AQA A Level Chemistry复习笔记8.1.1 Required Practical 7

Required Practical 7

 

Required Practical 7: Measuring the rate of a reaction

  • There are two ways to measure the rate of reaction
    • By an initial rate method
    • By a continuous monitoring method

     

An initial rate method: The reaction between magnesium and hydrochloric acid

  • This reaction can be used to investigate the effect of varying the concentration of the acid while keeping the temperature constant
  • When a gas is released in a reaction you can either try to measure the volume of gas given off or the mass change in the reaction flask
    • However, in this case the gas hydrogen is too low in density so the mass change will be far too small to register on a laboratory balance

     

  • Volume can be measured either by displacement of water into an inverted measuring cylinder or by using a gas syringe

6.1.3-Gas-collection-Set-Up

The rate of reaction between magnesium and dilute hydrochloric acid can be measured using a gas syringe and stopwatch

 

Steps in the procedure

  • Before you begin you need to check your apparatus is gas tight
    • This can be done by assembling everything without the acid or magnesium and trying to move the plunger
    • If you feel resistance the apparatus is gas tight

     

  • The acid will go in first because it is quicker to drop a piece of magnesium ribbon in than to pour in the acid
  • To vary the concentration of the acid you need to dilute it by measuring portions of acid in a measuring cylinder and then portions of distilled water in another measuring cylinder and adding them to the conical flask
  • Choose a suitable volume of acid to match the size of the flask, e.g. 40 cm3
  • You don't want to use acid that is stronger than 2 mol dm-3 because the reaction will be too fast, so its best to start with 2 mol dm-3 and perform a serial dilution, e.g. 40 cm3(acid) + 0 cm3 (water), 35 cm3 (acid) + 5 cm3 (water), etc

Practical tips

  • Make sure the plunger is fully inserted before you start the experiment otherwise you will have a volume error
  • If the magnesium does not look new and shiny, you may need to clean the surface with a bit of sandpaper
  • Make sure the plunger is secure and does not fall out of the barrel if the volume exceeds 100 cm3

Specimen Results

  • Here is a set of typical results for this experiment

Rate of reaction between magnesium and acid results table

8.1.1-Initial-Rates-Method-Results-Table

 

Graphing the results8.1.1-Initial-Rates-Method-Results-Graph

 

Graph showing typical results for the rate of reaction between hydrochloric acid and magnesium at different concentrations

 

Analysis

  • The lines of best fit are drawn for each concentration on the same graph
  • A tangent is then drawn starting from (0,0) since this method is to find the initial rate of reaction
  • The gradient of the tangent is determined which gives the rate of reaction
  • In the example above, the rate of reaction for 2.0 mol dm-3 acid is
    • Gradient 

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