2 REACTION RATE
The reaction rate of a chemical reaction is defined in terms of the time it takes to make a reactant disappear or for a product to appear. Sometimes, it is possible to determine the reaction rate from a change in another property where the chemical reaction occurs.
ä 2.1 Collision theory As we have seen, chemical reactions break and form bonds as a result of collisions between elemental units of the reactants. Not all collisions cause the breaking or forming of bonds; not all collisions are effective. Only collisions with sufficient energy and whose molecules have the right orientation may break or form bonds.
ä 2.2 Factors that affect reaction rates Many factors may change the rate of a reaction. Their importance depends on the state of matter of the reactants. They are: Temperature In reactions involving liquid or gas, raising the temperature accelerates the velocity of the elemental units in the reactant. This causes more collisions which have sufficient energy to contribute to the reaction. Concentration of reactants In reactions that are in aqueous solutions, increasing the concentration raises the probability of collisions, which accelerate the reaction rate. State of sub-division of reactants
Working with pictures
In cases where one of the reactants is in a solid state, the size of the reactant determines the reaction rate. You can see an example of this in the figure below.
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Imagine that one side of the cube is 1 cm. Calculate the available surface in all three cases. Work out a general rule as a function of the number of divisions.
Effect of sub-divisions The figure explains why the reaction rate is a function of the size of the solid reactant. Note that as the number of subdivisions grows, the surface area in contact with the other reactant (which may be a gas or liquid) grows. The number of effective collisions between molecules of both reactants will rise and so will the reaction rate.
