Mistakes

Remember to:

  • always relate answer with definition e.g. related with metallic bonding then mention electrostatic force of attraction between delocalised electrons and metal ions

  • ionic equations always think about whether the ion is spectator ion or not



Inorganic

  • Acid and Base

    • Why enthalpy of neutralisation for weaker acid is less exothermic

      • H+ ions only partially dissociated

      • some energy is used to completely dissociate the acid - some of the energy out of the originally released energy, so total enthalpy is less

    • As long as [OH-] = [H+], the solution is neutral, do not only look at pH value

    • When predicting the signs of enthalpy using Kw (Kw changes with temperature because ionisation is endothermic)- if K is positive, then temperature increases as Kw increasing which means the forward reaction is endothermic, so enthalpy positive sign

    • When finding ka from experiment, find the corresponding pH of the half volume of the equivalent point, then use Ka = [H+], so pH = pKa, Ka = 10^-pH

    • conjugate acid-base pair in a reaction label clearly/link them

    • titration curves - strong acid pH 1, strong base pH 13, weak acid pH 3, weak base pH 11

      • strong acid strong base - pH 7 centre, from 4 to 10

      • weak acid strong base - pH 9 centre, from 7 to 11 - buffer flat around pH 5

      • strong acid weak base - 5 centre from 3 to 7

      • weak acid weak base, no vertical

    • disproportionation reaction is an equilibrium reaction, so will some HA always dissociate to A-

    • why Ka for hydroxy acid is higher than e.g. ethanoic acid

      • the O of the extra OH group in hydroxy acid attracts electrons, which stabilises the anion/weakens the O-H bond in the acid so H+ lost more easily


  • Buffer

    • buffer solution always resists the change if small amount of acid/base added, weak acid+strong base, a large reservoir of weak acid and its conjugate base

    • buffer question when change added but pH stays approximately constant remember to mention so that the ratio of acid and salt is constant

    • why at half equivalence point, buffer solution is made

      • there is a large reservoir of HA and A-

      • so when add H+ to A-, HA is formed

      • add OH-, reacts with HA and A- is formed

      • so ratio of HA and A- is approximately constant, so pH is approximately constant


  • Topic 4 / Redox/ Transition Metals

    • remember that when explaining colours in complex ion, mention that the remaining light is transmitted

    • in standard cell set up, the acid used has to have 1M of [H+], so 1M of HCl, 0.5M of sulfuric acid (2 H+), but generally not ethanoic acid

    • why solution of complex ions (with water as ligand) has an acidic pH

      • the central metal ion distorts ligands’ bonds

      • this allows some water ligands to release H+ ions

    • square planar shape when the central atom bonded to four atoms and also have two un-bonded lone pairs


  • Bonding and Structure

    • in ionic compound soluble in water mention that the energy required to break apart the lattice is compensated by the enthalpy of hydration ; but ionic compound not soluble in organic liquid because the london forces that forms between ionic compound and organic liquid is weaker than the forces between ions in lattice

    • Always always always mention shielding in first ionisation energy and this outweighs the increase in proton number /nuclear number

    • ice less dense than water - hydrogen bond longer than covalent, molecules in a lattice, there are gaps between water molecules in ice; but in water, molecules are close together, water molecules fill the gaps

    • bond length

      • mention atomic radius

      • less/more shielding

      • strong/weak electrostatic force of attraction of the bonding electrons, so long/short bond length


  • Energetic

    • experimental value of lattice energy is more exothermic than the theoretical value due to presence of degree of covalent character, which is due to polarisation of ions

    • using value of K to determine whether a compound is soluble - K needs to be bigger than 1 for compound to be considered as soluble

    • Why first electron affinity of oxygen is exothermic and second electron affinity of oxygen is endothermic

      • exothermic when due to attraction between nucleus and incoming electron

      • endothermic because of repulsion between negative ion and incoming electron

    • Two assumptions used in model to calculate theoretical value of lattice energy

      • bonding is fully ionic

      • ions are perfect spheres

      • ions are point charge - charge distributed evenly across ions

      • ions in contact with each other

    • Quotient

      • calculate quotient, if quotient bigger than Kc, then equilibrium needs to shift to the left to maintain equilibrium

