Chemical Industry - Equilibria + Rates Flashcards
what is Kc?
equilibrium constant
how is Kc calculated?
product of concs of products / product of concs of reactants
what is dynamic equilibrium?
when rate of forward reaction = rate of back reaction in a closed system
if there is a change in pressure or concentration in the system, what will be the effect on Kc?
none - equilibrium will shift left or right to counteract change so overall Kc will not change
if there is a change in temperature, what effect will it have on Kc?
for endothermic forward - raise in temp - forward favoured = rise in Kc
for exothermic forward - raise in temp - back favoured = decreases Kc
how do catalysts affect equilibria?
increase rate at which equilibrium is reached but not equilibrium or Kc themselves
what is Le Chatelier’s principle?
if there is a change in a closed system at equilibrium, the system will counteract that change to remain at constant equilibrium
what is rate?
how quickly a quantity of something changes
what are the different methods for measuring rate?
volume of gas produced
change in mass
change in pH
colorimetry
chemical analysis
describe how colorimetry can be used to determine rate of reaction
a colorimeter can be used to measure change in colour
measure how long it takes for solution to change colour
how can chemical analysis be used to determine rate?
taking samples of mixture at regular intervals, stopping the reaction and determining how much reactant/product is formed
how can a sample be quenched?
sitting it on a bed of ice
adding acid/base to neutralise reaction
why are rate equations often used?
they allow estimates to be made about the impact of a factor on the rate of a reaction without having to carry out experiments
how can the rate be calculated from a graph?
gradient of the line
what does a straight line graph say about the rate in relation to the rate equation?
the rate is directly proportional to the concentration of the reagents
rate = k[reactant]
what is K in a rate equation?
the rate constant
what is the rate constant?
a constant within a rate equation that only varies with the temperature of the reaction
how can the units of the rate constant be calculated?
rearrange to make K the subject of the equation
rewrite the equation with the units instead of the terms
cancel out common units
what is the general formula of a rate equation?
rate = k[A]m[B]n
what are the powers in a rate equation?
they are the order of reaction with respect to the substance
how is a rate equation formed?
by doing experiments to find the effect of varying concentrations of reactants
what is a general rule for the units of the rate constant?
the overall order of reaction will determine the units
if a reaction is 0 order overall, what are the units of the rate equation?
mol dm-3 s-1
if a reaction is 1st order overall, what are the units of the rate constant?
s-1
if a reaction is 2nd order overall, what are the units of the rate constant?
dm3 mol-1 s-1
how can the initial rate be calculated?
drawing a tangent at t=0 on the graph
what does it mean if a reaction is 1st order?
doubling the concentration doubles the rate
what does it mean if a reaction is second order?
doubling the concentrations quadruples the rate
what does it mean if a reaction is 3rd order?
concentration has no effect on rate
what does it mean if a reagent is 0 order?
it does not affect the rate of reaction
what is R in the arrhenius equation?
the gas constant - on data sheet
what is e on the arrhenius equation?
mathmatical constant
what is A in the arrhenius equation?
frequency factor
what is the frequency factor?
term that includes factors such as frequency of collisions and their orientation
when is A a constant?
across small temperature ranges
what is the arrhenius equation used for?
calculating the impact of changing:
Ea
temperature
how can the rearrangement of the arrhenius equation fit the equation of a straight line?
rearrangement -> ln k = ln a - Ea/RT
ln k = - Ea/R x 1/T + ln A
y = mx + c
how can -Ea/R and ln A be determined from a graph?
plot ln k against 1/T using the y = mx+ c version of the equation
gradient = -Ea/R
y-intercept = ln A
if a reaction is raised by 10ºC, what will be the effect?
in many reactions:
rates are roughly doubled
what will always change the value of K?
changing the temperature
how is the order of reaction determined through experiments?
conduct several experiments exactly the same, changing the concentration of one substance at a time
ensure that all measurements are made at the same temperature
what are the 3 ways to determine the order of a reaction once the experiment has been carried out?
progress curve method
half lives
initial rate method
what is the progress curve method?
use a progress curve to draw tangents at different concentrations of a reactant
the gradient of each tangent gives the rate of reaction for a particular concentration
then find the order using initial rate method
what does a progress curve show?
how the concentration of a reactant or product changes as the reaction proceeds
what is the initial rate method?
drawing tangents at the origin of different progression curves
once you know initial rates for different concentrations, order can be found
what is the graph shape of a 0 order reactant?
how can half lives be used to determine the order of a reaction?
constant half life = first order
what is half life?
the time taken for the reactants to be halved
what is the rate determining step?
the slowest step in a mechanism
how can the rate determining step be found?
identify molecules in chemical equation that are in the rate equation
identify the order of reaction for each substance
order will give relative number of moles of each substance involved in the rate determining step
why are some steps in a mechanism slower than others?
steps have different Ea
which reaction will have the largets activation enthalpy in a mechanism?
the rate determining step
which has the most energy: reactants, intermediates or products?
intermediates
