Revision:Kinetics - 07

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7.1 Rates of reaction

7.1.1

Rate of reaction is concerned with how quickly a reaction reaches a certain point (not to be confused with how far a reaction goes - ie equilibrium).

7.1.2

Interpretation of rate graphs. Reaction rater graphs will generally be graphed with time on the x-axis and some measure of how far the reaction has gone (ie concentration, volume, mass loss etc) on the y-axis. This will generally produce a curve with, for example, the concentration of the products approaching zero.

7.2 Collision theory

7.2.1

Collision theory - reactions take place as a result of particles (atoms or molecules) colliding and then undergoing a reaction. Not all collisions cause reaction, however, even in a system where the reaction is spontaneous. The particles must has sufficient kinetic energy, and the correct orientation with respect to each other for the two to react.

(There is a diagram/picture missing here)

7.2.2

Higher temp -> greater average kE -> faster reaction.

Higher concentration -> more collisions -> faster reaction.

Catalysts -> lower activation energy / greater probability of proper orientation -> faster reaction.

In heterogenous reactions (where the reactants are in different states) the size of the particles of a solid may change reaction rate, since the surface is where the reaction takes place, and the surface area is increased when the particles are more finely divided...(therefore smaller solid particles in a heterogenous reaction -> faster reaction).

7.2.3

Most reactions involve several steps, which can be individually slow of fast, and which, all together, make up the complete reaction. The slowest of these steps is called the rate determining step, as is determines how fast the reaction will go. It is also not necessary that all the reactants are involved in every step, and so the rate determining step may not involve all the reactants. as a result, increasing their concentration (for example) of a reactant which is not involved in the rate determining step will not change the overall reaction rate.

Rate determining step is the stage of a reaction that determines the rate of reaction. For example , if the rate determing step is aA + bB ---> C , then the rate of reaction = k [A]^a [B]^b where k is the rate constant.

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