chem 1 xo

• covalent bondis electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms

• covalent structure types


  simple molecular I2 N2 CO2

  giant covalent SiO2
• simple molecular structures


  N2 I2 CO2

  MOLECULES held by weak intermolecular forces

  (Atoms within molecule held by strong covalent bonds)

  low m.p and b.p
• water has a concentration of 56 mol dm-3

  mol in water is mass/mr = 1000/18 roughly 56mol

  56moldm-3

  1dm3 of water is 1000g
• why would actual yield be less than theoretical yield?

  • incomplete reaction
  • reaction is reversible
  • side reactions
  • impurities
  • loss of reactants in handling/experimental methods
• the H-O-H bond angle in ice is about 109 but about 105 in gaseous H2O

  explain why there is a difference

  109.5 tetra 4 bond pairs

  104.5 non linear 2 bond pairs 2 lone pairs

  ice can form more hydrogen bonds

  water (H2O) has 2 bond pairs and 2 lone pairs

  lone pairs repel more than bond pairs
• explain why CF4 has polar bonds but NOT an overall dipole

  F is more electronegative than C so bond is polar

  BUT CF4 is tetrahedral so dipoles cancel out
• non symetrical

  difference in charge across

  overall dipole

  POLAR

  i.e. NON-LINEAR

  permanent dipol dipole interaction
• symmetrical

  effect of permanent dipole dipole is cancelled out

  linear

  trigonal planar

  tetrahedral

  octahedral

  NON POLAR
• partial pressure =


  mole fraction x total pressure
• 1st order wrt H+. At a pH of 1, the initial rate is 2.4x10^-3

  what is the initial rate at a pH of 3?

  10^3-1 = 10^2

  2.4x10-3/10^2
• how to reduce random error in titration

  Repeat titration until (two) titrations are concordant / within 0.1 cm3 (1) Calculate mean titre from concordant titres (1)
• salt


  i A hydrogen ion (of an acid) has been replaced by a metal ion ✔
• base vs alkali


  Base: A substance which readily accepts H+ ions (from an acid) ✔ Alkali: releases OH– ions into (aqueous) solution ✔
• assumptions of ideal gas equation

  molecules are in constant random motion in straight lines

  all collisions are perfectly elastic (all kinetic energy is maintained)

  pressure is caused by collisions between molecules and walls of container
• features of dynamic equilibrium

  • rate of forward = rate of reverse
  • closed system
  • @ equilibrium the conc. of the reactants and products remain constant (they will change if the conditions change)
• methyl orange in acid

  red
• methyl orange in alkali

  yellow
• methyl orange endpoint


  orange
• phenolphthalein in alkali


  pink
• phenolphthalein in acid


  colourless (silent but deadly)
• if manganate is in the burette then the endpoint of the titration will be the first permanent pink colour

  colourless=> purple
• reducing uncertainty in a titration


  replace measuring cylinders with pipettes and burettes which have lower apparatus uncertainty

  increase titre volume, done by increasing vol and conc of conical flask

  OR reducing the conc of the burette
• reducing uncertainties in measuring mass

  using balance with more decimal places

  using larger mass of solid

  weigh change in mass
• common errors in measuring enthalpies of combustion using Flame calorimetry

  • incomplete combustion of fuel
  • evapouration of fuel after weighing
  • measurements not carried out under standard conditions
  • heat capacity of calorimeter not included
• strongest oxidising agent is usually...

  bottom left (most positive as it is reduced)
• strongest reducing agent is usually...

  top right (most negative as it is oxidised)
• calorimetry


  process of measuring how much heat energy is taken in or given out during a chemical reaction
• group 2 solubility 


  increases down the group

  
  

  pH increases
• magnesium hydroxide use


  ingestion tablets to neutralise stomach acid
• calcium hydroxide use


  increase pH of acidic soil
• standard electrode potential


  voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode

  voltage measured is E.M.F electromotive force
• oxidation: Fe2+ => Fe3+ + e-

  reduction: 8H+ + MnO4- + 5e- => Mn2+ 4 H2O

  overall: 8H+ + MnO4- + 5Fe3+ => Mn2+ + 4H2O +5Fe2+ (purple to colourless)
• Oxidation: Fe2+ => Fe3+

  Reduction: Cr2O7^2- + 14 H+ + 6e- => 2Cr3+ + 7H2O

  Overall: 6Fe3+ + Cr2O7^2- + 14H+ => 6Fe2+ + 2Cr3+ + 7H2O
• colour of cr2o7^2-


  orange
• colour of Cr3+


  green
• colour change Fe3+ reduced by i- (KI)


  orange/brown to brown (fe2+ is green but brown of I2 overpowers)
• ligand substitution of haemoglobin

  oxygen ligands weakly bonded to iron

  can be broken off easily for transport to cells

  BUT can be replaced by CO, CO binds more strongly

  TOXIC
• cu2+ reduced by I- observation

  CuI white ppt

  I2 brown
• heterogenous catalysts transition metals


  haber process Fe

  hydrogenation of alkenes Ni

  catalytic converter Pt

  decomposition of hydrogen peroxide MnO2