Rates of Reaction

• Rates of reaction = the speed at which a reaction takes place - fast or slow.
• • Rate of a reaction is different to the reaction time - Reaction times are either long or short, whereas rates of reaction are fast or slow.
• To measure rate of reaction:

  • measure how quickly the reactants are used up.
  • measure how quickly the products are formed.
• Three main methods to measure the rate of reaction:

  1. Change in mass
  2. Volume of gas formed
  3. Formation of precipitate
• 1. Change in mass

  • Change in mass of the reaction mixture over time is used to measure the rate of reaction.
  • Mass of reaction mixture is measured using a balance.
  • Cotton wool is used in the neck of the flask to ensure only gas escape from reaction mixture.

  Mass of the apparatus and its contents decreases as gas produced leaves the flask.

  Recording loss in mass over time gives accurate rate measurement.
• Change in mass

  
• 2. Volume of gas formed

  Volume if gas can be measured using gas syringe or inverted measuring cylinder.

  Volume of gas collected over time is used to measure the rate of reaction.

  Recording the volume of gas produced over time gives an accurate rate measurement.
• Volume of gas

  
• Drawbacks - gas syringe

  • Delay in connecting syringe to flask, meaning done of the product is lost.
  • If reaction is too quick , the plunger may blow out of the syringe.
• Drawbacks - inverted measuring cylinder

  • Tube may slip from underneath measuring cylinder, so some product may be lost.
  • Its difficult to read the volume of gas produced accurately, due to bubbles moving in the water.
• 3. Formation of a precipitate

  • Rate at which the precipitate forms can be used to measure the rate of reaction.
  • Can he measure by placing reaction container in top of a black cross and timing how long it takes until the cross is no longer visible through reaction mixture.

  Recording the decrease in light intensity over time gives accurate measurement.

  Solution turns milky / cloudy as precipitate (insoluble solid) is produced.
• 3. Formation of a precipitate

  Alternative way:

  • Light sensor linked to a data logger measuring amount of light passing through reaction mixture.
  • Rate is measured timing how long it takes for light level to stop decreasing.
• Drawbacks - Light sensor

  • More expensive and so may not be readily available.
  • Light from surrounding area could possibly interfere with results from the light sensor.
• Drawbacks - Black cross

  • Human error can occur when judging the end point of reaction.
  • Method doesn’t provide data for entire reaction, meaning rate of reaction cannot be calculated for any point in the reaction.
• Equation:

  • rate of reaction = amount of product formed (or lost) / time
• Advantages - inverted measuring cylinder

  • No time delay in connecting the syringe.
  • All of gas produced is collected from the flask.
  • No gas escaped before syringe is connected.
  • Measuring cylinder is more precise than syringe.