A2

Cards (113)

  • Enthalpy change of solution is when 1 mole of solid solute dissolves to form a solution
  • Lattice enthalpy is the enthalpy change when one mole of an ionic solid is formed from its gaseous ions. This is an exothermic quantity.
  • Enthalpy change of hydration is the enthalpy change when 1 mole of a gaseous ion is hydrated by forming bonds to water molecules.
  • The enthalpy of hydration is more negative/exothermic if the charge density of an ion is greater because it forms stronger ion-dipole bonds, so the energy released by bond forming is larger
  • Lattice enthalpy becomes more exothermic if the ions in the lattice have a greater charge density because there is a greater electrostatic attraction.
  • Enthalpy change of solution = - Lattice enthalpy + Hydration enthalpies of cation + anion
  • Explain why ionic substances are soluble in water
    • Ionic bonds are broken when the ionic substance dissolves, some hydrogen bonds also break between water molecules. Ion-dipole bonds form between water molecules and the ions. The strength of the binds formed is similar to the strength of the bonds broken, so the energy released by bond formation is sufficient to compensate for the energy required to break the bond between ions
  • Why are ionic compounds insoluble in non-polar solvents?

    In order to dissolve, ionic bonds would need to break between the ions in the lattice. ID-ID bonds would also need to break between the organic solvent molecules. They do not have a permanent dipole so only weak ion-dipole bonds could form between the ions and the solvent molecules. The energy released by bond formation is not sufficient to compensate for the energy required to break the bond between ions.
  • Rate of reaction from a concentration-time graph can be found from the gradient. The shape of these curves shows the order: 
    • Zero order = straight line and half-life decreases
    • 1st = curve and half-life is constant
    • 2nd = steeper curve and half-life increases
  • Rate-concentration graphs: 
    • Zero order = horizontal line, 
    • 1st order = straight line through the origin 
    • 2nd order = a curve 
  • Rate = k [A]m[B]n
    At a higher temperature, the rate constant increases because the concentrations are constant but the rate increases.
  • The Arrhenius equation links rate constant and activation enthalpy. As Ea increases, k decreases, so a large Ea will mean a slow rate 
    K= rate constant 
    Ea= activation enthalpy (J mol-1)
    T= temperature (K)
    R= gas constant 
    If you plot a graph of 1/T against ln k, the gradient is -Ea/R and the y-intercept is A. 
    • Ammonia is formed from N2 and H2, leaving a lone pair on the N which allow it to act as a ligand
    • Ammonia is soluble in water as it can form hydrogen bonds
    • The lone pair means NH3 is a base because it forms dative covalent bonds with protons to form the ammonium ion 
    • NO, nitrogen monoxide, is a colourless gas 
    • N2O, dinitrogen monoxide, is a colourless gas with a sweet smell 
    • NO2, nitrogen dioxide, is a brown gas with a sharp odour and is toxic
  • Nitrate (V) ions are reduced by aluminium in the presence of NaOH when heated to produce NH3 gas. 
    3NO- + 8Al + 5OH- + 18H2O —> 3NH3 + 8[Al(OH)4]-
    • Nitrogen gas to ammonia = N2 + 3H2 —> 2NH3 
    • Ammonia to ammonium ions = NH3 + H+ —> NH4+
    • Ammonium ions to nitrate (III) ions = NH4+ + O2 —> NO2- + 4H+ + 2e-
    • Nitrate (III) ions to nitrate (V) ions= NO2- + H2O
    • Nitrate (V) ions to nitrogen gas = 2NO3 + 12H+ + 10e- —> N2 + 6H2O
    • Nitrogen gas to Nitrogen oxides= x N2 + x O2
    • The phosphate-sugar backbone in DNA is formed by condensation polymerisation to form phosphate-ester links. 
    • The phosphate -OH groups always attach to the -CH2OH group and the -OH group on the adjacent carbon. 
    • The bases join via a condensation reaction too, they all have an -NH group in their stricture which loses an H.
    • The N atom bonds to the sugar, eliminating an -OH group from the sugar to form water. The base always replaces the -OH group on the carbon adjacent to the -O- atom in the ring
  • The pharmacophore is the part of a drug molecule which binds to the target receptor site and makes it biologically/medicinally active.
    You can modify the groups around the pharmacophore to make it more effective or reduce side effects
  • The molecular recognition of a pharmacophore depends on:
    • Size and shape: particular structure to fit into the receptor site
    • Bond formation: functional groups in the pharmacophore form temporary bonds with functional groups in the receptor, for example dipole-dipole, hydrogen bonding e.g amines, alcohols or carboxylic acids, ionic interactions e.g acidic and basic functional groups can donate/accept proteins and become charged
    • Orentation: only one of E/Z stereoisomers can fit
  • Amines act as bases because they accept protons to form a cation. An amine has a LP of electrons on the nitrogen atom that can form a dative covalent bond with an H+ ion. You can neutralise an amine by reacting it with an acid to make an ammonium salt: RNH2 + HX —> RNH3 + X-
    Amides are carboxylic acid derivatives that can be either primary or secondary (one of H replaced with alkyl group). They are named using the suffix amide, and secondary amides have a prefix N-alkyl-. 
