Chapter 18 Rates of reactants Flashcards
Define rate of reaction.
Time taken for concentration of reactants to decrease and products to increase
Define order.
This is the power in which individual reactants are raised in the rate equation
Define overall order.
The sum of individual orders present on the rate equation
Define rate constant.
The constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation.
Define half-life
The time taken for the concentration of a reactant to decrease by half.
Define rate-determining step.
The slowest step in the reaction mechanism of a multi-step reaction
How do you calculate units from a rate equation.
Right down all units present
Cancel out the units that are repeated
Bring the units from the bottom to the top
Change the sign to its opposite
Name the order of this concentration-time graph.
0
Name the order of this concentration-time graph.
1st
Name the order of this concentration-time graph.
2nd
Name the order of this rate-concentration graph.
0
Name the order of this rate-concentration graph.
1st
Name the order of this rate-concentration graph.
2nd
what are the units for rate?
mol dm-3 s-1
Can orders be determined from a balanced chemical equation?
NO
How are orders calculated?
From experimental results.
A + B + 2C → D + 2E
when calculating order wrt A keep ___ and ___ constant.
B and 2C
what do you do when calculating orders and you cannot keep the other concentrations constant?
us the orders you know and substitute like an equation, if rate can be calculated and the order can be deduced as 1 or 2, if not it must be 0.
calculate the overall order:
rate = k[X]2 [Y]2 [Z]0
4
what does k equal?
rate constant
how do you calculate k?
rate = k[NO2] [O3]
rearrange:
k = rate /
[NO2] [O3]
describe the half-life of a first order reaction from a conc-time graph.
CONSTANT
what technique can be used to calculate concentrations of solutions?
colorimetry
what two methods are used for continuous monitoring of rate?
- gas collection
- mass loss
what does exponential decay refer to?
the pattern of 1st order reactions (c-t) having a constant half-life.
after a tangent is drawn hoe can you show constant half-life?
the times taken for concentration to half and half should both be equal
eg. 0.5 → 0.25 = 100s
0. 25 → 0.125 = 100s
how do you calculate k from t½?
- k* = ln 2 /
- t*1/2
how do you calculate k(rate constant) from the rate on a conc-time graph?
- rearrange rate equation
- substitute in rate( gradient change in y/x) and concentration (point where tangent is drawn(touching the line))
how do you calculate reaction constant of first order from a rate conc graph?
gradient = change in y / x
how do you calculate reaction constant of first order from a conc-time graph?
draw tangent at conc wanted and calculate gradient
What does a balanced equation show when calculating RDS?
the number of molecules of each species after cancelling the intermediates
If Ea is large k is …
SMALL and therefore rate is slow
If k is large Ea is …
SMALL
what do the two parts in the Arrhenius equation represent?
k = A e-Ea /RT
- A = pre-exponential factor (FREQUENCY of collision with correct orientation)*
- e-Ea /RT *= exponential factor (proportion of molecules that exceed Ea and correct temperature)
what do they stand for and units?
k = A e-Ea /RT
k-rate constant
A- Arrhenius constant
Ea- activation energy J/mol
R- gas constant 8.314 J mol-1 K-1
T- temperature Kelvin
A plot of ____ against ____ .
ln k (logarithm) against 1/T
A plot of ln.k again 1/T gives a straight line with …
gradient = -Ea/R
y-intercept = ln A on y axis
what sign will the gradient ALWAYS be. (hint ; exo)
NEGATIVE
if y-intercept = ln A how do you work out A?
A = e value at y-intercept
increase in A = ______ in k(rate constant)
INCREASE
increase in Ea = ______ in k(rate constant) and why
DECREASE less molecules have sufficient energy to exceed Ea and collide.
increase in T = ______ in k(rate constant) and why?
INCREASE more collisions when temperature increase
if gradient= -Ea/R how do you work out Ea?
Ea= gradient(negative value) x R(8.314)
if gradient= -Ea/R how do you work out Ea?
Ea= gradient(negative value) x R(8.314)
