Kinetics

Collision Theory

Reactions can only occur when collisions take place between particles having diffident energy. The energy is usually needed to break the relevant bonds in one or either of the reactant molecules, The minimum energy needed is called the Activation energy.

Activation Energy

Defined as the minimum energy which particles need to collide to start a reaction

Maxwells-Boltzmann Distribution curve

Shows the spread of energies that molecules of a gas or liquid have at a particular temperature

Features of the Curve

The energy distribution should go through the origin because there are no molecules with no energy

A few have low energies because collisions cause some particles to slow down

Emp is the most probable energy - on the peak of the curve

The mean energy of the particles is not at the leak of the curve but slightly off

Most molecules have energies between the two extremes but the distribution is not symmetrical

Only a few particles have energy greater energy than the Activation energy

The energy distribution should never meets the x axis as there is no maximum energy for the molecules

The area under the curve represents the total number of particles present

Temperature Increase on a Maxwell-Boltzmann curve

As the temperature increases the distribution shifts towards having more molecules with higher energies. The curve will shift to the right and become more flattened. The number of molecules with the energies of the Emp and mean energy decreases. The total area under the curve should remain constant because the total number her of particles is constant. At higher temperatures the molecules have a wider range of energies than at lower temperatures.

Rate of Reaction

Defined as the change in concentration of a substance in a unit time. The unit is normally mol dm-3s-1.

Rate Graphs

When a graph of concentration do a reactant is plotted against time, the gradient of the curve is the rate of reaction. The initial rate at the start of the reaction is where it is fastest. Reaction rates can be calculated from graphs of concentration of reactions or products by drawing a tangent to the curve at different times and calculating the gradient of the tangent

Increasing concentration and pressure

At higher concentrations and pressures there are more particles per unit volume and so the particles collide with greater frequency and there will be a higher frequency of effective collisions. If question mentions a doubling of concentration/rate then you double the number of particles per unit volume and hence double the frequency of effective collisions. If concentration increases, the shale of the energy distribution curve does not change so the Emp and Mean energy do not change. The curves will be higher up and the era under the curves will be greater as there are more particles and more molecules have energy greater rona the activation energy.

Comparing Rate curves

Different volumes of the same initial concentrations will have the same initial rate, if other conditions are the same, but will end at different amounts

The higher the concentration/temperature/surface area the faster the rate (steeper gradient)

Needs to calculate or compare the initial moles of reactants to distinguish between different finishing volumes

Increasing temperature

At higher temperatures the energy of the particles increases. The particles collide more frequently and more often with energy greater than the activation ever. More collisions result in a reaction. The graph shows that a significantly larger proportion of particles have an energy greater than the activation energy, so frequency of successful collisions increases.

Increasing Surface Area

Cause more successful collisions to occur more frequently between the reactant particles and this will increase the rate of reaction.

Catalysts

Catalysts i crease the rate of reaction without being used up in the reaction. They do this by providing an alternative route or mechanism with a lower activation energy. If the activation energy is lower more particles will have energy greater than it so there will be a higher frequency of effective collisions