Measuring Rates of Reaction (SL IB Chemistry)

Measuring rate of reaction

• To measure the rate of a reaction, we need to be able to measure either how quickly the reactants are used up or how quickly the products are formed
• The method used for measuring depends on the substances involved
• There are a number of ways to measure a reaction rate in the lab; they all depend on some property that changes during the course of the reaction
• That property is taken to be proportional to the concentration of the reactant or product, e.g., colour, mass, volume
• Some reaction rates can be measured as the reaction proceeds (this generates more data);
  • Faster reactions can be easier to measure when the reaction is over, by averaging a collected measurement over the course of the reaction
• Three commonly used techniques are:
  • Mass loss
  • Gas production
  • Colorimetry

Measuring the rate of reaction using colorimetry

• A colorimeter or spectrophotometer measures the amount of light that passes through a solution

Colorimetry Set Up

Colorimetry measures the light intensity of light passing through a sample

• If a solution changes colour during a reaction, this can be used to measure the rate
• The intensity of light reaching the detector is measured every few seconds and the data is plotted to show how the concentration of the reactants or products changes with time
• The light intensity is related to the concentration, so the graph represents a graph of concentration of products or reactants against time

Examples results from a colorimeter

Sketch graph of colour intensity against time (the coloured species is a reactant in this case)

• Note: Colorimetry cannot be used to monitor the formation of coloured precipitates as the light will be scattered or blocked by the precipitate

Measuring the rate of reaction using changes in mass

• When a gas is produced in a reaction it usually escapes from the reaction vessel, so the mass of the vessel decreases
  • This can be used to measure the rate of reaction
  • For example, the reaction of calcium carbonate with hydrochloric acid produces CO₂
  • The mass is measured every few seconds and the change in mass over time is plotted as the CO₂ escapes

Equipment used to measure the loss of mass

Measuring changes in mass using a balance. The cotton wool in the neck of the flask allows the gas to escape whilst preventing the other reactants and products from leaving the container

• The mass loss provides a measure of the amount of reactant, so the graph is the same as a graph of amount of reactant against time

A graph to show the change in mass with time

Mass loss of a product against 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)

Measuring rate using changes in volume of gases

• When a gas is produced in a reaction, it can be trapped and its volume measured over time
  • This can be used to measure the rate of reaction.
  • For example, the reaction of magnesium with hydrochloric acid produces hydrogen

Measuring rate of reaction using a gas syringe

Collecting gases experimental set up

• An alternative gas collection set up involves collecting a gas through water by displacement using an inverted measuring cylinder or burette
• This method can only be used if the gas produced has a low water solubility
• Hydrogen gas can be collected using this method

Measuring the rate of reaction using an inverted measuring cylinder

Alternative gas collection set up

• The volume can be measured every few seconds and plotted to show how the volume of gas varies with time
• The volume provides a measure of the amount of product, so the graph is a graph of the amount of product against time

Graph of gas volume evolved against time

 The volume of gas increases with time. The reaction has stopped when the volume of gas plateaus

Measuring concentration changes using titrations

• The concentration of a sample can be measured by performing a titration
• However, the act of taking a sample and analysing it by titration can affect the rate of reaction and it cannot be done continuously
• To overcome this, samples of the reaction mixture are taken at regular intervals during the course of the reaction
• The reaction in each of the samples is deliberately stopped - this is called quenching
  • Quenching 'freezes' the reaction at a specific point in time to allow the concentration to be determined by titration
• Based on the collected data, the rate of reaction can be calculated by determining the change in concentration with time

Measuring the rate of reaction using conductivity

• Conductivity can be used to measure the rate of a reaction by monitoring changes in the electrical conductivity of the reaction mixture over time
• As the reaction proceeds, the concentration of ions in the solution may change, affecting its conductivity
• By measuring the conductivity at different time intervals, the rate of the reaction can be determined based on how quickly the conductivity changes
• For example, in the reaction:

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

• During this reaction, HCl and NaOH dissociate into ions, increasing the conductivity of the solution
• As the reaction progresses, the concentration of ions changes which affects the conductivity

Measuring the rate of reaction using a 'clock reaction'

• Often it is more convenient to ‘stop the clock' when a specific (visible) point in the reaction is reached instead of continuously monitoring the change in rate
• 'Clock reactions' are non-continuous methods in which the time taken to reach a fixed point is measured 
  • For example, when a piece of magnesium dissolves completely in hydrochloric acid
  • Another common rate experiment is the reaction between sodium thiosulfate and hydrochloric acid which slowly produces a yellow precipitate of sulfur that obscures a cross when viewed through the solution:

Na₂S₂O₃ (aq) + 2HCl (aq)   →  2NaCl aq) + SO₂ (g) + H₂O (l) + S(s)

The reaction of sodium thiosulfate and hydrochloric acid

The disappearing cross experiment

• The main limitation here is that often it only generates one piece of data for analysis