Rates of reaction

• Rate of chemical reaction

  The speed with which the reactants get turned into products
• Measuring the rate of reaction


  1. Measure how fast the reactants are being used up

  2. Measure how fast the products are being formed
• Rate of reaction equation 1

  Rate = Quantity of reactants used / Time taken
• Rate of reaction equation 2


  Rate = Quantity of products formed / Time taken
• The rate of reaction starts off fast when there are lots of reactants, then slows down as the reaction progresses and reactants get used up
• Mass of reactant remaining over time

  Starts high, then decreases rapidly at first, then more slowly as reaction progresses
• Volume of product produced over time

  Starts at zero, then increases rapidly at first, then more slowly as reaction progresses
• Factors that affect the rate of chemical reactions are temperature, concentration, pressure, surface area, and the presence or absence of a catalyst
• Collision theory


  In order for particles to react, they have to collide with each other with sufficient energy, which is called the activation energy. If they collide with less energy than this, nothing will happen and they'll just bounce apart again.
• Rate of reaction


  Depends on the amount of energy the particles have and the frequency of the collisions
• How temperature affects the rate of reaction

  As temperature increases, particles gain more energy, move faster, and collide more frequently. They also collide with more energy, so are more likely to exceed the activation energy. This leads to a higher rate of successful collisions and a higher rate of reaction.
• How concentration and pressure affect the rate of reaction


  Increasing concentration or pressure means more particles per unit of volume, which makes the collisions more frequent and increases the rate of reaction.
• How surface area affects the rate of reaction

  Increasing the surface area provides more area over which collisions with other reactants can take place, leading to a higher frequency of collisions and a higher rate of reaction.
• Catalyst


  A substance that speeds up a reaction without being used up in the reaction itself. Catalysts work by lowering the activation energy, providing an alternative reaction pathway, and increasing the proportion of successful collisions.
• Common sources of catalysts

  • Transition metals like cobalt and nickel
  • Enzymes (catalysts made by living organisms)
• Catalysts are not included in the reaction equation as they are not reactants or products.
• Measuring the mean rate of a reaction and the rate of reaction at a specific time

  1. Plot a graph with time on the x-axis and the amount of product formed (or reactant used) on the y-axis

  2. To find the mean rate of reaction over a period, find the amount of product formed (or reactant used) in that period and divide by the time

  3. To find the rate of reaction at a specific time, draw a tangent to the curve at that time point and calculate the gradient of the tangent line
• The rate of reaction is fastest at the beginning and slows down as the reaction progresses
• Calculating the rate of reaction at a specific time

  1. Find the time point on the x-axis

  2. Draw a tangent to the curve at that point

  3. Calculate the gradient of the tangent line, which is the change in y (product/reactant) divided by the change in x (time)
• Calculating the mean rate of reaction over a period

  1. Find the amount of product formed (or reactant used) in that period on the y-axis

  2. Divide the amount by the time period
• As the reaction progresses

  The rate of reaction slows down
• The graph would start with the initial amount of reactant and decline rapidly at first, then more slowly