  • To hydrolyse an amide: 
    • Heat with a dilute acid to form a carboxylic acid and ammonium salt (or 2ndary = salt of primary amine)
    • -CONH2 + HCl + H2O—> -COOH + NH4Cl
    • Heat with dilute alkali to get a carboxylate ion and NH3 gas (2ndary = an amine)
    • -CONH2 + NaOH —> -COO-Na+ + NH3
  • Esters can be hydrolysed to form alcohols
    • Acid hydrolysis of an ester under reflux with a dilute acid forms a carboxylic acid and an alcohol, it is a reversible reaction which needs a lot of H2O to push the equilibrium to the right 
    • Base hydrolysis of an ester produces a carboxylate salt and alcohol, it is irreversible 
  • Acyl chlorides are named using the suffix -oyl chloride, and you number the carbons from the acyl functional group (same with carboxylic acids)
  • With esters, you number the carbons out from the ester link in the middle
  • Acyl chlorides react vigorously with alcohols to form an ester and HCl, and amides to form a secondary amide and HCl
  • Polyamides are made from dicarboxylic acid and diamine monomers
    Polyesters are made from dicarboxylic and diol monomers.
  • Nylons are a type of polyamide and are named nylon-x,y. Some nylons are made from one type of monomer when molecules have both an amine and carboxylic acid group which can react with themselves in condensation polymerisation reactions, e.g nylon-6
    • solar energy reaches Earth mainly as visible and UV
    • Earth absorbs some of this energy, heating up and radiates IR
    • greenhouse gases in the troposphere absorb some of this IR, in the ‘IR window’, increasing their kinetic energy and raising the temperature
    • greenhouse gas molecules also re-emit some of the absorbed IR in all directions, some of which heats up the Earth
    • increased concentrations of greenhouse gases lead to an enhanced greenhouse effect.
  • absorption of IR by greenhouse gas molecules increases the vibrational energy of their bonds, the energy is transferred to other molecules by collisions, thus increasing their kinetic energy and raising the temperature
    • In buffer solutions the ethanoic acid only slightly dissociates to H+ and CH3COO-, but the salt fully dissociates into its ions, CH3COO- and Na+. 
    • This equilibrium forms: CH3COOH <—> H+ + CH3COO-
  • What happens when you add acid to a buffered solution?
    • the H+ concentration increases, and the extra H+ ions combine with CH3COO- ions to form CH3COOH, which shifts the equilibrium to the left, reducing the H+ concentration.
  • What happens when you add base to a buffered solution?
    • If you add base, the OH- concentration increases, so the OH- ions react with H+ ions to form water, removing H+ ions from the solution, causing more CH3COOH to dissociate to form H+ ions, shifting the equilibrium to the right. 
  • In buffer calculations we assume 2 things:
    • The salt is fully dissociated, so the equilibrium concentration of A- is the same as the initial concentration of the salt 
    • The acid is only slightly dissociated so the equilibrium concentration is the same as the initial concentration. 
  • In weak acid calculations we assume:
    • It only slightly dissociates, so [HA]>>[H+], so [HA start] = [HA equilibrium]
    • The dissociation of the acid is much greater than the dissociation of water so all the H+ come from the acid, so [H+]=[A-]
  • In Ksp calculations we have to assume that the volume of solution does not change when solid dissolves in it
  • In Kw calculations we assume:
    • Water only dissociates slightly, and there is so much water compared to the amounts of H+/OH- ions that the concentration of water is considered to have a constant value 
    • Strong bases fully ionise in water
  • We can find the concentration of MnO4- by titrating against a reducing agent like Fe2+
    • Reducing agent e.g Fe2+ solution with an unknown concentration in conical flask, add excess dilute H2SO4 to ensure enough H+ ions for reduction
    • oxidising agent in burette with known concentration e.g MnO4- ions 
  • Carrying out a MnO4- titration:
    • Add the MnO4- ions in the burette to the conical flask, manganate ions from aqueous potassium permanganate solution are purple, they are reduced to colourless Mn2+ ions by the reducing agent until all the reducing agent is used up
    • End point is when the mixture just becomes tainted by the MnO4-, 1 drop will give it a pink colour 
  • Balancing half equations 
    • write down species before/ after a reaction
    • Balance any atoms apart from oxygen/hydrogen 
    • Balance any oxygens with H2O
    • Balance any hydrogens with H+ ions (this and step above may not be needed) 
    • Balance any charges with electrons
    • In FULL ionic equations there should be NO electrons 
  • The coordination number is the number of coordinate bonds formed with the central metal ion. The complex shape is dependent on the size of the ligands and the coordination number:
    • small ligands: 6 around a central metal ion (H2O, NH3, CN-)
    • larger= fits 4 e.g Cl-
    • only 3 of ethandioate/en can fit
    • Coordination number of 6= octahedral, 90’, 4= tetrahedral 109.5’ or square planar (specific example of cis-platin which is an anti-cancer drug [Pt(NH3)2(Cl)2](aq), bond angle of 90’