rates of reaction

Question 1

why is it important to measure reaction rates?

Answer 1

• to ensure that chemical reactors are safe
• to ensure industrial processes run as economically as possible
• to help determine how enzymes function

Question 2

measuring ROR for reactions where gas is produced

Answer 2

• 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

Question 3

measuring ROR for reactions where precipitate is formed

Answer 3

-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)

Question 4

other ways of measuring reactions

Answer 4

• 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)

Question 5

using tangents to calculate rates

Answer 5

-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

Question 6

collision theory

Answer 6

For two particles to react, they must:

• collide at the correct orientation
• collide with sufficient energy (energy >Ea)

Question 7

Ea (activation energy)

Answer 7

the minimum amount of energy a particle must have to successfully react upon collision

Question 8

factors affecting ROR

Answer 8

• 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

Question 9

catalyst definition

Answer 9

• 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