March 13th: Collision Theory and Rates of Reaction

In class, we learned about 6.1 Collision Theory and Rates of Reaction

The greater the probability that molecules will collide with sufficient energy and proper orientation, the higher the rate of reaction.

The key understandings for this topic are:

    · Species react as a result of collisions of sufficient energy and proper orientation

    · The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time

    · Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and color

    ·  Activation energy (E_a) is the minimum energy that colliding molecules need in order to have successful collisions leading to a reaction

    · By decreasing E_a, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically changed.

Collision Theory:

·         For a reaction to occur, particles must collide with a certain minimum energy (the activation energy) and have the correct orientation

Factors that affect the rate of a reaction include:

    1.      Concentration

·         Solutions:

·         If you increase the concentration of a solution = more particles in the space & more collisions = faster rate of reaction

·         Changing the volume of a solution WILL NOT affect  the rate of reaction

·         Also, increasing the pressure of a gaseous reaction-

-       If you take a gas and increase its pressure (decrease volume of container) = more collisions = faster rate of reaction

    2.    Surface Area

·         Surface area also plays a role in determining how fast a reaction will occur.

·         For example: Crushing a solid up into a powder will greatly increase its surface area and hence the number of collisions possible between reactant particles. One would also expect to see an increased rate of reaction

    3.    Temperature

·         KE of particles is proportional to temperature in Kelvin

·         Activation Energy: the minimum energy colliding molecules need in order to have successful collisions leading to a reaction

·         A match is needed to start a Bunsen burner

·         Maxwell-Boltzmann Distribution

-       If you increase the temperature, particles will have more energy

-       Drop in curve of T₂ is because the area under the curve is fixed – it has to stay the same. The area under the curve is representing the total number of particles (in the sample)

-       At a higher temperature (T₂), we can see that more particles have an energy greater than or equal to the activation energy

We also did a lab that involved measuring the rate of a reaction (and observing the change in the rate of reaction when the temperature is increased or decreased):

·         When sodium thiosulphate reacts with an acid, a yellow precipitate of sulphur is formed

·         In this experiment, we measured how long it took for the sulphur to form

-       This is done by observing the reaction through a conical flask while viewing a black cross on white paper

-       The “X” is eventually obscured by the sulphur precipitate and the time noted

·         In the experiment, we investigated the effects of temperature of the reactants on the rate of reaction by warming up the solutions beforehand in a water bath