Chemical+Reactivity

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 * RATES OF REACTION **

The rates of chemical reactions vary widely. When vinegar is added to baking soda, carbon dioxide is released rapidly because the reaction is very fast. In contrast, fresh concrete takes many hours to “set” while the rusting of iron is a very slow reaction that can take months or years to occur. Reaction rates describe how fast or slow a reaction is, and can be determined by measuring either how fast a reactant is used up or how quickly a product is formed. A variety of measurements can be made to determine how a concentration is changing with time e.g by intensity of colour, volume of gas produced, change in mass or pH. Depending on the reaction rate, measurements may have to be made every few seconds, minutes or even days.

=Collision theory = Temperature measures the **average kinetic energy** of the particles present in a particular sample. However, at any given instant, some particles will have considerably less energy than the average value and others will have considerably more. Before a chemical reaction can occur, the reactants must collide but usually only a small fraction of the total collisions result in a successful reaction occurring. i.e. most collisions are not successful either because they do not collide at the required orientation or position or more usually because they do not have sufficient kinetic energy to get over the activation energy barrier shown in the diagram below.



If there is insufficient energy, the reactants simply remain unchanged.

Reactions with a high activation energy tend to be slower (because fewer of the reactants will have sufficient energy to successfully get over the activation energy barrier) whereas in general fast reactions will have a low activation energy, //E//a.

Note: The nett energy change for a reaction, D r //H //, is independent of the size of the activation energy barrier so there is no correlation between the energy change and the rate of reaction. i.e. there are very exothermic reactions which occur very slowly and vice versa. = =

=Factors affecting reaction rate = If conditions are changed to allow more frequent, effective collisions to occur then the overall reaction rate will increase. There are 4 different ways of increasing the rate of reaction.

When the concentration of a reactant is increased, the frequency of collisions increases as there are more particles available for collision in a given volume at any one time as illustrated below for the reaction of zinc with hydrochloric acid.  zinc + hydrochloric acid ® <span style="font-family: Arial,sans-serif; font-size: 11pt;"> zinc chloride + hydrogen
 * **<span style="font-family: Arial,sans-serif; font-size: 11pt;">Concentration of reactants **



<span style="font-family: Arial,sans-serif; font-size: 11pt;">Note: All chemical reactions get progressively slower with time because as the reactants get used up their concentration decreases which reduces the number of collisions and therefore slows the rate.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The surface area of a solid reactant can be increased by crushing or powdering one big lump into many smaller pieces. This means that more particles will be exposed to collisions with another reactant and this greater frequency of collisions will result in an increased rate of reaction as shown below. A has a relatively low surface area, B has the same amount of material, but much more surface area.
 * **<span style="font-family: Arial,sans-serif; font-size: 11pt;">Surface area **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">NOTES: (i) Dissolving a solid reactant effectively increases its surface area available for collision to the maximum possible as the dissolving process separates the solid into individual particles. e.g. compare the rate of reaction of Pb(NO3)2 and KI in solution and as solids respectively. <span style="font-family: Arial,sans-serif; font-size: 11pt;">(ii) A combustible material may explode when in contact with a spark if the surface area is <span style="font-family: Arial,sans-serif; font-size: 11pt;">sufficiently large e.g. coal dust in mines and flour dust in silos or mills.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">If the temperature is increased the average kinetic energy of the particles also increases and this increases the rate for two reasons. <span style="font-family: Arial,sans-serif; font-size: 11pt;">(i) As the particles move faster they will collide more often with other particles so the collision **frequency** increases. <span style="font-family: Arial,sans-serif; font-size: 11pt;">(ii) More importantly, as the average kinetic energy increases a larger fraction of the collisions will have sufficient energy to successfully overcome the activation energy barrier as shown on the next page i.e. there is an increased frequency of **effective** collisions (those whose energy exceeds the activation energy).
 * **<span style="font-family: Arial,sans-serif; font-size: 11pt;">Temperature **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">A catalyst is a substance that participates in, but is not used up by a reaction and has the effect of increasing the rate of reaction. The catalyst achieves this by providing an alternative pathway that has a lower activation energy requirement than normal, as shown below. <span style="font-family: Arial,sans-serif; font-size: 11pt;">This lower energy requirement means that more particles will collide with sufficient energy to enable successful reaction i.e. the effectiveness of the collisions has been increased.
 * **<span style="font-family: Arial,sans-serif; font-size: 11pt;">Catalyst **