The rate and extent of chemical change

• how is rate of reaction calculated

  • amount of reactant used or product formed / time
  • (mol/s) = moles of reactant used or product formed / time
• what are the various units for rate of reaction

  • g/s (mass)
  • cm3/s (volume)
  • mol/s (moles)
• name 3 common ways of measuring rate of reaction

  • loss in mass of reactants
  • time for a solution to become opaque
  • volume of gas produced
• describe measuring the rate by monitoring mass loss

  • place the reaction flask on a balance
  • in these reactions a gas is given off, so record the decrease in mass in time intervals
  • plot a graph of mass vs time
• describe measuring the rate by monitoring the volume of a gas

  • connect a gas syringe to a reaction flask and measure the volume of a gas formed in time intervals
  • plot a graph of volume vs time
• describe measuring the rate by monitoring the disappearance of a cross


  • take a piece of paper and mark a X on it
  • put the reaction flask on this cross
  • mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross
• how to find a rate of reaction at some time, t, from a graph of amount of reactant vs time

  • pick a point corresponding to the time, t, and find the tangent to the curve at this point
  • the tangent is the gradient of this graph, tells you how fast the reaction proceeds at this point
  • the steeper the tangent line, the faster the rate
  • gradient of tangent can be expressed in change in y values over change in x values
• state 5 factors affecting the rate of a chemical reaction

  • concentration of reactants
  • pressure of gases (volume)
  • surface area
  • temperature
  • catalysts
• what is collision theory?

  chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to activation energy)
• describe and explain the effect of increasing temperature on the rate of reaction

  • Temperature increases = faster reaction
  • as Temperature increases, kinetic energy of particles increases (more energetic collisions)
  • also, they move faster so collide more frequently
  • there is no straight line relationship between the rate and temperature, not directly proportional to one another
• describe and explain the effect of increasing concentration on the rate of reaction

  • concentration increases = faster reaction
  • more reactants = more frequent collisions
• describe and explain the effect of increasing pressure of a gas on the rate of reaction

  • increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the rate of the reaction
  • volume and pressure are inversely proportional to each other
  • increasing the volume retards the reaction
• describe and explain the effect of increasing surface area

  • if solid reactants are in smaller pieces, they have a greater surface area
  • increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction
• what is a catalyst and how does it work? how does it affect the reaction profile?

  • changes the rate of reaction but is not used up
  • increases rate by providing a different pathway for the reaction that has a lower activation energy
  • reaction profile for a catalysed reaction will have a lower maximum of the curve (lower activation energy)
• what is an enzyme?

  molecule that acts as a catalyst in a biological system
• what is a reversible reaction?


  occurs when the products of a reaction can react backwards to produce the original reactants
• when is dynamic equilibrium reached?

  in a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant
• describe Le Chatelier’s principle

  if a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium
• describe the effect of changing the concentration of reactant and product on the position on the equilibrium

  • if the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again
  • if the concentration of a reactant is increased, more products will be formed until equilibrium is reached again
  • if the concentration of a product is decreased, more reactants will react until equilibrium is reached again
• describe the effect of changing temperature on the position of the equilibrium

  if the temperature of a system at equilibrium is increased:

  • the relative amount of products at equilibrium increases for an endothermic reaction
  • the relative amount of products at equilibrium decreases for an exothermic reaction
• describe the effect of changing pressure on the position of the equilibrium

  • this applies to equilibria that involve gases
  • increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction
  • decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction
  • pressure has no effect on the reactions where the numbers of gas molecules are equal on both sides of the equation
• describe the effect of a catalyst on the position of the equilibrium


  • no effect
  • speeds up both forward and backward reactions equally 
  • equilibrium is achieved faster