Calculating rates of reactions
- 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 (Mr = 44) but hydrogen would not (Mr = 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, Mr
- 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
The disappearing cross experiment
The chemicals are mixed which forms a a precipitate. The time taken for the precipitate to block the cross from view is recorded
- This method is susceptible to error though as they are subjective, given that different people may not agree on the exact moment that the cross disappears
- Another disadvantage is that only one data point is produced per experiment, so a rate of reaction graph cannot be plotted
