Cognito cards

• rate = k(A)m(B)n

  define all the terms in the rate equation above

  rate = the rate of the reaction

  k = the rate constant

  (A) = the concentration of reactant A

  m = the reaction order with respect to reactant A

  (B) = the concentration of reactant B

  n = the reaction order with respect to reactant B
• How do you determine the overall order of a reaction from the rate equation ?

  The overall order is the sum of the reaction orders for all reactants in the rate equation
• What is meant by the order of a reaction ?

  The order of a reaction specifies how the concentration of a particular reactant influences the rate of the reaction
• what are the units for the rate constant (k) ?

  The units for the rate constant (k) depend on the overall order of the reaction. For a first order reaction the units are s-1 for a second reaction the units are mol-1 dm3 s-1
• what does a rate equation show ?

  how the rate of a chemical reaction is affected by the concentrations of the reactants
• How does the reactant concentration affect the rate of reaction in a zero order reaction ?

  the rate of reaction is directly proportional to the reactant concentration
• which letter is conventionally used to represent the rate constant ?

  k
• How does increasing the temperature affect the value of the rate constant ?

  Increasing the temperature increases the value of the rate constant
• How does the reactant concentration affect the rate of reaction in a second order reaction ?

  the rate of reaction is proportional to the square of the reactants concentration
• How can measuring half life indicate if a reaction is first order ?

  Measure successive half lives. If the half lives are constant , teh reaction is first order
• what is the formula that links rate constant (k) and half - life (t1/2) of a first order reaction?

  k = ln2/t1/2 k = rate constant (s-1) and t1/2 = half life (s)
• How is a calibration curve prepared in colorimetry ?

  prepare standard solutions of known concentrations, measure their absorbance, and plot absorbance versus concentration. fit a trendline to generate the calibration
• What is a clock reaction ?

  A clock reaction is a reaction used to estimate initial rates by measuring the time it takes for a noticeable change, such as a colour change to occur. The shorter the time, the higher the initial rate.
• How is initial rate determined in the initial rates method ?

  the initial rate is determined by calculating the gradient of the tangent at time = 0 on a concentration time graph
• What is meant by the half life of a reaction ?

  the half life of a reaction is the time it takes for the concentration of a reactant to decrease to half of its original value
• What are the typical shapes of rate vs. concentration graphs for zero, first, and second order reactions ?

  Zero order: straight line First order: exponential decay Second order: exponential decay with steeper slope
• what are the typical shapes of concentration vs time graphs for zero, first and second order reactions ?

  Zero order: straight line (negative correlation First order: exponential decay Second order: exponential decay with steeper slope
• How are iodine clocks used in determining reaction rates?

  Iodine clocks are used to measure the initial rate of a reaction by timing how long it takes from the start of the reaction for the blue black colour to appear. The initial rate of reaction is inversely proportional to the time (t)it takes for the colour to change (1/t)
• How are the unknown sample conc. determined using a calibration curve in colorimetry ?

  to determine the concentration of an unknown sample, measure its absorbance locate this value on the calibration curve and read the corresponding, concentration for the x-axis
• What is meant by the rate-determining step in a chemical reaction ?

  The rate-determining step is the slowest step in a multi-step reaction mechanism, and it determines the overall rate of the reaction
• What chemical species are included in the rate equation of a reaction ?

  The rate equation includes only the chemical species that are involved in the rate determining step of the reaction
• how do you determine the orders of reactants in a rate equation based of the rate determining step ?


  The order of reactants in a rate equation correspond to the number of times each reactant appears in the rate- determining step. These orders are determined experimentally
• What is a reaction intermediate ?

  A reaction intermediate is a chemical species that is formed and consumed during a reaction, but does not appear in the overall equation
• what is the effect of increasing temperature on the rate constant (k)?

  The rate constant increases exponential as a temperature increases
• state and explain the effect of increasing temp on the rate of reaction

  this is because:

  1. at higher temperatures, molecules move faster, resulting in more frequent collisions
  2. at higher, molecules higher kinetic energy, allowing a higher proportion to exceed the activation energy barrier
• what is the arrhenius equation ?

  k = A e (-Ea/RT)

  k = rate constant

  A = pre-expontial factor

  Ea = activation energy (kj mol-1)

  R = gas constant (JK-1mole-1)

  T= temperature (k)
• What is the logarithmic form of the Arrhenius equation ?

  ln(k) = - (Ea / RT) + lnA
• What information does the gradient and y-intercept of an Arrhenius plot tell you ?

  Gradient = -Ea/R

  y-intercept = ln(A)
• Explain why a small increase in temperature results in a large increase in rate of reaction

  A small increase in temperature results in a significantly higher fraction of molecules having energy greater than, or equal to, the activation energy. This means a much higher proportion of collisions are successful and the rate of reaction increases by a large amount