[inc.] Rates and Equilibria

T/F a successful collision is particles colliding in the correct orientation

FALSE. they also need to have enough energy

what happens during an unsuccessful collision

particles collide and bounce off each other

define rate of reaction, and give its units

speed at which reactant concentration decreases or product concentration increases

mol dm^-3 s^-1

define collision theory

that particles will only react if

• they collide in the right orientation

• they have energy greater than or equal to the activation energy

define activation energy

the minimum amount of energy required for a successful collision between particles

ways of measuring the rate of a reaction

VISUAL: time taken for precipitate to disappear or form, colour change (colorimeter/light sensor and data logger)

MASS: measuring loss of mass of a reaction over time when a gas is produced (open system apparatus)

VOLUME: measuring volume of gas given off by a reaction over time (gas syringe/balance)

CONC: pH, titration

T/F titration is a way of measuring the rate of reaction

FALSE, during titration you keep adding reactant until there is a colour change, time is not measured

although i think uplearn says it does… keep going to find out !!

T/F the rate of reaction during an experiment is never constant

explain why or why not

TRUE. as reactant concentration decreases, there are less reactant molecules to react with each other, so lower frequency of successful collisions, so a lower rate of reaction. rate is constantly changing.

why is the clock experiment a useful experiment

as previously discussed the rate of reaction changes during the reaction

so the true rate of reaction is the initial rate, before the concentration of substances starts to change

in a clock reaction, a colour change happens when a small proportion of reactants have reacted

so the rate of reaction is barely affected by decreasing reactant concentration

describe an iodine clock reaction (3)

I^- ions react with hydrogen peroxide to form I₂.

these I₂ immediately reacts with S₂O₃^2- ions to form I^- again, preventing them from turning starch indicator blue black

only after all the thiosulfate ions have reacted, is there any I₂ remaining which turns starch indicator blue black

factors that affect rate of reaction

temperature

concentration (liquids)

pressure (gases)

surface area (solids)

describe how to carry out an iodine clock reaction

add K₂S₂O₈ into the small beaker

add KI, H₂O, Na₂S₂O₃ and starch into the large beaker

pour the small beaker into the big beaker, immediately start the stopwatch and stir

wait for the colour change into blue-black

stop the timer and record the time

repeat the experiment with different volumes of water and potassium iodide in the big beaker.

explain how temperature affects the rate of reaction

• particles have more kinetic energy

  • so more particles will have the required activation energy and can react

  • so there will be a higher proportion of collisions with enough energy to react

  • so rate increases

• and,

  • increasing kinetic energy means particles move faster

  • this increases the frequency of collisions, thus frequency of successful collisions increases

  • so rate increases

define catalyst

a substance that increases the rate of reaction

that is not used up in the reaction

or permanently chemically changed.

it lowers the activation energy by providing an alternative reaction pathway /

OR it provides lower activation energy through alternative reaction pathway with lower E_a

so that more a greater proportion of molecules have more energy greater than/equal to activation energy ✓

UPLEARN SAYS IT DOES NOT LOWER E_A except PMT accepts that

explain how concentration affects the rate of reaction

increasing concentration means there are more reactant particles in the same volme of liquid

so more frequent collisions

and the rate increases

this is the graph of a reaction with a catalyst

copy this graph, and draw the rate graph for without the catalyst

In this reaction, which substance is a catalyst?

One mole of E-but-2-ene gas is heated with one mole of solid platinum. When one mole of hydrogen gas is added, a reaction takes place. At the end of the reaction, there is one mole of butane gas, and one mole of nickel.

Pt is the catalyst

its not hydrogen, because the reaction will not take place without hydrogen

there is the same amount at the end.

T/F higher concentration means there will be a higher proportion of successful collisions

FALSE. because concentration does not affect the kinetic energy or orientation of particles

temperature does! pressure does not, neither does surface area

sure, though, if there’s a higher freq of collision there’s a higher freq of successful collisions, but the PROPORTION does not increase

explain how pressure affects the rate of reaction [3]

explain how surface area affects the rate of reaction

larger surface area means higher frequency of collisions that can take place

Zn + 2HCl → ZnCl₂ + H₂

sketch a graph of volume of hydrogen produced over time for this reaction

eplain the shape of the graph

at the start, there is a high concentration of reactant particles, that collide most frequently to produce a high rate of reaction. steepest gradient.

