Reaction rates and equilibrium

• What is meant by rate of reaction?

  The rate of a chemical reaction measures how fast a reactant is being used up or how fast a product s being formed. It can be defined as the change in concentration of a reactant or a product in a given time
• rate= concentration/time
• units = mol dm-3/s

  mol dm-3 s-1
• The rate of reaction is fastest at the start of the reaction, as each reactant is at its highest concentration.

  The rate of reaction slows down as the reaction proceeds, because the reactants are being used up and their concentrations decrease.

  Once one of the reactants has been completely used up, the concentration stops changing and the rate of reaction is zero
• What are the factors that can affect the rate of a chemical reaction?

  concentration (pressure when reactants are gases)

  temperature

  use of a catalyst

  surface area of solid reactants
• What does collision theory state?

  Two reacting particles must collide for a reaction to occur.
• When will a collision be effective?

  If two conditions have been met: the particles collide with the correct orientation and the particles have sufficient energy to overcome the activation energy barrier of the reaction
• How does increasing the concentration affect the rate of reaction?

  When the concentration of a reactant is increased, the rate of reaction generally increases. An increase in concentration increases the number of particles in the same volume. The particles are closer together and so collide more frequently. In a given period of time, there will be more effective collisions (correct orientation and sufficient energy) and an increased rate of reaction
• How does increasing the pressure of a gas affect the rate of a reaction?

  When a gas is compressed into a smaller volume the pressure of the gas is increased and the rate of reaction increases. The concentration of the gas molecules increases as the same number of gas molecules occupy a smaller volume. The gas molecules are closer together and collide more frequently, leading to more effective collisions in the same time
• What are the methods for following the progress of a reaction?

  monitoring the removal (decrease in concentration) of a reactant

  OR

  Following the formation (increase in concentration) of a product
• How do you choose a method of following the progress of a reaction?

  Dependent on the properties and physical states of the reactants and products in the reaction.
• What other measurable properties might change as the reaction proceeds?

  gas volume, mass of reactants or products, colour
• What two methods can be used to determine the rate of reaction, if a reaction produces a gas?

  1. monitoring the volume of gas produced at regular intervals using gas collection
  2. monitoring the loss of mass of reactants using a balance
• volume of gas produced and mass loss are both proportional to the change in concentration of a reactant or product - therefore change in volume with time or mass loss with time both give a measurement of rate of reaction
• Monitoring the production of gas using gas collection

  1. hydrogen peroxide is added to the conical flask and the bung is replaced
  2. initial volume of gas in the measuring cylinder is recorded
  3. manganese dioxide MnO2 is then quickly added to the conical flask and the bung replaced. Stop clock started
  4. Volume of gas produced in the measuring cylinder is recorded at regular intervals until reaction is complete
  5. the reaction is complete when no more gas is produced
  6. plot a graph of total volume of gas produced against time (x).
• How do you calculate initial rate?

  draw a tangent at t=0

  gradient of the tangent gives the reaction rate