Rate and Extent of Chemical Change

• How is rate of reaction calculated?

  Rate of reaction = amount of reactant used ÷ time

  Rate of reaction = amount of product formed ÷ time

  Rate of reaction (mols/s) = moles of reactant used ÷ time
• What are the various units for rate of reaction?

  g/s , cm³/s or mol/s
• Name three common ways of measuring rate of reaction

  • Loss in mass of reactants
  • Volume of gas produced
  • Time for a solution to become opaque (not able to be seen through; not transparent.)
• Measuring the rate by monitoring mass loss

  1. Place the reaction flask on a balance

  2. In these reactions (e.g. metal carbonate + acid) a gas is given off

  3. Record the decrease in mass in time intervals (note hydrogen is too light)

  4. 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 on one axis and time on another.
• Measuring the rate by monitoring the disappearance of a cross

  1. Take a piece of paper and mark a cross (X) on it

  2. Put the reaction flask on this cross

  3. Mix the reagents

  4. Measure how long it takes for a cloudy mixture to conceal a cross
• How to find a reaction at some time (t) from a graph of amount of reactant vs time? (Higher)


  Pick a point corresponding to the time (t), and find a tangent to the curve at this point.

  The tangent is the gradient of the graph- it tells you how fast the rate of the reaction proceeds at this point. The steeper the tangent line, the faster the rate.
• State five factors affecting the rate of a chemical reaction?

  • Concentration of reactants
  • Pressure of gases (volume)
  • Surface Area
  • Temperature
  • Catalysts
• Describe and explain the effect of increasing temperature on the rate of reaction.

  Temperature (T) = faster reaction.

  As T increases, kinetic energy of particles increases, i.e. more energetic collisions. Also, they move faster, so they collide more frequently.

  However, there are no directly proportional.
• Describe and explain the effect of increasing concentration on the rate of reaction.

  Concentration increase = faster reaction.

  More reactants = more frequent collisions.
• Effect of increasing pressure of a gas on the rate of reaction

  1. Increasing the pressure of reacting gases

  2. Increases the number of gas molecules in the same volume

  3. Increases the frequency of collisions

  4. Increases the rate of reaction
• Volume and pressure are inversely proportional to each other
• Increasing the volume reduces the rate of reaction
• Surface area

  The total area of the exposed surface of a solid reactant
• 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
• Increasing the surface area of solid reactants

  Increases the rate of reaction
• Solid reactants

  • Block of magnesium reacts slower with acid than magnesium powder
• What is a catalyst how does it work? How does it affect the reaction profile?

  A catalyst changes the rate of the reaction but it is not used up. It increases rate of reaction by providing an alternate pathway for the reaction that has lower activation energy. The reaction profile for a catalysed reaction will have lower maximum of the curve (lower activation energy).
• What is an enzyme?

  An enzyme is a molecule that acts as a catalyst in a biological system.
• What is a reversible reaction?

  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 concentration of reactant 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 equilibrium.
• Equilibrium

  The state where the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of reactants and products remain constant
• Effect of changing concentration of reactant or product on equilibrium

  1. If concentration of reactant is increased, more products will be formed until equilibrium is reached again

  2. If concentration of product is decreased, more reactants will react until equilibrium is reached again
• 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
• Describe the effect of changing temperature on the position at 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 at equilibrium.

  This applies to equilibria that involves gases.

  An increase in pressure causes the equilibrium position to shift toward 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 the side with a larger number of molecules as shown by the symbol equation. Pressure has no effect on the reaction that has equal number of gas molecules on both sides.
• Describe the effect of a catalyst on the position of the equilibrium.

  No effect.

  It speeds up both forwards and backwards reactions equally so the equilibrium is achieved faster.