Rates and orders of reaction

The Rate Expression and Orders of Reaction

Simplistically one would imagine that doubling the concentration of a reactant would double the rate of a reaction. This seems logical; there will be double the number of collisions. However, although this is often the case it is not always true. Indeed, sometimes doubling the concentration of a reactant could have no affect on the rate and sometimes it will increase the rate by more than double, for example the rate could be quadrupled.

How can this be so? The answer lies in the fact that reactions usually take place by a series of steps and one of the steps (the slowest one (rate determining step RDS)) will control the rate. Only reactants involved in this step will affect the rate.  For reactants that are not involved in the RDS their concentration will have no affect on the rate. The order with respect to this reactant will be zero and this reactant will not appear in the rate expression. If a reactant is involved once in the RDS then doubling its concentration will cause the rate to double. It is said to be first order with respect to this reactant. The rate expression would be rate ≈ [conc] or Rate = k [conc]. It could be the case that a reactant is involved twice in the RDS, in which case, doubling its concentration will cause the rate to go quadruple i.e. go up by 4. The rate expression would be: Rate ≈[conc]²  or  Rate = k[conc]². K = the rate constant and the powers are the orders for each of the reactants. The overall order is simply the addition of all of the powers in the rate expression.

For example for the rate expression:  Rate = k  [BrO₃^-] [Br^-] [H⁺]²

What is the order with respect to each reactant?

What is the overall order?

What units will k have in this rate expression.  Note that the units of k will vary depending on the expression.

Remember that the rate expression can only be determined experimentally by following the rate and changing one reactant at a time. The stoichiometry of a reaction tells you nothing about the rate expression or possible mechanism.

Deducing  a Mechanism from the Rate Expression

This can be quite complex. However, simplistically if a reactant has zero order then it is not in the RDS but must be in a faster step. If a reactant is first order then it is directly or indirectly involved once in the RDS. If a reactant is second order then it’s either directly or indirectly involved twice in the RDS. Indirectly means that it could be involved with producing an intermediate, which is then involved in the RDS.

It can be fun proposing mechanisms that fit with the rate expression. There may well be several possibilities.


Mechanisms and orders

Rate expression

Guide to writing a lab report

Guide to writing a lab report

Rates of Reaction

You've seen how concentration and surface area affect the rate of a reaction. let's now look at the effect of catalysts and how heterogeneous and homogeneous catalysts can work;

The following is a good chem guide link:

Chem guide rates

Chem guide Catalysts

Maxwell Boltsmann distribution curve
temperature and the Maxwell Boltsmann curve

Measuring the rate of a reaction

Some useful notes on rates

IB Syllabus link to rate expression

VIRTUAL EXPERIMENT

Virtual exp results

Another virtual experiment

Chem Wiki

Energetics definitions

This is from the interactive syllabus. Remember for standard values you must state under standard conditions i.e. 25 0C 1 atm and any solutions 1 mol/dm3.

Energy Definitions

It should be noted that many of the energy definitions can be considered in the reverse direction with a corresponding change of sign for the energy. Bond formation = exothermic ΔH negative, bond cleavage = endothermic ΔH positive

Enthalpy of vaporisation

The energy required to vaporise one mole of a liquid

Enthalpy of atomisation

The energy required to produce one mole of gaseous atoms from an element in its standard state

Bond dissociation enthalpy

The energy change when one mole of a specific bond is broken or created

Bond enthalpy

The average energy change when one mole of a specific type of bond is broken or created.

Enthalpy of Combustion

The energy released when one mole of a compound is burned in excess oxygen

Enthalpy of formation

The energy change when one mole of a compound is formed from its constituent elements in their standard states

Enthalpy of solution

The energy change when one mole of a substance is dissolved in an infinite amount of water

Hydration enthalpy

The energy change when a particle is taken from infinite separation in the gaseous state to its position in an aqueous lattice

1st Ionisation energy

The energy required to produce one mole of gaseous ions from one mole of gaseous atoms by removal of one mole of electrons

Lattice enthalpy

The energy change when one mole of an ionic substance is broken into its constituent atoms at infinite separation.

Energetics Useful links

The following are some useful chemguide links:

Definitions

Using bond enthalpies

using Hess's law

Using specific heat capacities

Homework- using Bond energies

Please finish the sheet on bond energies. Also read the next part in your book so that you are ready for the lessons after academic travel. Hess's law and Born Haber cycles.

Have a good week.

Allotropes of carbon

The following video is interesting about the possible future of graphene: