Calculating Rates of Reactions (AQA GCSE Chemistry: Combined Science)

• Reactions take place at different rates depending on the chemicals involved and the conditions
  • Some are extremely slow e.g. rusting and others are extremely fast e.g. explosives
• The rate of reaction can be measured in two different ways:
  • How fast a reactant is used up
  • How fast a product is made
• Three methods used to determine the rate of reaction are:
  • Mass loss - measuring the mass loss in the reactants over time
  • Gas collection - measuring the amount of a gas formed over time
  • Precipitation - measure the formation of a precipitate over time
• Depending on the method used, the rate of reaction can be calculated using the appropriate equation:

Rate =    OR   Rate =

Formula triangle for calculating the rate of reaction

This formula triangle can help with rearranging rate equations for calculations 

Measurements for rate

• Time is always measured when determining the rate of a chemical reaction
  • Time is usually in seconds as many reactions studied in the lab are quite quick
• The amount of reactant used up or product formed is the other measurement 
  • Several of these measurements are made to ensure that there is sufficient data to establish a conclusion 
  • Measuring the amount of product forming is usually easier than measuring the amount of reactant being used up
• The calculation and units for rate depend on the reaction
  • If mass is being measured in grams, then the units for rate would be g/s
  • If volume is being measured in cm³ or dm³, then the units for volume would be cm³/s or dm³/s

Mass loss method

• When a gas is produced in a reaction it usually escapes from the reaction vessel, so the mass decreases
• For example, the reaction of calcium carbonate with hydrochloric acid producing carbon dioxide:

Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide 

• The mass is measured every few seconds and the change in mass over time is recorded as the gas escapes

• The reaction is typically performed in a conical flask placed on top of a balance to measure the loss in mass 
  • Cotton wool can be placed in the neck of the flask to allow the gas to escape while stopping any materials from being ejected  

The set-up for measuring the reaction rate by mass loss

A timer is started when the reactants are combined and the mass is recorded over time

• However, one limitation of this method is the gas must be sufficiently dense or the change in mass is too small to measure on a 2 or 3 decimal place balance
  • So, carbon dioxide would be suitable (M_r = 44) but hydrogen would not (M_r = 2)

Gas collection method

• When a gas is produced in a reaction, it can be trapped and its volume measured over time
  • This method can be used for any reaction that produces a gas but is particularly useful when the gaseous product is hydrogen or another gas with a small relative formula mass, M_r
• For example, the reaction of magnesium with hydrochloric acid producing hydrogen:

Magnesium + hydrochloric acid → magnesium chloride + hydrogen  

• The volume of gas produced over time is recorded

• One gas collection set-up involves collecting a gas through water using an inverted measuring cylinder 
  • This technique is called the downward displacement of water

The set-up for measuring the reaction rate by gas collection

The gas displaces the water from the inverted measuring cylinder and the volume of gas produced is recorded over time 

• Alternatively, the gas could be captured in a gas syringe which measures its volume

Alternative set-up for measuring the reaction rate by gas collection

The gas fills the gas syringe and the volume of gas produced is recorded over time 

Precipitation method

• Precipitation reactions form a solid precipitate when two clear solutions are mixed together
• The precipitate clouds the reaction mixture so if the flask is placed over a piece of paper with a cross on it, the time it takes for the cross to disappear from view (due to the formation of the precipitate) can be measured
• For example, the reaction of sodium thiosulfate and hydrochloric acid:

Sodium thiosulfate + hydrochloric acid → sodium chloride + sulfur dioxide + water + sulfur

• The time taken for the cross to disappear is measured