18.1-Rates of Reaction

The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed.

• Collision theory:based on the assumption that for a reaction to occur it is necessary for the reacting species (atoms or molecules) to come together or collide with one another.

• For a chemical reaction to happen:

  • Reactant particles must collide with each other

  • The particles must have enough energy for them to react

• A collision that produces a reaction is called a successful collision.

• The activation energy is the minimum amount of energy needed for a collision to be successful. It is different for different reactions.

Colliding particles

• Two pairs of particles move towards each other

• There are different ways to determine the rate of a reaction.

  • The method chosen usually depends on the reactants and products involved, and how easy it is to measure changes in them.

• The mean rate of reaction can be calculated using either of these two equations:

  • Mean rate of reaction=quantity of reactant used time taken

  • Mean rate of reaction=quantity of product formed time taken

  • e.g.v(o)-k(a)^x(b)^y

  • so if a = 3

  • b=4

  • x=2

  • y=3

  • so 576

Measuring mass

• The change in mass of a reactant or product can be followed during a reaction.

• This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small relative formula mass, Mr.

  • The units for rate are usually g/s or g/min.

Measuring volume

• The change in volume of a reactant or product can be followed during a reaction.

• This method is useful when a gas leaves the reaction container.

• The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder.

• The units for rate are usually cm3 s-1 or cm3 min-1.

Two ways to measure the volume of a gas produced in a reaction

• The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time.

• The graph shows this for two reactions.

• The steeper the line, the greater the rate of reaction.

• Faster reactions - where the line becomes horizontal - finish sooner than slower reactions

• The gradient of the line is equal to the rate of reaction:

• the steeper the line, the greater the rate of reaction

• fast reactions - seen when the line becomes horizontal - finish sooner than slow reactions

18.2-Factors Affecting Rates of Reaction

Orientation of reacting species

• The reaction between the reactants occurs only when they collide in correct orientation in space.

• Greater the probability of collisions between the reactants with proper orientation, greater is the rate of reaction.

• The orientation of molecules affect the probability factor, p.

• The simple molecules have more ways of proper orientations to collide. Hence their probability factor is higher than that of complex molecules.

• The orientation factor also affect the interaction between reactants and catalysts.

  • For example in case of biological reactions, which are catalyzed by enzymes, the biocatalysts.

• The enzymes activate the reactant molecules (or substrates) at a particular site on them. These sites are called as active sites and have definite shape and size.

• The size, stereochemistry and orientation of substrates must be such that they can fit into the active site of the enzyme.

• Then only the reaction will proceed. This is also known as lock and key mechanism.

• The enzymes lose their activity upon heating or changing the pH or adding certain chemical reagents.

• This is due to deformation of the configuration of active site.

Surface area

• The rate of a reaction increases with increase in the surface area of solid reactant, if any used.

• The surface of a solid can be increased by grinding it to a fine powder.

Intensity of light

• The rate of some photochemical reactions, which occur in presence of light, increases with increase in the intensity of suitable light used.

• With increase in the intensity, the number of photons in light also increases.

• Hence more number of reactant molecules get energy by absorbing more number of photons and undergo chemical change.

• However, some photochemical reactions involving the free radicals, generated in a chain process, are not greatly affected by the intensity of the light.

• Just one photon is sufficient to trigger the formation a free radical.

• This in turn initiate a chain process in which more free radicals are formed repeatedly in each cycle without the need of extra photons.

Nature of solvents

• The solvent may affect the rate in many ways as explained below:

• The solvents are used to dissolve the reactants and while doing so they help in providing more interactive surface between reactant molecules which may be otherwise in different phases or strongly bonded in solid phase.

• Usually solvents help in breaking the cohesive forces between ions or molecules in the solid state.

• The polar molecules tend to dissolve more in polar solvents with more dielectric constants and react faster in them. Whereas non polar molecules prefer non polar solvents.

18.3-Measuring Rates of Reaction

Finding rate of reaction

• The rate of reaction can be found by measuring the amount of product formed in a certain period of time.

• The mass of a solid product is often measured in grams, while the volume of a gaseous product is often measured in cm3.

• The time period chosen may depend upon the rate of the reaction.

  • For example, it may be a few seconds for a fast reaction or a few minutes for a slow reaction.

• The units for rate of reaction are commonly written as:

  • g/s or g/min

  • cm3/s or cm3/min

• There are three main methods of measuring rate:

  • measuring the volume of gas given off by a reaction over time

  • measuring the loss of mass of a reaction over time when a gas is produced

  • measuring the amount of light that passes through a reaction mixture (as a precipitate forms) over time – this can be done by inspection or using a light sensor and data logger

  • The rate of a chemical reaction can also be measured in mol/s.

    • For example, if two moles of a product were made during ten seconds, the average rate of reaction would be 2 ÷ 10 = 0.2 mol/s.

Measuring the rate where a gas is produced

• The apparatus needed depends on the nature of the product being measured.

• Mass

• The mass of a substance – solid, liquid or gas – is measured with a balance.

  • This mass balance could be attached to a data logger to measure the decrease in mass against time continuously and produce a graph instantly.

  • The data logger does not make the mass balance any more accurate.

  • Cotton wool is used in the neck of the flask as it allows the gas to escape.

Practice Questions:

• What’s two things that would speed up a rate of a reactions?

  • Enzymes and increase concentration of acid

• Why is a digital thermometer more suitable to measure temperature rather than an analogue thermometer?

  • Higher resolution

• What piece of apparatus could be used to measure the volume of gas produced in a reaction?

  • Gas syringe

• What is a catalyst?

  • Something that speeds up a chemical reaction