Rates and Equilbira

• What is the rate of a chemical reaction?

  The speed at which reactants are changed into products
• The speed at which the reactants are changed into products


  mean rate of reaction = quantity of reactant used / time taken , mean rate of reaction = quantity of product formed / time taken
• What units are used to measure the quantity of reactant or product?

  Mass in grams (if it is a solid), or volume in cm3 (if it is a gas)
• What units can be used to represent the rate of reaction?


  Grams per second (g/s) or cubic centimetres per second (cm3/s)
• What is ‘collision theory’?

  Chemical reactions only occur when the reacting particles collide with each other with sufficient energy
• What is the ‘activation energy’?

  The minimum energy particles need to collide with to react
• What four factors can affect the rate of reaction?

  Temperature, concentration or pressure, surface area, use of a catalyst
• What happens to the rate of reaction if the temperature is increased?

  Increases
• What happens to the rate of reaction if the temperature is increased?


  There are more successful collisions because the particles have more energy

  There are more frequent collisions because they are moving faster
• What happens to the rate of reaction if concentration or pressure is increased?

  Increases
• Why does the rate of reaction increase if the concentration or pressure of the reactants is increased?


  There are more frequent collisions because there are more particles in the same volume
• What happens to the rate of reaction if the surface area of the reactant is increased??

  Increases
• How can you increase the surface area of a reactant?

  Cut it into smaller pieces
• Why does the rate of reaction increase if the surface area of the reactant is increased?


  There are more frequent collisions because there is a higher surface area to volume ratio meaning there are more particles exposed
• What is a catalyst?


  A substance used to speed up a chemical reaction
• Why does using a catalyst increase the rate of reaction?

  They provide an alternative pathway with a lower activation energy
• What happens to a catalyst during a reaction?


  Nothing, they are not used up
• An exothermic reaction before and after a catalyst has been used

  
• What are three different ways we can measure the rate of a reaction?

  Time how long it takes for the colour of a solution to change, or for a substance to lose mass, or to collect gas in a gas syringe
• Why might the colour of a solution change during a reaction?

  If one of the products of the reaction is a precipitate (a solid)
• What would be plotted on the axis of a graph if you were recording the volume of gas produced at regular time intervals?

  Time on the x-axis, volume of gas on the y-axis.
• A graph showing the volume of gas produced over time during a chemical reaction

  
• How do we tell when the reaction has stopped on a rate of reaction graph?

  The line becomes horizontal (the line is flat - no more gas is produced)
• How can you calculate the rate of a chemical reaction at a certain point, from a graph?

  The gradient of the graph at that point
• What is a reversible reaction?

  A reaction in which the products of the reaction react to produce the original reactants
• When does a reversible reaction reach equilibrium?

  When the forward and reverse reactions occur at exactly the same rate
• What is needed for equilibrium to be achieved in a reaction?

  A closed system - none of the reactants can escape, and nothing else can get in
• What happens to the concentration of the products if the equilibrium of a reaction lies to the right?

  The concentration of products is greater than that of the reactants
• What happens to the concentration of the products if the equilibrium of a reaction lies to the left?

  The concentration of the products is less than that of the reactants
• What factors can change the position of equilibrium?

  Temperature, pressure, changing the concentration of reactants or products
• What sort of energy transfers take place in a reversible reaction?

  If it is exothermic it goes in one direction (gives out energy), if it is endothermic it goes in the opposite direction (takes in energy)
• What happens to the total amount of energy in the forward and backward reaction in a reversible reaction?

  It remains the same
• What is Le Chatelier’s Principle?

  If you change the conditions of a reversible reaction at equilibrium, the system will move to counteract that change
• What happens to the reaction’s equilibrium if the temperature of the reaction is decreased?

  It will move in the exothermic reaction to produce more heat
• What happens to the reaction’s equilibrium if the temperature of the reaction is increased?

  It will move in the endothermic direction to try to decrease the temperature
• What happens to the reaction’s equilibrium if the pressure of the reaction is decreased?

  It will move towards the side where there are more molecules of gas
• What happens to the reaction’s equilibrium if the pressure of the reaction is increased?

  It will move towards the side where there are less molecules of gas
• What happens to the reaction’s equilibrium if the concentration of the reactants is increased?

  The position of equilibrium moves to the right, so the reaction makes more products and reduces the concentration of reactants
• What happens to the reaction’s equilibrium if the concentration of the products is increased?

  Equilibrium moves to the left, so products react and increase the concentration of reactants
• Collecting gas

  1. Use a measuring cylinder to measure out 50cm3 of 0.5M hydrochloric acid
  2. Pour this into a conical flask
  3. Add 5g of calcium carbonate small chips and immediately attach a bung connected to a gas syringe
  4. Start a timer at the same time as the calcium carbonate is added
  5. Swirl the conical flask
  6. Record the volume of gas produced every 10 seconds until no more gas is produced
  7. Repeat these steps and calculate a mean
  8. Repeat these steps using the same volume of hydrochloric acid at different concentrations and 5g of calcium carbonate small chips