Rates of Reaction

• Rate of reaction 


  Is the rate of change of the reactants into the products in a chemical reaction
• Factors affecting rate of reaction


  The rate depends on:

  • surface area or size of solid particles
  • concentration of solution
  • temperature
  • presence of a catalyst

  Different methods of measuring rate, all of which measure a quantity against time
• Reactions used in rate of reaction experiments

  • A metal reacting with dilute acid e.g zinc or magnesium with dilute hydrochloric acid or sulfuric acid. These produce hydrogen gas: Mg+2HCl-> MgCl2 + H2 or Zn+H2SO4->ZnSO4+H2
  • Calcium Carbonate reacting with dilute hydrochloric acid, producing carbon dioxide gas: 2H2O2->2H2O+O2
  • The catalyst is manganese (IV) oxide, MnO2
  • The reaction between sodium thiosulfate solution and HCl produces a precipitate of sulfur making solution go cloudy. Sulfur causes cloudiness.
• Ways to measure the rate of reaction

  • Measuring a change in mass
  • Measuring gas volume
  • Measuring time for a reaction to be completed
  • Measuring production of a solid precipitate
• Measuring a change in mass

  • The reaction between CaCO3 and HCl can be used to investigate how the size of solid particles affects the rate of the reaction
  • Mass is lost because CO2 escapes from the reaction vessel. Recording the loss over a certain time period gives an indication of the rate of reaction
  • Cotton wool stops any liquid lost in the flask. There is bubbling which causes solution to splash out
• Change in mass equipment

  • Conical flask
  • Cotton wool
  • Top ban/electric balance
  • Stop clock
• Change in mass results

  • The reaction occurs more rapidly with smaller chips. These have a much larger surface area to volume ratio. There is more contact between the surface of small chips and acid which causes greater rate of reaction
  • Same mass and same volume and same concentration of acid was used. Both reactions start and level off at the same masses
  • The steeper initial slope of curve for small marble chips show mass is decreasing quickly therefore rate of reaction is higher
• Measuring gas volume 


  If a reaction produces gas, collecting gas in a gas syringe over a period of time is a good way to measure a rate of reaction. Volume is taken from readings on gas syringe at time intervals.
• Gas volume equipment

  • Delivery tube
  • gas syringe
  • stop clock
  • conical flask
• Gas volume results

  • The volume of gas produced increases more rapidly when small chips are used
  • Curves start and end at same gas volume because same volume and concentration of HCl is used
  • Both curves of small and large chips are steepest at the start. This indicates the reaction is faster at start and gradually gets slower because reactants are being used up. Rate of reaction decreases as reaction proceeds
• Measuring time for a reaction to be completed


  1. For a reaction that produces gas:

  2. Reading the time when the mass first reaches its minimum on a graph of mass against time

  3. Reading time when gas volume first reaches its maximum on a graph of gas volume against time

  4. Timing the reaction until you see the production of gas has stopped

  5. For a reaction in which a solid disappears: Timing the reaction until no more solid can be seen in reaction vessel
• Measuring Production of a solid precipitate

  • Sodium thiosulfate solution reacts with dilute hydrochloric acid and the reaction produces solid sulfur
  • If solutions are mixed in a conical flask placed on a piece of white paper with a black cross, then as the solid sulfur is produce, it obscures the cross
  • Time is measured until cross can no longer be seen from above
  • The higher the concentration of sodium thiosulfate solution of HCl acid, the less time taken for the cross to disappear, meaning a higher rate of reaction
• Production of solid precipitate equipment

  • Conical flask
  • white paper
  • stop clock
  • sodium thiosulfate and HCl
• Calculating rate from time

  Rate = 1/time

  The unit of rate is S-1 (small minus 1)
• Planning an investigation

  • This involves several steps but making an initial prediction and ensuring the investigation is a fair test is important
  • For an investigation into effects of changing the concentration of HCl on the rate of reaction with Mg ribbon - the prediction could say as concentration increases, the rate of reaction increases
  • A fair test is used when one variable is changed, one is measured and all others are kept the same
• Validity of data collection 


  Validity is part of the overall design of the experiment e.g using two concentrations to determine the link between concentration and rate of reaction. Validity can be improved by using a greater number and range of concentration
• Reliability of data collection 


  Reliability depends on whether or not the same result could be obtained if the experiment was repeated. Measuring the gas volume or change in mass can produce more reliable results because time when the reaction finishes is determined by apparatus used
• Anomaly 


  Any anomalles are indicated by points that don’t meet the line of best fit. It is a piece of data that doesn’t match the pattern shown
• Explaining effects of temperature and concentration on reaction rate

  Reactions can only happen when reacting atoms, molecules or ions collide with each other. Only some of the collisions result in a reaction - these are successful collisions

  An increase in temperature increases the rate of most reactions
• Activation energy 


  The minimum energy required in a collision in order to cause a reaction
• At higher temperatures

  • Particles have more energy and move faster
  • This leads to more collisions between particles
  • And so more successful collisions in a given time - more particles have more energy than the activation energy
  • Which increases the rate of reaction
• At a higher concentration of solution

  • There are more particles present in the same volume
  • Leads to more collisions between particles
  • More successful collisions in a given time
  • Increasing rate of reaction
• Investigating the effect of the presence of a catalyst


  Reaction commonly used for the effect of a catalyst is the decomposition of hydrogen peroxide using manganese (IV) oxide, MnO2 as a catalyst. Reaction can be monitored using a gas syringe to collect and measure volume of oxygen gas produced.
• Presence of catalyst

  • The catalysed reaction happens at a faster rate, and the catalyst reaction at a higher temperature occurs at the highest rate of reaction
  • A change in concentration of hydrogen peroxide solution would change the volume at which the graph levels off
  • This is usually simple e.g if a concentration is doubled, the volume at which graph levels off doubles
• Graph at high temperature when all factors are the same

  • Gas volume starts at 0
  • Gas volume is higher at each time interval
  • Line on graph levels off earlier
  • Line ends at the same final gas volume
• Metal with acid reaction 


  Effects of changed of both temperature and concentration can be seen using two different reactions:

  • zinc with dilute acid
  • magnesium with dilute acid

  Both of these produce hydrogen gas, so gas volume can be used to measure the rate of reaction

  • A change in temperature will alter the shape of the gas volume against time graph - it doesn’t make any difference whether the acid or metal is in excess
  • A change of concentration of acid will alter the shape of the gas volume graph but the final gas volume will be changed only if all acid is used up
• Catalyst


  A catalyst is a substance that increases the rate of a chemical reaction without being used up
• Catalysts work 


  Catalysts work by providing an alternative reaction pathway of lower activation energy which increase the number of successful collisions in a given time - this increases the rate of reaction

  • The mass of a catalyst will be the same at the end of the reaction as the start as it is not used up
  • Transition metals and their compounds are often used as catalysts e.g manganese (IV) oxide in the catalytic decomposition of hydrogen peroxide and iron in the Harber process