C6 The Rate And Extent Of Chemical Change

• Rate of reaction


  Amount (e.g. grams, cm3) of reactant used or product formed / time
• Rate of reaction (mol/s)


  Moles of reactant used or product formed / time
• Units for rate of reaction

  • g/s
  • cm3/s
  • mol/s
• Common ways of measuring rate of reaction

  • Loss in mass of reactants
  • Volume of gas produced
  • Time for a solution to become opaque
• Measuring rate by monitoring mass loss

  Place the reaction flask on a balance. In these reactions (e.g. metal carbonate + acid) a gas is given off, so record the decrease in mass in time intervals (note hydrogen is too light). Plot a graph of mass vs time.
• Measuring rate by monitoring volume of 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.
• Measuring rate by monitoring disappearance of a cross


  Take a piece of paper and mark a cross (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.
• Finding rate of reaction at time t from 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 - it 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.
• Factors affecting rate of reaction

  • Concentration of reactants
  • Pressure of gases (volume)
  • Surface area
  • Temperature
  • Catalysts
• Collision theory


  Chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to activation energy)
• Increasing temperature


  Faster reaction
• Increasing concentration


  Faster reaction
• Increasing pressure of gases

  Faster reaction
• Increasing surface area


  Faster reaction
• Catalyst


  Changes the rate of reaction but is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
• Enzyme


  A molecule that acts as a catalyst in a biological system.
• Reversible reaction


  A reaction where the products can react backwards to produce the original reactants.
• Dynamic equilibrium


  In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.
• 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 of 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

  An 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. A 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.

  It just speeds up both forward and backward reactions equally.

  i.e. equilibrium is achieved faster.