rates of reaction
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why is it important to measure reaction rates?
• to ensure that chemical reactors are safe • to ensure industrial processes run as economically as possible • to help determine how enzymes function
measuring ROR for reactions where gas is produced
• over water in an inverted measuring cylinder, the rate of gas release (‘evolution’) can then be measured over the course of the reaction • in a gas syringe, the evolution can then be measured over the course of the reaction • open container placed on an electronic balance, gas is allowed to escape and the consequent mass loss can be measured against time
measuring ROR for reactions where precipitate is formed
-cross drawn under beaker containing reaction mixture, time taken for cross to disappear due to the gradual opaquing can be measured (this is proportional to the initial ROR)
other ways of measuring reactions
• an indicator can be used (e.g. starch indicator which forms a blue-black colour with iodine), this is proportional to the initial ROR • measuring change in pH (e.g.in neutralisation reactions) • measuring change in temp (e.g. in exothermic/endothermic reactions)
using tangents to calculate rates
-if a graph of the change of reactant/product against time is plotted, then the ROR at any point = the gradient of the tangent to the curve at that point
collision theory
For two particles to react, they must: • collide at the correct orientation • collide with sufficient energy (energy >Ea)
Ea (activation energy)
the minimum amount of energy a particle must have to successfully react upon collision
factors affecting ROR
• temp: particles have more kinetic energy and move faster -> more frequent collisions -> more frequent successful collisions, an increase in 10 °C will ~double the ROR • surface area: more particles exposed for reaction with another reactant’s particles -> more frequent collisions -> more frequent successful collisions • concentration: greater number of particles in a given volume -> more frequent collisions -> more frequent successful collisions • catalysts: lowers the Ea -> more particles with energy greater than or equal to Ea -> more successful collisions • pressure: greater number of particles in a given volume -> more frequent collisions -> more frequent successful collisions
catalyst definition
• a substance that alters the rate of a chemical reaction - it is not used up in the reaction and is chemically unaltered at the end of reaction