Chapter 18: Kinetics (Rates)

Question 1

What are the main factors that affect the rate of chemical reactions

Answer 1

• Temperature
• Concentration
• Pressure
• Surface Area
• Catalysts

Question 2

What is a reaction rate and how do you calculate the rate of reaction at any instant

Answer 2

• Is the change in the concentration of reactants and products per unit time
• Draw a tangent to the curve and find its gradient

Question 3

What is the rate expression

Answer 3

• Tells us about the contributions of species that affect the reacyion rate
• The detail of how each species contributes to the reaction can only be found by experiment
• Describes how the rate at a particular temperature depends on the concentration of species involved

Question 4

What do square brackets mean

Answer 4

• Mean the concentrations

Question 5

What is the general rate equation

What are the different reaction orders

Answer 5

• Rate = k[A]^m[B]ⁿ
• If [A] changes and the rate stays the same - the order of reaction with respect to A is 0
• If the rate is proportional to [A] e.g if A doubles the rate doubles then the order of reaction with respect to A is 1
• If the rate is proportional to [A]² e.g if A doubles the rate quadruples then the order of reaction with respect to A is 2

Question 6

How do you calculate the overall order of the reaction

Answer 6

• Add the orders of each of the reactants in the rate equation

Question 7

What is the rate constant

Answer 7

• By putting a rate constant you can remove the use of proportionality sign
• k is the rate constant and is different for every reaction and varies with temperature
• Bigger it is the faster the reaction - increase temperature rate constant increases.
• units of the rate constant depend on the overall order of the reaction

Question 8

How do you find the order of a reaction by using rate-concentration graphs

Answer 8

• One method of finding the order of a reaction with recpect to a particular species is to plot a graph of rate against concentration
  
  • Plot the original graph of [A] against time and draw tangents at different values of [A] - the gradient of these tangents are the reaction rates
  • The values for these can be used to create a second graph of rate against concentration

Question 9

What are the different shapes of rate-concentration graph which tell you the order of that reactant

Answer 9

• If the graph is horizontal (straight line) this means the rate is unaffected by [A] - order is 0
• If the garph is a sloping straight line through the origin then the order is 1
• If the graph is not a straight line the order cant be found directly - it could be 2

Question 10

What is the initial rate method

Answer 10

• A series of experiments is carried out at constant temperature
• Each experiment starts with a different combination of initial concentration of reactants, catalyst
• Concentration of 1 species varies - rest stays the same.
• For each experiment, the concentration of 1 reactant is followed and a concentration-time graph is plotted.

Question 11

What is the effect of temperature on k

Answer 11

• Small changes in temperature produce large changes in reaction rates
• The rate constant allows you to compare the speeds of different reactions at a given temperature
• The larger the k the faster the reaction

Question 12

What is the Arrhenius equation

Answer 12

• Increasing the temperature increases the number of collisions that have energy greater than the activation energy.
• The fraction of molecules with energy greater than the activation energy is e^-E_a/RT
• The activation energy can be linked to the rate constant by the Arrhenius equation

Question 13

What is the rate-determining step

Answer 13

• The rate of the slowest step will govern the rate of the whole process.
• The slowest step may form a “bottleneck” called the rate determining step
• In a chemical reaction, any step that occurs after the rate determining step will not affect the rate.