ch 1 chem

11A 1 TECHNIQUES FOR MEASURING 11.10

THE RATE OF REACTION

LEARNING OBJECTIVES

Understand the term 'rate of reaction'.

Select and justify a suitable experimental tecÜique to obtain rate data for a given reaction, including:

(i) titration

(ii) colorimetry

(iii) mass change

(iv) volume of gas evolved

(v) other suitable tecÜique(s) for a given reaction.

RATE OF REACTION

The rate of a reaction can be expressed in two ways:

(1) How the concentration of a product increases with time.

change in concentration of product

time

rate =

(2) How the concentration of a reactant decreases with time.

change in concentration of reactant

time

rate =

The negative sign in the second expression shows that the concentration of the reactant is decreasing and therefore gives a positive value for the rate.

Rate is measured in units of concentration per unit time, and the most common units are mol dm-3 s-1.

The expressions in calculus notation are:

rate=

rate

d[product]

dt d[reactant]

This rate of reaction is sometimes called the 'overall rate of reaction'.

TECHNIQUES FOR MEASURING THE RATE OF REACTION

Before investigating the rate of a particular reaction, it is necessary to know the overall equation, including state symbols. for the reaction so that we can decide what technique to use to follow the reaction.

There are various techniques available to use, such as:

1 measuring the volume of a gas evolved

2 measuring the change in mass of a reaction mixture

3 monitoring the change in intensity of colour of a reaction

mixture (colorimetry)

4 measuring the change in concentration of a reactant or

product using titration

SPECIFICATION REFERENCE

11.1(i) 11.3()

11.3(i) 11.3(ii)

11.3(iii) 11.3(iv) 11.3(v)

5 measuring the change in pH of a solution.

6 measuring the change in electrical conductivity of a reaction

mixture.

The technique chosen to follow the reaction will depend on the nature of the reactants and products, as well as the conditions under which the reaction is carried out.

For example, the reaction between calcium carbonate and dilute hydrochloric acid,

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

could conveniently be followed by measuring the volume of gas (technique 1) given off at regular time intervals, or by measuring the change in mass of the reaction mixture with time (technique 2).

However, the reaction between propanone and iodine in aqueous solution,

CH,COCH3(aq) + I2(aq) → CH,COCH2l(aq) + H*(aq) + (aq)

could not be followed by measuring the change in mass because all products of the reaction remain in solution. It would be possible, however, to follow the reaction by monitoring the decrease in intensity of colour of the reaction mixture, since 12(aq) is the only coloured species present (technique 3).

TECHNIQUE 1: MEASURING THE VOLUME OF A GAS EVOLVED

The two most common techniques for collecting and measuring the volume of a gas evolved during a reaction are:

1 collection over water into a measuring cylinder (fig A), and 2 collection using a gas syringe (fig B).

The technique chosen will depend partly on the level of precision required. The gas syringe has a greater degree of precision, but if a large volume of gas is being measured, the difference in the degree of measurement uncertainty becomes so small that either. instrument is sufficiently precise.

-conical flask

-reaction mixture

fig A Collecting a gas over water.

EXAM HINT

measuring cylinder

gas evolved

Reactions producing gases that are very soluble in water, such as sulfur dioxide, cannot use the gas collection over water tecÚique.

water

TOPIC 11

gas evolved

gas syringe

-conical flask

11A.1 MEASURING THE RATE OF REACTION

Сл

-reaction mixture

A fig B Collecting a gas in a gas syringe.

TECHNIQUE 2: MEASURING THE CHANGE IN MASS OF A REACTION MIXTURE

This is another technique applicable to reactions in which a gas is evolved.

The reaction flask and contents are placed on a digital balance and the decrease in mass is measured as the reaction proceeds (fig C).

This technique is most precise when the gas given off has a relatively high density, such as with carbon dioxide. With a low-density (i.e. low relative molecular mass) gas such as hydrogen, the mass changes are so small that the measurement uncertainties become significant.

TECHNIQUE 3: MONITORING A COLOUR CHANGE (COLORIMETRY)

Colour change can sometimes be monitored using observation only. However, using a colorimeter gives more precise results (fig D). A colorimeter can detect far more subtle changes than the human eye can observe, and provides a quantitative (rather than a subjective) measurement.

18493.

fig C Cotton wool is placed in the neck of the flask to prevent the loss of liquid spray.

ogo chip

DIGITAL COLORIMETER

A fig D A colorimeter.

TECHNIQUE 4: ANALYSIS BY TITRATION

This technique involves using a pipette to remove small samples (aliquots) from a reaction mixture at regular intervals. The reaction in the aliquot can either be stopped by adding another substance to it or slowed down to almost zero by immersing it in an ice bath. The aliquot is then titrated to determine the concentration of a reactant or product species.

The process of stopping or slowing down the reaction in an aliquot is known as 'quenching'.

For example, if the reaction involves an acid, the aliquot, after quenching, could be titrated against a standard solution of sodium hydroxide to determine the concentration of the acid. This technique is used to investigate the reaction between iodine and propanone, which is catalysed by acid. Sodium hydrogen carbonate is added to the aliquot to remove the acid catalyst and, as a result, effectively stops the reaction. The remaining iodine is then titrated against a standard solution of sodium thiosulfate (fig E).

CH,COCH3(aq) + 12(aq) → CH3COCH2l(aq) + H+(aq) + 1-(aq)

12(aq) + 2S2O3(aq) → 21-(aq) + SO2 (aq)

A fig E Titrating iodine against

sodium thiosulfate.

11A.1 MEASURING THE RATE OF REACTION

SKILLS CREATIVITY

TOPIC 11

TECHNIQUE 5: MEASURING THE ELECTRICAL CONDUCTIVITY

If the total number, or type, of ions in solution changes during a reaction, it might be possible to follow the reaction by measuring changes in the electrical conductivity of the solution using a conductivity meter. For example, it could be used to follow this reaction:

5Br (aq) + BrO3(aq) + 6H (aq) → 3Br2(aq) + 3H2O(1)

TECHNIQUE 6: MEASURING ANY OTHER PHYSICAL PROPERTY THAT SHOWS A SIGNIFICANT CHANGE

Possible physical properties that have not already been mentioned include changes in the volume of liquid ('dilatometry'), chirality and refractive index.

CHECKPOINT

1. State suitable tecÚiques to collect rate data for each of the following reactions.

(a) Magnesium with dilute sulfuric acid:

Mg(s) + 2H (aq) Mg2+(aq) + H2(g)

(b) Ethyl ethanoate with sodium hydroxide:

CH3COOCH2CH3(1) + OH (aq) → CH3COO (aq) + CH3CH2OH(aq)

H2(g) +12(g) 2HI(g)

2. Why would the tecÚique of measuring the change in mass of a reaction vessel and contents not

work well in the reaction between magnesium and dilute sulfuric acid?

3. The reaction between calcium carbonate and dilute hydrochloric acid can be followed by collecting

and measuring the volume of gas produced. The gas could be collected over water in a measuring cylinder or in a gas syringe. Which tecÚique would be the more suitable for this reaction? Explain your answer.

SUBJECT VOCABULARY

(overall) rate of reaction the change in concentration of a species divided by the time it takes for the change to occur. All reaction rates are positive

(chemical) species an atom, a molecule or an ion that is taking part in a chemical reaction