Rates equations and Equilibrium

• What is Kc affected by?

  Only temperature
• What is a zero order reaction?

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

  The rate depends on the concentration raised to the power of 1
• What is a second order reaction?

  The rate depends on the concentration raised to the power of 2
• What are the equations for the 0th, 1st and 2nd order reaction?

  rate = [A]^0

  rate = [A]^1

  rate = [A]^2
• What is the rate equation?

  Rate = k[A]^m[B]^n

  k - rate concentration

  m - order with respect to A

  n - order with respect to B
• What is the overall order and how do you calculate it?

  Overall effect of concentrations of all reactants on the rate of reaction

  overall order = sum of orders with respect to each reactant
• What is the shape of a zero order concentration time graph?

  Straight line with a negative gradient

  Reaction rate doesn’t change at all

  Gradient = rate constant k
• What is the shape of a first order concentration time graph?

  Downward curve with a decreasing gradient

  Reaction slows down with time

  Half life is constant

  Half life = rate constant k
• What is the shape of the second order concentration time graph?

  Downward curve which is steeper at the start
• How do you calculate k from a concentration time graph?

  draw a tangent to the curve and calculate gradient

  calculate from half life - ln2/t1/2
• What is the shape of the zero order rate concentration graph?

  Horizontal line - gradient 0

  y intercept = rate constant k
• What is the shape of the first order rate concentration graph?

  Rate is directly proportional to concentration - straight line through origin

  gradient = rate constant k
• What is the shape of the second order rate concentration graph?

  An upward curve with increasing gradient

  Plot a second graph of rate against the concentration squared - straight line through the origin

  Gradient of straight line = rate constant k
• What is the initial rate and how is it found?

  rate at the start of the reaction

  found by measuring gradient of a tangent drawn at t=0 on concentration time graph
• What is the more useful way of obtaining initial rate?

  a clock reaction - time t is measured for a visual change to be observed
• What does it mean if there is little to no change during the time for the clock reaction?

  The initial rate is the average rate of reaction

  The initial rate is then proportional to 1/t
• What is a common clock reaction?

  Iodine and starch

  Colour change from orange-brown to blue-black
• What are the conditions for dynamic equilibrium?

  Closed system

  Rate of forward = Rate of backwards reaction

  Concentrations of reactants and products are constant
• What is le Chatelier’s principle and how is it applied?

  If the conditions of a reaction in equilibrium change, then the equilibrium will shift to oppose the change

  If the conc of a reactant increases - equ shifts to products

  If the conc of a reactant decreases - equ shifts to reactants

  If the pressure increases - equ shifts to side with lowest g moles

  If the pressure decreases - equ shifts to the side with the most g moles

  If the temp increases - equ shifts to products if endo, opposite if exo

  If temp decreases - equ shifts to products if exo, opposite if endo
• What is the effect of a catalyst on the position of equilibrium?

  Increases the rate of the forwards and backwards reaction the same amount

  Doesn’t effect the position of equilibrium
• What is the importance in the chemical industry of a compromise between chemical equilibrium and reaction rate in deciding the operational conditions?

  The Haber process

  The reaction was exothermic so increasing temp - 450C - decreased yield

  Compromise - yield was decreased but was done to increase rate

  A catalyst was also used to increase the rate

  Pressure of 200 atm - compromise between yield and cost
• What is the equation for Kc?

  Kc value is only affected by temperature

  Kc = conc of products^moles/conc of reactions^moles
• How do you estimate the position of equilibrium from the value of Kc?

  Kc>1 - likely lies towards the products

  Kc<1 - likely lies towards the reaction