Chemical Industry - Equilibria + Rates

Question 1

what is Kc?

Answer 1

equilibrium constant

Question 2

how is Kc calculated?

Answer 2

product of concs of products / product of concs of reactants

Question 3

what is dynamic equilibrium?

Answer 3

when rate of forward reaction = rate of back reaction in a closed system

Question 4

if there is a change in pressure or concentration in the system, what will be the effect on Kc?

Answer 4

none - equilibrium will shift left or right to counteract change so overall Kc will not change

Question 5

if there is a change in temperature, what effect will it have on Kc?

Answer 5

for endothermic forward - raise in temp - forward favoured = rise in Kc

for exothermic forward - raise in temp - back favoured = decreases Kc

Question 6

how do catalysts affect equilibria?

Answer 6

increase rate at which equilibrium is reached but not equilibrium or Kc themselves

Question 7

what is Le Chatelier’s principle?

Answer 7

if there is a change in a closed system at equilibrium, the system will counteract that change to remain at constant equilibrium

Question 8

what is rate?

Answer 8

how quickly a quantity of something changes

Question 9

what are the different methods for measuring rate?

Answer 9

volume of gas produced

change in mass

change in pH

colorimetry

chemical analysis

Question 10

describe how colorimetry can be used to determine rate of reaction

Answer 10

a colorimeter can be used to measure change in colour

measure how long it takes for solution to change colour

Question 11

how can chemical analysis be used to determine rate?

Answer 11

taking samples of mixture at regular intervals, stopping the reaction and determining how much reactant/product is formed

Question 12

how can a sample be quenched?

Answer 12

sitting it on a bed of ice

adding acid/base to neutralise reaction

Question 13

why are rate equations often used?

Answer 13

they allow estimates to be made about the impact of a factor on the rate of a reaction without having to carry out experiments

Question 14

how can the rate be calculated from a graph?

Answer 14

gradient of the line

Question 15

what does a straight line graph say about the rate in relation to the rate equation?

Answer 15

the rate is directly proportional to the concentration of the reagents

rate = k[reactant]

Question 16

what is K in a rate equation?

Answer 16

the rate constant

Question 17

what is the rate constant?

Answer 17

a constant within a rate equation that only varies with the temperature of the reaction

Question 18

how can the units of the rate constant be calculated?

Answer 18

rearrange to make K the subject of the equation

rewrite the equation with the units instead of the terms

cancel out common units

Question 19

what is the general formula of a rate equation?

Answer 19

rate = k[A]^m[B]ⁿ

Question 20

what are the powers in a rate equation?

Answer 20

they are the order of reaction with respect to the substance

Question 21

how is a rate equation formed?

Answer 21

by doing experiments to find the effect of varying concentrations of reactants

Question 22

what is a general rule for the units of the rate constant?

Answer 22

the overall order of reaction will determine the units

Question 23

if a reaction is 0 order overall, what are the units of the rate equation?

Answer 23

mol dm^-3 s^-1

Question 24

if a reaction is 1st order overall, what are the units of the rate constant?

Answer 24

s^-1

Question 25

if a reaction is 2nd order overall, what are the units of the rate constant?

Answer 25

dm³ mol^-1 s^-1

Question 26

how can the initial rate be calculated?

Answer 26

drawing a tangent at t=0 on the graph

Question 27

what does it mean if a reaction is 1st order?

Answer 27

doubling the concentration doubles the rate

Question 28

what does it mean if a reaction is second order?

Answer 28

doubling the concentrations quadruples the rate

Question 29

what does it mean if a reaction is 3rd order?

Answer 29

concentration has no effect on rate

Question 30

what does it mean if a reagent is 0 order?

Answer 30

it does not affect the rate of reaction

Question 31

what is R in the arrhenius equation?

Answer 31

the gas constant - on data sheet

Question 32

what is e on the arrhenius equation?

Answer 32

mathmatical constant

Question 33

what is A in the arrhenius equation?

Answer 33

frequency factor

Question 34

what is the frequency factor?

Answer 34

term that includes factors such as frequency of collisions and their orientation

Question 35

when is A a constant?

Answer 35

across small temperature ranges

Question 36

what is the arrhenius equation used for?

Answer 36

calculating the impact of changing:

E_a

temperature

Question 37

how can the rearrangement of the arrhenius equation fit the equation of a straight line?

rearrangement -> ln k = ln a - E_a/RT

Answer 37

ln k = - E_a/R x 1/T + ln A

y = mx + c

Question 38

how can -E_a/R and ln A be determined from a graph?

Answer 38

plot ln k against 1/T using the y = mx+ c version of the equation

gradient = -E_a/R

y-intercept = ln A

Question 39

if a reaction is raised by 10ºC, what will be the effect?

Answer 39

in many reactions:

rates are roughly doubled

Question 40

what will always change the value of K?

Answer 40

changing the temperature

Question 41

how is the order of reaction determined through experiments?

Answer 41

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

Question 42

what are the 3 ways to determine the order of a reaction once the experiment has been carried out?

Answer 42

progress curve method

half lives

initial rate method

Question 43

what is the progress curve method?

Answer 43

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

Question 44

what does a progress curve show?

Answer 44

how the concentration of a reactant or product changes as the reaction proceeds

Question 45

what is the initial rate method?

Answer 45

drawing tangents at the origin of different progression curves

once you know initial rates for different concentrations, order can be found

Question 46

what is the graph shape of a 0 order reactant?

Answer 46

Question 47

how can half lives be used to determine the order of a reaction?

Answer 47

constant half life = first order

Question 48

what is half life?

Answer 48

the time taken for the reactants to be halved

Question 49

what is the rate determining step?

Answer 49

the slowest step in a mechanism

Question 50

how can the rate determining step be found?

Answer 50

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

Question 51

why are some steps in a mechanism slower than others?

Answer 51

steps have different E_a

Question 52

which reaction will have the largets activation enthalpy in a mechanism?

Answer 52

the rate determining step

Question 53

which has the most energy: reactants, intermediates or products?

Answer 53

intermediates