Core Practical: Investigating Rate of Reaction (Edexcel GCSE Chemistry: Combined Science)

• 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:
  • measuring mass loss on a balance
  • measuring the volume of a gas produced
  • measuring a reaction where there is a colour change at the end of the reaction

Changes in Mass

• When a gas is produced in a reaction it usually escapes from the reaction vessel, so the mass 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 change in mass over time is plotted as the CO₂ escapes

Measuring mass changes on a balance

Volumes 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 changes in gas volume

Measuring concentration changes

• Measuring concentration changes during a reaction is not easy; the act of taking a sample and analysing it by titration can affect the rate of reaction (unless the reaction is deliberately stopped- this is called quenching).
• Often it is more convenient to ‘stop the clock’ when a specific (visible) point in the reaction is reached
  • 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 disappearing cross experiment

Calculating rates of reaction

• Reactions take place at different rates depending on the identities and conditions
• Some are extremely slow e.g. rusting and others are extremely fast e.g. explosives
• Rates of reaction can be measured either by how fast a reactant is used up or by how fast the product is made
• Rate is concerned with amounts of substances and time and can be calculated using the formula:

A formula triangle for calculating the rate of reaction

• In order to provide sufficient data to establish a conclusion several measurements need to be made during the reaction
• The product is usually the one that is measured as it is usually easier to measure a product forming than it is a reactant disappearing
• The quantity to be measured depends on the reaction and may be in grams for mass or cm³ or dm³ for volume if the product is a gas
• The units of the rate of reaction would therefore be g/s or cm³/s or dm³/s
• Time is usually in seconds as many reactions studied in the lab are quite quick
• If one of the products is a gas which is given off, then the reaction can be performed in an open flask on a balance to measure the loss in mass of reactant
• Cotton wool is usually placed in the mouth of the flask which allows gas out but prevents any materials from being ejected from the flask (if the reaction is vigorous)

Calculating Gradients

• Often a curved graph is obtained or a graph which starts out as a straight line but then curves to form a horizontal line as the reaction peters out, usually due to one of the reactants running out
• The curved section signifies that the relationship between rate and the factor being measured is not directly proportional, so the rate of reaction is different along each point of the curve
• For a curve graph a tangent must be drawn to calculate the change in x and y so the rate of reaction at a particular point during the reaction can be calculated
• Place a ruler on the point being studied and adjust its position so the space on either side of the point between the ruler and curve are equal:

Drawing a tangent to a curve using a ruler

• Use the tangent to calculate the rate of reaction as shown below:

Obtaining a tangent on a curve

• The gradient at that point is

GRADIENT = ∆ (PRODUCT) ÷ ∆ (TIME)

• You can use this formula to calculate the gradient at any particular point in the curve

Part A- Measuring the Production of a Gas

Aim:

To investigate the effect of changing surface area of marble chips in the reaction between marble chips and hydrochloric acid

Materials:

• Marble chips, small and large
• Hydrochloric acid 1 mol dm^-3 
• Conical flask (100 cm³)
• Safety goggles
• Gas syringe
• Stop clock

Diagram:

Investigating the effect of different size marble chips on the rate of reaction between calcium carbonate and hydrochloric acid

Method:

• Add hydrochloric acid into a conical flask
• Use a delivery tube to connect this flask to an inverted measuring cylinder
• Add marble chips into the conical flask and close the bung
• Measure the volume of gas produced in a fixed time using the measuring cylinder
• Repeat with different sizes of marble chips 

Result:

• Increase in the surface area of the marble chip, the rate of reaction will increase
• This is because more surface area particles of the marble chips will be exposed to the dilute hydrochloric acid so there will be more frequent and successful collisions, increasing the rate of reaction

Part B- Observing a Colour Change

Aim:

To investigate the effect of changing concentration in the reaction between sodium thiosulfate and hydrochloric acid

Materials:

• 40 g dm^-3 sodium thiosulfate solution
• 1.0 mol dm^-3 dilute hydrochloric acid
• Conical flask (100 cm³)
• Black cross on paper
• White paper or white tile
• Stopwatch or timer

Diagram:

Diagram showing the apparatus needed to investigate the effect of concentration on the rate of reaction

Method:

• Measure 50 cm³ of sodium thiosulfate solution into a flask
• Measure 5 cm³ of dilute hydrochloric acid into a measuring cylinder
• Draw a cross on a piece of paper and put it underneath the flask
• Add the acid into the flask and immediately start the stopwatch
• Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
• Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)

Result:

• With an increase in the concentration of a solution, the rate of reaction will increase
• This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions, increasing the rate of reaction

Hazards, risks and precautions

Hazard symbols to show substances that are flammable and toxic

• Magnesium is a flammable metal
• Dilute hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin
• The reaction between sodium thiosulfate and hydrochloric acid produces sulfur dioxide which is toxic if inhaled
  
  
• Magnesium should be kept away from naked flames, e.g. a Bunsen burner
• For dilute hydrochloric acid, avoid contact with the skin and use safety goggles
• Take care not to inhale sulfur dioxide gas; asthmatics need to be especially careful and a fume cupboard can be used to avoid exposure