms in a molecule

Carbon-13 nmr- Carbons with different adjacent species will be affected differently.                                              Carbon-13 nmr is simpler than proton nmr

Chemical shift-  chemical shift is a measure of the movements caused by shielding and                                       deshielding, chemical shift is measured relative to an internal standard this                                   is tetramethylsilane (CH3)4Si, TMS it has 12 equivalent H atoms and so                                     produces a single intense peak its peak is upfield from nearly all other H atom                             peaks it is non-toxic and inert and easily vapourised.

Peaks- the number of peaks shows the number of different H atom environments, the area under               the peaks shows the number of equivalent H atoms, the position of the peaks shows                     information about the position of H atoms in the molecule using the H NMR shift data.

N+1 Rule: N hydrogens on an adjacent carbon atom will split into N+1 smaller peaks.

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## Chromatography

Stationary Phase: Thin layer of liquid or solid coated inside the capillary tubing. The liquid used in gas chromatography is usually a long-chain alkane because they have a high boiling point and wont evaporate.The solid used in gas chromatography include silicone polymersThe stationary phase depends on what compound is being tested.

Mobile Phase: A carrier gas which moves through the column. The gas is unreactive such as helium or nitrogen so the components of the mixture being tested in gas chromatography wont react with the mobile phase.

Column Chromatography-  The stationary phase is normally a powder, this is packed into a narrow tube and a solvent is added at the top. as solvent runs down column the components of the mixture move down at different rates. these separated parts of the component can then be collected separately.

Gas-Liquid Chromatography- Separates volatile components in a mixture. Useful for organic compounds with a low boiling-point & evaporate easily. Takes place in a gas chromatograph.  Measures retention time(time taken for component to pass from column inlet to detector). Takes place in a controlled oven.

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