      • concentration of products then need to decrease and concentration of reactants need to increase as quotient = concentration of products/concentration of reactants


  • Definition:

    • d-block element: elements that have last added electron in the d orbital

    • orbital: region that holds electrons, contains a pair of electrons with opposite spins

    • lattice energy - the energy change when one mole of compound forms from its gaseous ions



Organic

  • Silver nitrate when testing for halide ions must be acidified because the acid reacts and remove carbonate ions

  • Halides displacement reactions: talk about whether they are a good oxidising agent or not e.g. iodine is a worse oxidising agent than bromine, so displacement reaction cannot occur

  • a nucleophile is a lone pair of electrons donor, a base is a proton acceptor

  • In CH3COOH, only the H in OH is ionised because there is a lack of CH polarity - not polar low electronegativity, harder to donate the H+

  • When a molecule is branched, london forces decreases because there is less surface area of close contact

  • aldehyde less yield - gets further oxidised to carboxylic acid, no gas lost because get condensed back to the flask - reflux

  • chiral molecule rotates the plane of plane polarised light whereas non-chiral molecule do not

  • acid hydrolysis is reversible, but alkaline hydrolysis is nonreversible (ester hydrolysis)

  • If concentration vs time graph shows zeroth order, rate should be constant, but if shows first order, half life should be constant - time taken for concentration to halve should be constant

  • When mention amino acids, think about zwitterions, especially when asked about boiling points of amino acids, how they have both charges

  • Honey comb thing in catalyst increases surface area so allows more gases to flow through

  • How to distinguish between two enantiomers

    • use a polarising filter and determine the direction of the angle of rotation

    • they rotate the plane of plane polarised light in the opposite direction

  • NMR - use TMS as reference, (CH3)4Si

  • halogenoalkane to alcohol uses aqueous KOH - for safety do not mention ethanolic

  • PO3- called phosphate ion

  • the mixture (NaBr + water to make HBr(this is why some Br2 is formed, orange gas) then + alcohol) is cooled in the reflux experiment before adding drop by drop potassium dichromate because the reaction is very exothermic, releases lots of heat

  • in reflux setup, there should be no gap between the round bottled flask and condenser to prevent gases escape

  • when filtering and weighing, the final mass may not be accurate that yield may be less because of incomplete reactions and handling loss e.g. mass loss on funnel; yield may be larger due to impurities on the solid

  • Grignard six marker points:

    • halogenoalkane plus mg and in dry ether

    • presence of HCl / H+ acidic work up

    • + CO2 then form carboxylic acid

    • plus methanal then forms primary alcohol

    • plus ethanal forms secondary alcohol

    • plus ketone forms tertiary alcohol

  • In mass spec to work out fragments, usually if asked to draw structure, there is only one bond can be split. But sometimes structure can split twice i.e. break two bonds

  • purifying organic liquid that is insoluble in water use a separating funnel

    • e.g. add sodium carbonate to remove any acidic impurities

    • then follow by addition of water to remove any soluble impurities

    • then add sodium sulfate as drying agent, solution turns from cloud to clear

    • then use distillation to obtain the purified liquid

  • recrystallisation

    • the greater the tendency for molecules to be adsorbed onto the surface of silca (stationary phase), the slower the movement of these molecules

  • colour of iodine - purple vapour, brown in solution, black solid

  • disadvantage of radical substitution - unwanted product made, and difficult to separate the wanted products from the unwanted products

  • Why is benzene less reactive that ethene

    • electrons are delocalised in pi bond of the ring for benzene

    • benzene is more stable

    • ethene has localised electron in double bond, double bond has high electron density

    • so more susceptible for electrophilic to attack

  • In experiment trying to find the rate of reaction - which order reactants are, how to ensure that when trying to find order for one reactants, the others stay the same

    • by using large volume of the others relative to the wanted reactant

  • why is sodium hydrogen carbonate used in experiment of finding orders

    • to neutralise any H+ acids and to quench the reaction