Topic 6

Cards (30)

  • What is the rate of chemical reaction?
    How fast the reactants are changed into products
  • rate of reaction graphs
    steeper line = faster rate
    flat = reaction finished
    higher line = more reactants and products
  • Collision theory

    For a reaction to occur:
    - the particles must collide,
    - they must collide with the appropriate orientation,
    - and they must collide with sufficient energy.
  • Rate of chemical reactions are based on:
    How often particles collide
    Whether the particles collide with enough energy
  • Factors affecting rates of reaction
    temperature,
    concentration,
    pressure,
    surface area,
    whether there is a catalyst
  • Increase of temp on reaction rate
    - particles move faster
    - collide more frequently
    - faster they move = more energy so more successful collisions
  • increase of concentration on reaction rate
    - more concentrated = more particles
    - more likely to collide
  • increase of pressure on reaction rate
    - pressure increase = more gas molecules in same volume of gas
    - more likely to collide
  • increase of surface area on reaction rate
    - solid = broken down = larger surface area
    - increase surface area : volume ratio
    - particles have more area to work on
    - more frequent collisions
  • use of a catalyst on reaction rate
    - decrease activation energy - easier for reaction to start
  • what is a catalyst

    A substance which increases the speed of a reaction, without being changed or used up in the reaction
  • what are the ways to tell if a reaction has occurred?
    - colour change
    - form of precipitation
    - change in mass
    - gas produced
    - change in temp
  • Measuring rate of reaction: Precipitate forms
    1) if original solution is transparent you can see if a precipitate forms at the bottom of the flask
    2) done by placing an 'X' on bottom of flask - see how long it takes to disappear.
  • measuring rate of reaction: colour change
    1) you can see how long it takes for the colour to change in a reaction
  • measuring rate of reaction: change in mass

    1) Measuring the speed of a reaction that produces a gas can be carried out using a mass balance
    2) As the gas is released, the mass disappearing is quickly measured on the balance - the quicker the reading on the balance drops, the faster the reaction
    3) Measurements can be taken at regular intervals for a rate of reaction graph to easily find the rate
    4) Most accurate but the gas is released straight into the room
  • measuring rate of reaction: volume of gas produced

    1) use of gas syringe to measure volume of gas produced
    2) more gas given off in a time interval the quicker the rate of reaction.
    3) gas syringes are usually quite accurate
  • Reaction of HCl and Magnesium: practical
    1) add a set volume of DILUTE HYDROCHLORIC ACID to a conical flask - measure the mass
    2) add some MAGNESIUM RIBBON to the acid, quickly plug with cotton wool
    3) start the stopwatch and record the mass of the balance at regular intervals.
    4) plot results on a table
    5) the loss of mass is from the reaction creating Hydrogen gas

    6) you can try with different acid concentrations
  • reaction of Sodium Thiosulfate and HCl: practical
    1) add a set volume of DILUTE SODIUM THIOSULFATE to a conical flask
    2) place the flask on a piece of paper with a BLACK CROSS DRAWN
    3) add some DILUTE HCl acid
    4) watch the black cross disappear (as the yellow precipitate forms) and time how long until fully covered.

    5) can be repeated with different concentrations
  • finding reaction rate from graphs
    find the overall change

    MEAN RATE OF REACTION = CHANGE IN Y / CHANGE IN X
  • finding the reaction rate for a particular point

    - find your point
    - draw a tangent
    - calculate gradient of the tangent
  • Reversible reactions

    can go in either direction under different circumstances
  • reversible reactions at equilibrium
    At equilibrium the forward and backward reactions will be going at exactly the same rate.
    The concentration of products and reactants are balanced and will not change.
    Can only happen if the reaction takes place in a closed system.
  • What is a closed system?
    A system where neither matter nor energy can enter or leave
  • Equilibrium lies to the right
    The concentration of products is greater than that of the reactants
  • equilibrium lies to the left
    The concentration of reactants is greater than that of the products
  • What conditions effects the position of equilibrium?
    - temperature
    - pressure
    - concentration of reactants/products
  • Le Chatelier's Principal

    The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to COUNTERACT that change
  • Effect of temperature on the equilibrium
    - If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.

    - If temperature is decreased the equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat.
  • Effect of pressure on the equilibrium
    - Increasing the pressure causes the equilibrium position to shift towards the side with the smaller number of moles of gas.

    - Decreasing the pressure causes the equilibrium to shift towards the side with the larger number of moles of gas.
  • Effect of concentration on the equilibrium
    - If the concentration of a reactant is increased, equilibrium will shift towards the products
    - If the concentration of a product is decreased, equilibrium will shift towards the reactants