as time goes on, there are fewer and fewer Zn and HCl particles remaining as many have reacted to form the product. the concentration of HCl decreases, so fewer successful collisions. rate of reaction decreases

the graph plateaus eventually when all the reactant has been used up and the reaction has finished

in his exam paper, john reads the question:

“the graph shows that a rate of reaction decreases over time. explain why.”

john writes this in response to a question:

concentration decreases, so there are less collisions

improve his answer

concentration (of acid) decreases

DO NOT ACCEPT LESS COLLISIONS, BECAUSE THAT ISN’T LINKED TO RATE

instead: less frequent collisions

explain three types of catalysts

homogeneous — catalyst is in the same phase/physical state as reactants

heterogeneous — catalyst is in a different phase/physical state as reactants

autocatalysis — one of the products acts as a catalyst

draw this graph at a lower temperature

explain why

decreasing temperature decreases the rate of reaction, so gas is produced more slowly, so gradient decreases

the final volume of hydrogen is not changed because this doesn’t affect how much is being produced

this shows the volume of product formed during a reaction.

describe and explain, in words, what this graph would look like at lower pressure.

(orange)

less particles per volume OR less volume same particles

less frequent collisions

so (initial) rate is slower and curve is less steep

so takes longer time to plateau

volume of product is not changed, so plateaus at the same volume of hydrogen produced

john has written the following statement:

a catalyst provides an alternative route. this has a lower activation energy. all reactant particles now undergo this reaction

improve his answer

homogeneous

particles can still undergo the direct route — its just not common

explain how homogeneous catalysts work (4)

• same phase as reactants

• produces an intermediate

• provides an alternative route which has a lower activation energy

• so more particles have the required activation energy or greater and can react

• so a higher proportion and frequency of successful collisions

explain how heterogeneous catalysts work

• the reactant forms weak bonds with atoms on catalyst surface (ADSORPTION ONTO ACTIVE SITE)

• the reactant breaks down/react on the surface of the catalyst

• products form on the active site

• the products desorp/dettach from the surface

interesting

T/F changing the surface area, pressure or concentration of catalyst can change the rate of reaction

TRUE

what graph is this shape, used in kinetics?

and what does it show?

a Maxwell-Boltzmann distribution

shows the distribution of energy of all the particles in a sample of gas

draw a generic maxwell boltmann distribution (4)

(1) for levels - number of particles and energy

(1) for the shape (skewed to the left) and starting at 0,0

(1) for NOT touching the x axis at the end

(1) for labelling E_a

do maxwell boltzmann distribution curves start at the origin? why?

yes, because all particles have some kinetic energy, so 0 particles have 0 kinetic energy

why does the curve not touch the x-axis

a small number of particles have extremely high energy levels

is HBr a homo- or heterogeneous catalyst

homogeneous (reactants in same phase)

draw and label a maxwell distribution curve

what does the area under the maxwell-boltzmann distribution represent

the total number of particles in the gas

if there are so few particles with the required activation energy, why isn’t the percentage yield like 1%

particles to the left of the Ea can gain kinetic energy through unsuccessful collisions (1).

as a result they have the required activation energy or more and can react (1)

what is the effect of increasing the temp on a max-bolt curve. NOT the rate

you may use a diagram to help you.

the peak moves to the right, get lower and wider.

the E_mp (most probable energy) will increase

the number of particles with the E_mp will decrease _{(I guess the energy of particles is more evenly distributed, and because the area under the curve has to stay the same.)}

the area under the curve will remain the same

DIAGRAM:

look outside!!

see the lights as they shine on the seeeea it caaaalls meeee…

okaay

what is the effect of decreasing the number of particles on a max-bolt curve and thus the rate

you may use a diagram to help you (4)

the peak moves downwards but the overall graph stays the same.

the E_mp (most probable energy) will stay the same

the number of particles with the E_mp will decrease

the area under the curve will decrease

EXPLANATION:

decreasing the number of particles decreases the pressure, so there are fewer collisions and a lower frequency of successful collisions. so the rate decreases

T/F: the area under the curve decreases if gas particles are released from the container

TRUE. volume increases, pressure decreases, and there are less successful collisions

apparently. idak 😭😭

what is the outcome of a lower activation energy?

a faster rate of reaction, because more particles have the required activation energy or more, so there is a higher proportion of successful collisions

explain why the peak of the curve is higher

a large number of particles will have low energies

give an example of a homo and heterogeneous catalyst reaction

homogeneous: alcohol and carboxylic acid to form an ester and water with a H+ (aq) catalyst

heterogeneous: contact process uses solid V₂O₅ as a catalyst OR haber process uses solid Fe

draw a general max-bolt distribution curve.

