Kinetics+Summary

A reaction occurs when two particles at the right orientation collide with each other with enough energy. Rate of reaction is concerned with how quickly a reaction reaches a certain point (not to be confused with how far a reaction goes...i.e. equilibrium). It is measured in mol dm -3 s -1 (i.e. concentration per sec) Rate = Change in concentration or amount / change in time = (c 2 - c 1 )/(t 2 - t 1 ) = ∆[Concentration of reactants]/∆time or ∆[Concentration of products]/∆time For the reaction aA + bB --> cC + dD Reaction rate: The Maxwell-Boltzmann distribution curve To speed up the reaction, you need to increase the number of the very energetic particles - those with energies equal to or greater than the activation energy. Increasing the temperature has exactly that effect - it changes the shape of the graph. In the next diagram, the graph labelled T is at the original temperature. The graph labelled T+t is at a higher temperature. If you now mark the position of the activation energy, you can see that although the curve hasn't moved very much overall, there has been such a large increase in the number of the very energetic particles that many more now collide with enough energy to react. Remember that the area under a curve gives a count of the number of particles. On the last diagram, the area under the higher temperature curve to the right of the activation energy looks to have at least doubled - therefore at least doubling the rate of the reaction. Factors affecting reaction rate: Concentration: As the concentration of one or both reactants increase, the rate of reaction also increases as there is more of a chance of the reactants colliding with each other. This explains why the greatest rate of reaction occurs when the two solutions are initially mixed. As the reaction proceeds, there are less and less reactants colliding with each other. [] Pressure: When one or more reactants are gases, an increase in pressure can lead to an increase in reaction rate as the particles have less space to move in and so more collisions will occur leading to more reactions. [] Temperature: An increase in temperature means the reactants move faster and with more energy. So an increase in temperature not only means the particles have more of a chance of colliding with each other, but also have enough energy to react. [] Surface area: Reactions occur on the surface area of a reactant, so when the reactants are cut smaller, there is more surface area and so more reactions will occur, leading to a higher reaction rate. Catalyst: A catalyst helps speed up a reaction by offering a pathway needing less activation energy (E a ) to react. Light: Some reactions are greatly increased by exposure to light like silver halides, silver nitrate, hydrogen peroxide and nitric acid etc. More info: []