MODULE 5: Rates of Reactions Flashcards
Name the two equations for rate of reaction?
rate = quantity/time, rate = change in concentration/change in time
What is the order of reaction?
The relationship between the rate of reaction and the change in concentration of the product(s)
What is the rate of reaction proportional to?
The concentration of the reactant(s) to the power of the order (rate = k[A]ⁿ)
What is zero order?
Concentration does not have an effect on rate
What is first order?
When concentration is doubled, rate is doubled
What is second order?
When concentration is doubled, rate is quadrupled
What is the rate equation that shows the relationship between concentrations of reactants and rate?
rate = k[A]ⁿ[B]ⁿ
What is the overall order?
The sum of all of the orders of reactants in the rate equation
How do you determine the units of the rate constant?
Rearrange the equation so you are finding ‘k’, add units to the equation, cross out common units on the denominator and numerator
What is continuous and discontinuous monitoring?
Continually measured and only measured at the beginning and end
What are some methods of continuous monitoring?
Monitoring by gas production, mass loss, absorbance change with a colourimeter
What are the shapes of each order in a CONCENTRATION-TIME graph?
Zero = straight line on a negative gradient First = downward curve with decreasing gradient over time Second = steeper downward curve than first order, tails off more slowly
What is half-life?
Time taken for half of a reactant to be used up (t1/2) (exponential decay)
What two equations can you use to calculate rate constant? (k)
Rearranged rate equation (k = rate/[A]), or half-life equation k = In2/t1/2
What are the shapes of each order in a RATE-CONCENTRATION graph?
Zero = horizontal line with no gradient First = straight line through the origin Second = upward curve with increasing gradient over time
What is initial rate?
Instantaneous rate at the beginning of a reaction (when t=0) which is proportional 1/t
What is a clock reaction?
A reaction where the initial rate can be obtained by taking a single measurement, usually time taken for an observable or visual change (colour/precipitate). This reaction is repeated over several concentrations so 1/t are calculated for each run
Describe an iodine clock
Aqueous iodine is orange-brown, and when sodium thiosulfate is added it delays the colour change of iodine. Once all of the sodium thiosulfate has all reacted, the colour disappears, the iodine can form a starch-iodine complex that turns the solution from colourless to blue-black
What is the rate-determining step?
In a multi-step reaction, the rate determining step is the slowest step
How do you know if the reaction mechanism you have predicted is correct?
The rate equation only involved reacting species involved in the rate-determining step, and the orders in the rate equation match the number of species reacting
What effects k (rate constant)?
Temperature
Why does temperature effect k?
Increasing temperature increases the proportion of molecules with equal or more than the activation energy, and causes an increase in kinetic energy which causes particles to move faster and increases rate of successful collision
What is the Arrhenius equation?
k = A e^-Ea/RT k = rate constant A = pre-exponential factor (frequency) e = exponential factor Ea = activation energy T = Temperature R = gas constant
What do ‘A’ and ‘e^-Ea/RT’ in the Arrhenius equation represent?
A = frequency of collisions with correct orientation (successful) e^-Ea/RT = proportion of molecules that exceed the activation energy
