Rate of a reaction

• What is the definition of the rate of reaction?

  Change in concentration of a reactant or a product per unit time

  
• How can you calculate the rate of reaction at a given instant?

  Rate of reaction = change in concentration of reactants or products / time

  
• What does it mean if the order is zero with respect to a reactant?

  Changing the concentration of the reactant has no effect on the rate
• What is the effect on rate in a first order reaction?


  Rate is directly proportional to the concentration

  
• What is the effect on rate in a second order reaction?

  Change in rate = change in concentration squared
• What is the generic rate equation and what does each term mean?

  Rate = k [X]x [Y]y; k = rate constant, [X] and [Y] are concentrations, x and y are the orders of reaction

  
• Do zero order reactants appear in the rate equation? Why?

  No, because they don’t have an effect on the rate

  
• How is the overall order of a reaction calculated?

  Sum of individual orders
• How would you calculate the units of the rate constant?

  Units of rate are mol dm-3s-1 and units of concentration are mol dm-3; rearrange rate equation to get k
• When the overall order is 3, what would be the units for the rate constant?

  dm6 mol -2 s-1
• How could you measure the rate of reaction experimentally?

  Use a colorimeter for color change, or a gas syringe to collect gas volume or measure mass change

  
• What does half-life mean?


  The time taken for the concentration of a reactant to decrease by half
• What is the symbol for half-life?

  t1/2
• How would you draw a concentration-time graph for a zero order reactant?

  It would be a straight line decreasing over time

  
• How would you draw a concentration-time graph for a first order reactant?

  It would be a curve that decreases exponentially over time

  
• What is the relationship between first order reactions and half-life?

  First order reactants have constant half-lives

  
• What is the equation used to determine the rate constant using half-life in a first order reaction?

  k = ln2 / t1/2

  
• How would you draw a rate-concentration graph?


  Plot [A] against time, draw tangents, then plot rate against [A]

  
• What is the relationship between rate and time?

  Rate ∝ 1/t
• How would you draw a rate-concentration graph for a zero order reactant?

  It would be a straight line with a negative slope

  
• How would you draw a rate-concentration graph for a first order reactant?

  It would be a curve that increases steeply at first and then levels off

  
• How would you draw a rate-concentration graph from a second order reactant?

  It would be a curve that increases more steeply than a first order graph

  
• How to determine the rate constant from a rate-concentration graph of first order?

  k = rate / concentration

  
• What is the rate determining step?

  The slowest step in a reaction with multiple steps
• How does the rate determining step link to the species involved in the rate equation?

  Any species involved in the rate determining step appear in the rate equation
• For a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate determining step?

  The order of the reactant

  
• In the rate equation rate = k[NO]2, how many molecules of NO will be present in the rate determining equation?

  2
• What affects the value of the rate constant for a given reaction?

  Temperature, nothing else

  
• What is the effect of a 10℃ temperature increase on the rate of reaction, roughly?

  Doubles the rate of reaction
• What is the Arrhenius equation and what does each term mean?

  Answer not provided in the material

  
• How can you convert the Arrhenius equation into a useful form for plotting a graph?


  ln k = -Ea/RT + ln A; graph of lnk against 1/T is a straight line
• What is the full form of the Arrhenius equation?


  k = A e^(-Ea/RT)

  
• What do the terms in the Arrhenius equation represent?

  k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin

  
• k is the rate constant which depends on temperature and pressure
• The rate equation is the relationship between the concentration of reactants, products and time.