then draw the same curve at a lower temperature

(1) for axis leabels

(1) for shape of general

(1) for shape of T_lower but it must be roughly the same area

draw this reaction with a catalyst

draw this with a catalyst

trick question! you’ll see why in a minute

draw this energy profile with a HETEROGENEOUS catalyst

draw an energy profile with a HOMOGENEOUS catalyst

advantages of catalysts

speed the rate of reaction so lower temperatures and pressures can be used

thus saving energy costs as lower energy demand

thus meaning less CO₂ emissions from burning fossil fuels

uses different reactions with better atom economy and less waste

don’t require specific conditions

A reaction in the presence of a ____ catalyst has a lower _____ than the original reaction, so the curve on the reaction profile is ____

A reaction in the presence of a _____ catalyst takes place via an _____ so it has two curves and two _____

a reaction with heterogeneous catalyst has a lower activation energy that original so the curve is lower

a reaction with homogeneous catalyst takes place via a intermediate so has two curves and 2 transition states

what condition change(s) has happened from curve 1 to curve 2, and how does this affect the rate

• temperature decreased

• number of particles decreased

• rate decreased

these graphs all show the max-bolt distribution of particles at different temperatures. some of these curves are wrong.

explain which and why

A is wrong- area under curve is not the same (OR peak is meant to be higher at low temperatures)

B is wrong for the same reason

C is correct!

D is wrong because the graphs shouldn’t touch near the end

E is wrong because the graphs should onyl cross each other once

F is wrong for the same reason (they don’t cross at all, they should cross 1nce)

G is wrong because the curves don’t diverge immediately

five rules for drawing max-bolt curves

1. at low temps, the peak must be higher, narrower and left

2. area under the curve must be the same at all temps

3. curve must start at the origin and diverge almost immediately from the original curve

4. the second cuve should cross the original curve EXACTLY once

5. the second curve shouldn’t touch the original curve as it nears the horizontal axis

iron is submerged in hydrochloric acid solution. The reaction forms a yellow iron (III) chloride solution as well as another product. How might we measure the rate of this reaction?

Fe + 3HCl → FeCl₃ + 1.5H₂

• use a gas syringe to measure the volume of hydrogen gas produced

• use a colorimeter to identify when the colour change occurs

• use a pH meter to measure acid strength

• use titration to measure concentration of the solution (of acid i’m guessing)

• use a mass balance to measure the change in mass

UNITS

1.67 cm³ min^-1

find the initial rate of this reaction

I got 0.0125

from this, find the orders of reaction

rate = k[A]^X[B]^Y

when the conc of B doubles, the rate stays the same.

so the reaction is 0th order with respect to B

when the conc of A (and B, but that doesn’t affect as we’ve just worked out) doubles, the rate doubles

this the reaction is 1st order w respect to A

so rate = k [A]¹ [B]⁰

label the letters.

what is this diagram called?

is this for an endo or exothermic reaction?

A- energy

B- progress of reaction

C- reactants

D- products

E- activation energy

energy profile

exothermic

which of these reactions happens at the fastest rate, and why?

B. because the reaction has the lowest activation energy.

draw AND LABEL the reaction profile for a 1. endothermic and 2. exothermic reaction

(1) for axis labels - energy and progress

(1) for line labels

(1) for deltaH and E_a correct

(1) for endo and exo the correct way around

what is this point called?

what happens here?

the transition state is the point of

• highest energy

• where reaction is halfway between reactants and products

• reactant bonds have broken, and products start to form

explain why the transition state is the point of highest energy

it is when the reactants’ bonds have broken

bond breaking is an exothermic process so requires more energy

it is just before bonds are made/product formed

which releases energy

the start of equilibria

okaybro

features of dynamic equilibrum

• closed system

• concentration of reactants and products are CONSTANT, (not necessarily the same)

• rate of forward and backward reaction is equal

N₂ + O₂ ⇌ 2NO +66.4kJmol^-1

what happens to the equilibrium position if

• we add NO

• the temperature increases

• the pressure increases

• add O₂

• volume of reaction mixture increases

• we add a catalyst

• temperature decreases

• shifts left

• shifts right

• no change

• shifts right

• no change

• no change

• shifts left

define le chat’s principle

when equilibrium is disturbed, the equilibrium shifts to minimise effects of the change

how does a catalyst affect the rate of the forward and backwards reaction?

