Chem - 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


  • Look at loss of mass
  • Collect the gas produced
  • Look at color change
• Reaction between sodium thiosulfate and hydrochloric acid

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


  Potential error from gas already in the cylinder
• Increasing temperature


  Increases the rate of reaction
• Increasing surface area


  Increases the rate of reaction
• Increasing pressure or concentration

  Increases the rate of reaction
• Catalyst


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


  Reaction can go both ways, endothermic and exothermic
• Le Chatelier's Principle - a system will shift to counteract changes in conditions
• Hydrocarbon


  Compound made up of only hydrogen and carbon
• Alkanes


  Hydrocarbons with only single bonds, formula CnH2n+2
• Fractional distillation of crude oil

  Heats up crude oil to gas, condenses at different temperatures to separate components
• Cracking long hydrocarbons produces useful shorter alkanes and alkenes
• Test for alkenes


  Bromine water turns from orange to colorless
• Complete combustion of hydrocarbons produces water and carbon dioxide
• Melting point


  Pure substances melt at a single point, mixtures melt over a range
• Testing melting point


  Put crystals in a thin tube and observe in a melting point apparatus
• Catalytic cracking


  Gives short alkanes and alkenes
• Alkenes


  Have double bonds, indicated by two E's
• Testing for alkenes


  Use bromine water, goes from orange to colorless
• Colorless is required, clear is not enough
• Complete combustion of hydrocarbons

  Hydrocarbon + oxygen -> water + carbon dioxide
• Melting point of pure substance is at a single temperature, melting point of mixture is over a range
• Melting point determination


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


  Draw start line in pencil, end of paper just in solvent, cover to prevent evaporation
• Rf value


  Distance moved by spot / distance moved by solvent
• Gases that can be produced in experiments

  • 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 radiation, trapping heat in atmosphere and warming planet