rates (A2) Flashcards
link between orders and half life shown on a graph
0 order: decreases
1st order: constant
2nd order: increases
half life
the time taken for the concentration of a reactant to decrease to half its original value
calculating rate constant, k, from half life
k = ln2 / t1/2
shape of conc/time graph
zero order = straight line with negative gradient
first order = downward curve with decreasing gradient over time
second order = same as first order but steeper at the start and tailing off more slowly
skeleton answer from initial rates table
between experiment ___ and ___, [X] has multiplied by ___, and the initial rate has multiplied by ___. therefore X is ___ order
what do you do if the concentration of 2 chemicals changes? (halfway house method)
- use the chemical you know the order of to find how much the rate changes from just that chemical (‘halfway’)
- the chemical you don’t know the order of must change the rate from the ‘halfway’ point to the final rate, so find the factor that the second chemical changes the rate by
- compare this to the factor the second chemical changes by to find the order
how do you find rate of reaction on a conc-time graph?
draw a tangent to the point where you want to find the rate and then find the gradient of the tangent (change in y - change in x)
how to find the half life from a conc-time graph
divide time by 2 and draw a line across to the graph, and then down at that point to find the time. repeat, dividing again by 2 (and again), to find successive half lives
shape of rate/conc graphs
zero order: horizontal straight line with no gradient
first order: a straight line graph through the origin
second order: an upward curve with increasing gradient
finding k from 0 and 1st order rate/conc graphs
zero order: y intercept gives rate constant
first order:
1. determine the order and the rate equation
2. find the gradient of the line (change in rate over change in conc)
how to make 2-step mechanisms
- the rate equation tells you the reactants in the rate determining step (RDS)
- by comparing these reactants with the overall equation, the second step can be deduced. any 0 order reactants must be in a step after the RDS
- the reactants in the first step produce an intermediate (and possibly some products) which then reacts with the reactants in the second step to form the overall equation
2 step mechanism example
CO2 + 2NO2 —> C + 2NO3
Rate = k [CO2][NO2]
- CO2 + NO2 —> NO3 (product) + CO (intermediate)
- NO2 + CO —> NO3 + C
how to find the overall order of a reaction
the sum of the individual orders e.g. [A]2[B] : 2+1 = 3
what is the Arrhenius equation?
k = Ae^-Ea/RT
where k is the rate constant
A is the ‘pre exponent factor’
Ea is the activation energy in J mol-1
R is the gas constant, 8.314 J mol K-1
T is the temperature in K
Arrhenius equation in the y = mx + c form
lnK = -Ea/R x 1/T + lnA
y = m x X + c
calculating A graphically
A = e^y-intercept
calculating Ea graphically
Ea (J mol -1) = -R x gradient
direct substitution method (when no y intercept)
A = e^y value - (gradient x X-value)
y = mx + c and the Gibbs equation
deltaG = -deltaS x T + deltaH
y = m x X + C
