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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Kinetics - 17

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17.1 Rate expression

17.1.1

The rate expression will be of the form:

Rate etc (for each reactant).

• is a constant, and
• etc show the order of the reaction for each reactant.

17.1.2

Order of reaction is the total of above. is the rate constant, which will change due to any factor affecting the rate other than concentration (ie temp, catalysts...)

17.2.3

If temperature increases, then the rate will increase and so the rate constant must increase also to account for this. If the temp decreases, the rate constant will decrease.

17.2.4

A zero order reaction will be a straight line where the slope gives the rate constant (like ). A first order reaction will be a curve where the half lives are constant (ie the time taken for the concentration to halve from any given point is constant). A second order reaction is a curve where the half lives are not constant.

17.1.5

By substituting data into Rate , it is possible to solve the resulting simultaneous equations for and . For example, if the concentration of A was doubled, B was kept constant and rate doubles as a result, then the reaction would be first order with respect to A (ie m=1). If the rate increased by a factor of 4, then the reaction would be second order with respect to a (since (2a)² = 4a² and rate is proportional to a²). The overall order of the reaction can be calculated from graphical data as above.

17.1.6

The rate constant can be found from a graph by taking a point, finding it's concentration, then finding a point on the graph which corresponds to half this concentration. The half life is the time between thee two points. The half life is also equal to where is the rate constant (equation given in data book).

17.2 Reaction mechanism

17.2.1

Rate determining step - the slowest step in a reaction which determines the rate for the overall reaction.

Molecularity - The number of molecules reacting in an elementary step of a reaction.

Activated complex - as two particles collide (with sufficient energy to react and in the correct orientation) they form an intermediate called the activated complex...not literally a chemical substance, but an intermediate in which the bonds are in the process of being broken and formed.

17.2.2

The order of the reaction is defined by the particles involved in the rate determining step (which is one step in the mechanism). For example, if two of one type of particle are colliding, the order with respect to that particle will be 2 (and zero to any others). The activated complex is formed by (in this example) the two colliding molecules, weakening the bonds, which then break and form to create the new molecule(s).

17.3 Collision theory

17.3.1

Maxwell-Boltzman energy distribution curve - sometime I'll actually draw a picture of it, but for now:

The curve is on a graph of kinetic energy (x-axis) against fraction of molecules with this kE (y-axis). The curve rises sharply to a peak, then falls off with the x-axis as an asymptote (ie is is possible, though statistically unlikely, for a molecule in any gas to be traveling at very high speed.

(Diagram of the curve needed)

17.3.2

At higher temperatures, the peak of the curve is lower, and further to the right. The graph is overall 'flatter' resulting in a greater proportion being above any given temperature and so whatever the required activation energy, a greater proportion of the particles will have sufficient activation energy for the reaction to proceed. Thus making the reaction occur faster.

17.4 Activation energy

17.4.1

Arrhenius equation:

.

(equation in data book)

The in this equation is a constant related to the number and frequency of collisions occurring between the the reactants.

17.4.2

Enthalpy level diagrams showing a curve demonstrating the path between the reactants and products on the graph. Energy is shown on the y-axis, and presumably the reactants and products are of different energies...the curve goes between the two levels (the reactants and products) peeking above both between them (with the distance between the highest point ant the reactants being the activation energy). A catalysed reaction graph is the same, only the curve peaks lower, representing a lower activation energy requirement.

17.4.3

Reactions only occur when the reacting particles have energy greater than the activation energy.

17.4.4

Homogeneous catalysts - catalysts in the same state (phase -- ie solid, liquid or gas) as the reactants.

Hetrogeneous catalysts - catalysts in a different phase (usually a solid) from the reactants.

17.4.5

Homogeneous catalysts operate by reacting with the reactants and eventually producing a reaction pathway of lower activation energy (and also being regenerated at the end of this process). Hetrogeneous catalysts provide a reactive site on which an activated complex forms, weakening the bonds and increasing the rate of collisions thus increasing the rate of reaction.

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