Chemistry paper 2

• When measuring the rate of a reaction, we need to look carefully at the units used
• Units for rate of reaction


  • Volume in cm^3 over time in minutes
  • Time in seconds and mass in grams
• First graph shows volume of carbon dioxide being produced

  It is going up
• Second graph shows mass being lost

  It is going down
• Finding the rate at a particular point

  1. Draw a tangent

  2. Work out the gradient of the tangent
• Comparing rates of reaction at different points - the tangent is steeper at the start and shallower later on
• Ways to follow a reaction

  • Loss of mass
  • Collecting the gas
  • Color change
• Reaction between sodium thiosulfate and hydrochloric acid

  • Need to be careful about contamination, temperature, and skin irritation
• Collecting gas in an inverted measuring cylinder


  1. Careful about gas already in the cylinder

  2. Gas will move from conical flask through delivery tube
• Increasing temperature


  Increases the rate of reaction
• Reason temperature increases rate

  Particles have more energy so they can move around faster, leading to more frequent successful collisions
• Increasing surface area


  Increases the rate of reaction
• Reason surface area increases rate

  More particles available to react, leading to more successful collisions
• Increasing pressure or concentration

  Increases the rate of reaction
• Reason pressure/concentration increases rate

  More particles in a fixed volume, higher chance of successful collisions
• Catalyst


  Lowers the activation energy, making the reaction easier to happen
• Reversible reaction


  Reaction can go both ways, represented by half arrows
• Changing temperature, pressure or concentration of a reversible reaction


  Reaction will shift to compensate according to Le Chatelier's Principle
• Hydrocarbon


  Compound made up of hydrogen and carbon only
• Alkanes


  Hydrocarbons with single bonds only, general formula CnH2n+2
• Separating crude oil by fractional distillation

  1. Crude oil heated to gas, condenses at different temperatures to separate fractions

  2. Short hydrocarbons at top, long hydrocarbons at bottom
• Cracking


  Breaking down long hydrocarbons using heat and catalyst to produce shorter alkanes and alkenes
• Testing for alkenes


  Use bromine water, goes from orange to colorless
• Complete combustion of hydrocarbons

  Hydrocarbon + oxygen -> water + carbon dioxide
• Pure substances melt at a single melting point, mixtures melt over a range
• Catalytic cracking


  Gives short alkanes and alkenes
• Alkenes


  Have double bonds, indicated by two E's
• Colorless is required, clear is not enough
• If a pure substance, it melts at its melting point. If a mixture, it melts over a range of temperatures.
• Testing melting point


  Put crystals in thin tube, heat and observe through window
• Chromatography


  Separate compounds, draw start line in pencil, use lid to prevent solvent evaporation
• Rf value


  Distance moved by spot / distance moved by solvent
• Gases that may be produced in an experiment

  • Hydrogen
  • Oxygen
  • Carbon dioxide
  • Chlorine
• Tests for gases


  Hydrogen - squeaky pop

  Oxygen - relights glowing splint

  Carbon dioxide - turns lime water cloudy

  Chlorine - bleaches damp litmus paper
• Composition of modern atmosphere vs early atmosphere
• Ammonia


  Smells like hair dye or old baby nappies
• Methane


  Smells like farts
• Changes in early atmosphere

  Water vapor decreased as it rained to form oceans

  Carbon dioxide decreased as it dissolved in oceans and was locked up in rocks

  Oxygen increased with evolution of photosynthetic plants
• Main greenhouse gases


  • Carbon dioxide

  Water vapor

  Methane
• Greenhouse gases


  Absorb infrared/heat radiation, trapping it in atmosphere and warming the planet