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.
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.
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
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.
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.
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
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)
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.
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.
Organic Synthesis & Analysis
Relationships between Chemical Functional Groups:
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
Mass Spectrometry / Infa-red spectroscopy
vaporisation --> Ionisation --> Acceleration --> Deflection --> Detection
some compounds fragment to form different species : M+• → X+ + Y•
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.
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 polymers. The 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.