physical chem; rate equations Flashcards
how is rate calculated from a graph?
rate be found from the gradient
gradient = change in y / change in x
what are the orders of reaction?
an order is a power to which a concentration is raised in the rate equation - tells us how the concentration of the substance affects the rate
what is the zero order?
changes in conc do not affect the rate
e.g. if [A] doubles then the rate doesn’t change
what is the 1st order?
changes in conc have a proportional change on the rate
e.g. if [A] doubles then rate doubles
what is the 2nd order?
changes in conc have a squared proportional change on the rate
e.g. if [A] doubles the rate quadruples
what is the rate constant (k)?
a constant number that allows us to equate rate & conc
why is the rate fixed at a certain temp?
as changes in temperature causes rate to change
how is the rate calculated?
rate = k [A] [B]
how is the rate constant calculated?
- write the rate expression
- rearrange to make k the subject: k = rate / [x]
- work out the units
what is the iodine clock experiment?
it’s a reaction that times how long it takes for a colour change to occur
- add sodium thiosulfate & starch (acts as an indicator) to reaction mixture
- sodium thiosulfate reacts immediately with I₂
- reaction is monitored by sitting reaction vessel on paper with a cross on it - time how long it takes for the cross to disappear
write the balanced equation for the iodine clock reaction including states
H₂O₂ + 2I⁻ + 2H⁺ + → I₂ + 2H₂O
why does the +ve test for starch produce a blue/black colour?
when there is no more sodium thiosulfate left the I₂ reacts with starch - produces blue/black colour
how can we calculate the order of reaction for the iodine clock experiment?
by varying the conc of I₂ &/or H₂O₂ & keeping everything else constant - results in time taken for the blue/black colour to appear changing - we can work out the order of reaction
state the 3 ways in which rate can be calculated
- by change in pH of a reaction
- by calculating the amount of mass lost
- by calculating volume of gas produced
explain how the change in pH of a reaction allows us to calculate rate
- the pH of a reaction may change over time if H⁺ ions are used up/produced
- a pH meter measures pH of a reaction at regular intervals - can then calculate H⁺ ions conc
explain how using the amount lost in a reaction allows us to calculate rate
- for reactions that produce gas - place reaction on balance & measure the mass loss (which is gas loss)
- mole calculations can be used to work out no.of moles of gas lost therefore amount of reactants left
explain how using the volume of gas produced allows us to calculate rate
- measure the amount of gas produced using a gas syringe & measure over a specified time
- use the ideal gas equation to work out the no.of moles of gas produced - then use molar ratio in equation to work out reactants conc’s
for which reactions is a colorimeter used?
reactions with colour changes are measured with a colorimeter
what does a colorimeter measure?
measures the absorbance of light by a coloured sample - the more concentrated a sample is = darker in colour
what is a rate-concentration graph?
is created by knowing the rate. the rate is found by taking the gradient at various points on the conc-time graph
describe the rate-concentration graph for the zero order
straight horizontal line (as its constant)
describe the rate-concentration graph for the 1st order
straight diagonal line (as changes are in equal amounts)
describe the rate-concentration graph for the 2nd order
curved line (as changes are in unequal amounts)
what is the rate determining step?
it’s the slowest step in a multi-step reaction
why is the rate determining step important?
as the whole reaction rate depends on how quick the rate determining step is
what affects the rate of reaction?
reactants that appear in the rate equation - these reactants must appear in the rate determining step & catalysts
what is the arrhenius equation?
k = Ae (-Ea/RT)
k - rate constant
Ae - arrhenius constant
Ea - activation energy (J)
R - gas constant (8.31 JK-1 mol-1)
T - temperature (k)
explain what happens to the rate constant of Ea gets smaller & why
Ea gets smaller = rate constant gets bigger - means that as Ea drops, the rate of reaction increases so many more particles have enough energy to react
explain what happens to the rate constant as temperature increases & why
temp increases = rate constant increases - as when temp increases, particles have more KE & are more likely to collide with at least the Ea so rate of reaction increases
rearrange the arrhenius equation to make Ea the subject
Ea = (InA - Ink) x RT
rearrange the arrhenius equation to make k the subject
Ink = InA - (Ea/RT)
