Chemistry p2

• Rate of reaction

  How quickly the reactants turn into the products
• Calculating rate of reaction

  1. Measure how quickly a reactant is used up

  2. Measure how quickly a product is produced
• Mean rate of reaction (reactant)

  Quantity of reactant used / Time taken
• Mean rate of reaction (product)

  Quantity of product formed / Time taken
• Measuring rate for reactions involving gas

  Measure change in mass or volume of gas given off
• Volume of gas produced

  Rate = Volume of gas produced / Time taken
• Change in turbidity or colour

  Reaction produces an insoluble solid (precipitate) making the solution cloudy
• Change in mass

  Reaction mixture placed on mass balance, mass decreases as gaseous product given off

  Rate = Change in mass / Time taken
• Calculating rate from graphs

  Steep gradient = high rate of reaction

  Shallow gradient = low rate of reaction
• Mean rate between two points in time

  Loss in mass / Change in time
• Measuring rate by disappearance of cross

  Reaction mixture placed on white paper with black cross

  Time how long it takes for cross to no longer be visible

  Shorter time = faster rate of reaction
• Collision theory


  For a reaction to occur, the reactant particles need to collide. When the particles collide, they need to have enough energy to react or they will just bounce apart. This amount of energy is called the activation energy.
• Increasing the rate of a reaction

  1. Increasing the frequency of collisions

  2. Increasing the energy of the particles when they collide
• Factors affecting rate of reaction

  • Increasing the temperature
  • Increasing the concentration of solutions
  • Increasing the pressure of gases
  • Increasing the surface area of solids
• How the condition increases the rate

  • Heat the container in which the reaction is taking place
  • Use a solution with more solute in the same volume of solvent
  • Increase the number of gas particles you have in the container or make the container smaller
  • Cut the solid into smaller pieces, or grind it to create a powder, increasing the surface area
• Why the condition has that effect

  • Particles move faster, leading to more frequent collisions
  • Particles have more energy, so more collisions result in a reaction
  • There are more reactant particles in the reaction mixture, so collisions become more frequent
  • Less space between particles means more frequent collisions
  • Only reactant particles on the surface of a solid are able to collide and react
• Catalysts

  Some reactions have specific substances called catalysts that can be added to increase the rate. These substances are not used up in the reaction. A catalyst provides a different reaction pathway that has a lower activation energy. As such, more particles will collide with enough energy to react, so more collisions result in a reaction.
• Reversible reactions

  In some reactions, the products can react to produce the original reactants. This is called a reversible reaction. When writing chemical equations for reversible reactions, use the symbol ⇌.
• If the forward reaction is endothermic

  The reverse reaction will be exothermic
• Dynamic equilibrium

  In a closed system no reactants or products can escape. If a reversible reaction is carried out in a closed system, it will eventually reach dynamic equilibrium - a point in time when the forward and reverse reactions have the same rate. At dynamic equilibrium, the reactants are still turning into the products and the products are still turning back into the reactants, but the rates of these two processes are equal, so the amount of reactants and products are constant.
• At dynamic equilibrium the amounts of reactant and product are constant, but not necessarily equal
• What happens during a reaction over time

  • Start of reaction: High frequency of collisions A+B, high rate of forward reaction, amount of A+B high
  • Middle of reaction: Decreasing frequency of collisions A+B, decreasing rate of forward reaction, amount of A+B decreasing
  • At dynamic equilibrium: Constant frequency of collisions A+B and C+D, constant rate of forward and reverse reactions, amounts of A+B and C+D constant
• At dynamic equilibrium the rate of the forward reaction is the same as the rate of the reverse reaction
• Crude oil

  Crude oil is formed from the remains of ancient biomass-living organisms (mostly plankton) that died many millions of years ago. Raw crude oil is a thick black liquid made of a large number of different compounds mixed together. Most of the compounds are hydrocarbons of various sizes.
• Alkanes

  One family of hydrocarbon molecules is called alkanes. Alkane molecules only have single bonds in them.
• First four alkanes

  • Methane
  • Ethane
  • Propane
  • Butane
• The different alkanes have different numbers of carbon atoms and hydrogen atoms. You can always work out the molecular formula of an alkane by using CH2n+2.
• Combustion

  Hydrocarbons are used as fuels. This is because when they undergo combustion, complete combustion is a type of combustion where the only products are carbon dioxide and water.
• Properties of hydrocarbons as fuels

  • Boiling point
  • Volatility
  • Viscosity
  • Flammability
• Short chain hydrocarbons

  Have lower boiling point, higher volatility, very runny (low viscosity), higher flammability
• Long chain hydrocarbons

  Have higher boiling point, lower volatility, thick (high viscosity), lower flammability
• You can check if you've drawn hydrocarbon compounds correctly since carbon always forms four bonds and hydrogen always forms one bond.
• Hydrocarbons

  • Molecules made of carbon and hydrogen only
• Combustion

  1. Hydrocarbons react with oxygen to release energy

  2. Complete combustion produces carbon dioxide and water
• Properties of hydrocarbons

  • Depend on the length of the molecule
  • Shorter chain has lower boiling point and higher volatility
  • Longer chain has higher boiling point and lower volatility
• Viscosity

  How easily a liquid flows
• Flammability

  How easily a substance burns
• Fractional distillation

  1. Crude oil is vaporised

  2. Hydrocarbon gases enter the column

  3. Gases rise up the column and cool

  4. Hydrocarbons condense at their boiling points

  5. Fractions are collected
• Products from fractional distillation

  • Fuels: petrol, diesel, kerosene, heavy fuel oil, LPG
  • Feedstock: raw material for other processes
  • Useful materials: solvents, lubricants, polymers, detergents
• Alkenes

  Hydrocarbons with double bonds between carbon atoms