5.1.1 rates of reaction Flashcards
what is the rate of reaction?
the change in concentration of a reactant or a product in a given time
what is the order of reaction?
the power to which the concentration of a reactant is raised in the rate equation
what is the overall order?
the sum of individual orders of reactants in the rate equation
what is zero order?
the concentration of a reactant has no effect on the rate
what is first order?
a reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of one
what is second order?
a reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of two
rate equation for reaction: A + B → C with orders x for A and n for B
rate = k [A]ˣ [B]ⁿ
how are orders of reactions found?
from experimental results - compare concentration change with change in initial rate
what is shape of a concentration-time graph for a zero order reaction?
a straight line with a negative gradient
- the value of the gradient is equal to the rate constant k
what is shape of a concentration-time graph for a first order reaction?
a downward curve with a decreasing gradient
- the time for the concentration of the reactant to halve is constant (constant half lives)
what is shape of a concentration-time graph for a second order reaction?
a downward curve with a decreasing gradient, steeper than the downward curve of a first order reaction
what is a half-life?
the time taken for the concentration of a reactant to decrease by half
how to determine k for a first order reaction? method 1
calculating the rate constant from the rate
- tangent to curve is drawn
- gradient of tangent is calculated = rate
- k calculated by rearranging the rate equation
how to determine k for a first order reaction? method 2
calculating the rate constant from the half-life
- uses exponential relationship for a constant half-life
- k = ln2 / t(half-life)
what is shape of a rate-concentration graph for a zero order reaction?
a horizontal straight line with no gradient
- y intercept gives rate constant k
- reaction rate doesn’t change with increasing concentration
what is shape of a rate-concentration graph for a first order reaction?
a straight line through the origin
- rate is directly proportional to concentration
- the gradient is equal to the rate constant
what is shape of a rate-concentration graph for a second order reaction?
a upward curve with increasing gradient
- k can be found by plotting another graph of rate against concentration squared, producing a straight line graph through origin - k is equal to gradient
what is a clock reaction?
the time t from the start of an experiment is measured for a visual change top be observed (colour or precipitate)
- initial rate proportional to 1/t
- repeated at different concentrations
- a graph of 1/t against concentration is plotted
what are methods of continuous monitoring?
- monitoring by gas collection
- monitoring by mass loss
- colorimetry - the amount of light absorbed by a solution is measured
what is continuous monitoring used for?
to plot concentration-time graphs
how accurate are clock reactions?
- reasonably accurate provided less than 15% of the reaction has taken place
what is the rate-determining step?
the slowest step in the reaction mechanism of a multi-step reaction
what does the rate equation show about the rate determining step?
the orders show the number of species in the rate determining step
what factors affect the rate constant as temperature increases?
- increasing temperature shifts Boltzmann distribution to right, increasing the proportion of particles that exceed the activation energy
- particles move faster and collide more frequently
what is the Arrhenius equation?
the exponential relationship between the rate constant k and temperature T
k = A e^(- Ea/RT)
what does the exponential factor e^(- Ea/RT) represent?
the proportion of molecules that exceed Ea and that have sufficient energy for a reaction to take place
what does the pre-exponential term A represent?
takes into account he frequency of collisions with the correct orientation
what is the logarithmic form of the Arrhenius equation?
ln k = -Ea/RT + ln A
how is the logarithmic Arrhenius equation shown graphically?
plot ln k against 1/T
- downwards straight line with gradient of -Ea/R and y intercept of ln A
