Rate Equations

• What is the rate of reaction?

  The change in the amount of reactants or products per unit time
• How do you find the rate from a curve?

  Draw a tangent on the curve and find the gradient of the line
• What is the rate equation?


  K[A]^m[B]^n
• What does the order of reaction tell you?


  How the concentration of reactant A affects the rate
• When will the order of reaction be 0?


  If [A] doubles, the rate will stay the same
• When will the order of reaction be 1?


  If [A] doubles, the rate will double
• When will the order of reaction be 2?


  If [A] doubles, the rate will be 2^2 = 4 times faster
• What changes when you increase temperature?

  Rate constant rises, rate of reaction increases
• What doesn't change when you increase temperature?

  Concentrations and orders of reaction don't change
• What does a concentration-time graph show?


  The amount of reactant decreases with time because it gets used up in the reaction. You can then draw a tangent at 0 and find the gradient
• How do you find the initial rate?

  You don't have to monitor the reaction to the end, plot the graph, draw a tangent and find the gradient
• What is a clock reaction?


  Measuring how the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants
• What assumptions do you have to make with clock reactions?

  Concentration of each reactant doesn't change significantly over time period, temperature stays constant, when endpoint is seen the reaction has not proceeded too far
• What can you assume if your assumptions are reasonable?


  The rate of reaction stays constant during the time period of your measurement
• What is the rate-determining step?


  The step with the slowest rate
• What will be in the rate-determining step?


  If a reactant appears in the rate equation, it will be part of the rate determining step
• What does the order of reaction tell you about the rate-determining step?


  If a reaction's second order with respect to X, there'll be two molecules of X in the rate-determining step
• What does the Arrhenius equation show?

  How the rate constant (k) varies with temperature and activation energy
• What does a large activation energy mean?


  K gets smaller, so the rate of reaction is slow - not many particles have the energy to react, so only few collisions occur
• What does a high temperature mean?


  K increases, so rate of reaction is faster - more particles have activation energy to collide
• What do you do if you want to rearrange to something in the fraction?

  Use ln(k) = ln(A) - E / RT
• What is the Arrhenius equation in terms of y = mx + c?


  ln(k) = -E/R + ln(A)
• What is a partial pressure?


  In a mixture of gases, each individual gas exerts its own pressure
• What is a mole fraction?


  The proportion of a gas mixture that is made up of a particular gas
• How do you calculate mole fractions?


  Number of moles of gas / Total number of moles
• How do you calculate partial pressure of a gas in a mixture?

  Mole fraction of gas x total pressure of the mixture
• What is the equilibrium equation for Kp?

  aA + bB > dD + eE
• When will a temperature equilibrium shift change Kp?

  If the shift decreases the amount of product formed, then Kp will decrease too , if it increases the product, Kp will increase too
• Will a pressure equilibrium shift change Kp?

  Doesn't change it, partial pressures of reactants and products keep Kp constant
• Will a catalyst change Kp?


  No, increases both reactants and products so equilibrium is reached faster
• What is La Chatelier's principle?

  If a reaction is subject to a chance in pressure, temperature or concentration, the equilibrium will shift to counteract the change
• How do you know when a reaction isn't a one step mechanism?


  Because one of the products isn't produced in the first equation
• How do you work out the rate determining step?


  Because only the reactants are in the rate equation
• What are the units for k?

  Mol-1dm3s-1
• What does the rate equation tell you?


  How the rate is affected by the concentration of reactants
• Concentration-time graph for zero order

  
• Concentration-time graph for first order

  
• Concentration-time graph for second order


  Same as first
• Rate-concentration graph for zero order

  
• Rate-concentration graph for first order