Topic 17 - Kinetics
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[edit] 17.1 Rate expression
[edit] 17.1.1
The rate expression will be of the form Rate = k[A]m[B]n... (with a term for each reactant). k is a constant, and m, n etc. show the order of the reaction for each reactant.
[edit] 17.1.2
The order of a reaction is the total of m+n+... above. k is the rate constant, which will change due to any factor affecting the rate other than concentration (i.e. temperature, catalysts etc.)
[edit] 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 temperature decreases, the rate constant will decrease.
[edit] 17.2.4
A zeroth order reaction will be a straight line where the slope gives the rate constant (like y = mx + c). 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.
[edit] 17.1.5
By substituting data into Rate = k[A]m[B]n, it is possible to solve the resulting simultaneous equations for m and n. 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.
[edit] 7.1.6
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 ln2/k where k is the rate constant (equation given in data book).
[edit] 17.2 Reaction mechanism
[edit] 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. This is not literally a chemical substance, but an intermediate in which the bonds are in the process of being broken and formed.
[edit] 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).
[edit] 17.3 Collision theory
[edit] 17.3.1
Maxwell-Boltzman energy distribution curve. This would be much easier to explain if we had a graph of it, but we'll stick with text for now.
The curve is a graph of kinetic energy (x-axis) against the number of molecules with this kE (y-axis). The curve rises sharply to a peak, then falls off with the x-axis as an asymptote (i.e. is is possible, though statistically unlikely, for a molecule in any gas to be traveling at very high speed.
[edit] 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. This means that a greater proportion of the particles will have an energy greater than the activation energy, meaning that the reaction will occur more quickly.
[edit] 17.4 Activation energy
[edit] 17.4.1
Arrhenius equation : k = Ae(-Ea/RT) (in the data book)
The A in this equation is a constant related to the number and frequency of collisions occurring between the the reactants.
[edit] 17.4.2
Enthalpy level diagrams show 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.
[edit] 17.4.3
Reactions only occur when the reacting particles have energy greater than the activation energy.
[edit] 17.4.4
Homogeneous catalysts : Catalysts in the same phase (i.e. solid, liquid or gas) as the reactants.
Hetrogeneous catalysts : Catalysts in a different phase (usually a solid) from the reactants.
[edit] 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.
