# Edexcel A Level Chemistry:复习笔记1.9.2 Calculating Rates of Reaction

### Calculating Rates 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

• Commonly used techniques are:
• mass loss
• gas production

#### 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 changes in mass using a balance

• 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 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 d.p. balance
• So carbon dioxide would be suitable (Mr = 44.0) but hydrogen would not (Mr = 2.0)

#### 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 Collecting gases experimental set up

• An alternative gas collection set up involves collecting a gas through water using an inverted measuring cylinder (as long as the gas is not water soluble) 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 amount of product against time Graph of gas volume evolved against time

#### 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

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

#### Worked Example

Using the results shown below, calculate the initial rate of reaction for the reaction using 2.0 mol dm-3 HCl (aq)

Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) Step 1: Draw a graph of the results • The gradient can be used to give the rate of reaction, however, the graph has produced a curve

Step 2: Draw a tangent to the curve at time = 0 seconds

Step 3: Calculate the gradient

• Gradient 1.05 mol dm-3 s-1

#### Exam Tip

You should be familiar with the interpretation of graphs of changes in concentration, volume or mass against time and be able to calculate a rate from a tangent to the graph 