and thus does it affect equilibrium position

it increases the rates of both forward and backward reactions, equally

so does not change EP

a reaction runs under 600^oC and has 85% yield. data shows that when 100^oC is used, there is a 90% yield.

explain why manufacturers carry out the reaction at 600C

to increase the rate of reaction

and the yield does not improve that much with lower temperatures

a reaction runs under 2000kPa and has 85% yield. data shows that when 5000kPa is used, there is a 95% yield.

explain why manufacturers carry out the reaction at 2000kPa

the equipment and energy needed to reach and maintain the high pressure is too expensive

too high pressure is dangerous and high risk of explosions and leakage

What you do need to know is that Le Chatelier’s Principle allows us to predict how the equilibrium position will move. It doesn’t give a full explanation as to why. (because it don’t got no brain so it can’t choose to counteract change!)

👍👍👍👍

model answer

1- state relevant features

• if temperature: which is exothermic

• if pressure: which has more moles?

2- movement of EP

3- why the EP moves

• to counteract the increase/decrease in pressure/temperature

4- yield

• if left, decreases equilibrium yield

• if right, increases equilibrium yield

• yield of reactants/products increases

F Me Will You

K_C = [F]³ / [E] [D]²

SO₃ g + H₂O g ⇌ H₂SO₄ l

at equilibrium [SO₃] = 0.400 mol dm^-3

[H₂O] = 0.480 mol dm^-3

and [H₂SO₄] = 0.600 mol dm^-3

1. express the equilibrium constant

2. find the value of the equilibrium constant

H2SO4 IS A LIQUID AND LIQUIDS DON’T COUNT

1 / [SO₃] [H₂O]

1 / 0.48 × 0.4

5.21 dm⁶ mol^-2

sigfigs and units mark

I₂ + H₂ ⇌ 2HI

there are 0.21 moles of HI, 0.03 moles of H₂ and 0.03 moles of I₂.

calculate K_C

you CAN calculate it, despite the lack of concentration!

because the SUM of the MOLES on both sides of the reaction is the SAME

so the CONCENTRATION doesn’t matter!

SO 0.21² / 0.03²

49 no units

what is K_c and how does it link to the equilibrium position

Kc is the equilibrium constant.

when Kc < 1 the equilibrium position is on the left

when Kc = 1 the equilibrium position is in the middle

when Kc > 1 the equilibrium position is on the right

explain the two types of equilibria

homogeneous: when the substances in a reaction at equilibrium are at the same phase

heterogeneous: when the substances in a reaction at equilibrium are in different phases

co (g) + h₂o (g) ⇌ co₂ (g) + h₂ (g)

1.60 moles of CO and 4.00 moles of CO₂ and 4.00 moles of H₂ and 1.60 moles H₂O are found in an 8000cm³ container at 690C at equilibrium. calculate the value of the equilibrium constant.

convert moles to concentration (mol/vol)

[CO] = 0.2 mol dm^-3

[H₂O] = 0.2 mol dm^-3

[CO₂] = 0.5 mol dm^-3

[H₂] = 0.5 mol dm^-3

then: 0.5 × 0.5 / 0.2 × 0.2

= 6.25 no units

at these K_C values, where does the equilibrium lie?

-1

0

2

0.5

3.5

K_C CANNOT BE NEGATIVE !

K_C CANNOT BE ZERO! EQUILIBRIUM IS AT 1!

2 > 1 so right

0.5 < 1 so left

3.5 > 1 so right

express K_C

and find the units

why do we not include solids or liquids in the K_C expression

their concentrations do not change throughout the reaction

give the units for the K_C value

what affects the equilibrium position

what affects the equilibrium constant

changes in temperature, pressure and concentration affect the equilibrium position (as per le chat’s)

ONLY temperature changes affect K_C

N₂O₄ ⇋ 2NO₂ +57.9kJmol^-1

explain how does increasing temperature affect K_C of this reaction (3)

when temperature increases, the equilibrium position moves in the direction of endothermic reaction

so for an endothermic forward reaction, it shifts to the right

products concentration increases as a result of that

K_c = products↑↑↑ / reactants↓↓↓

so Kc increases

H₂+ I₂ ⇋ 2HI -10.4kJmol^-1

how does increasing temperature affect the rate of this reaction

forward reaction is exothermic

increasing temp favours endothermic reaction

so equilibrium position shifts to the left

so concentration of reactants increases

K_C = products↓↓↓ / reactants↑↑↑

so K_C decreases

!! QUESTION IS BASED OFF MANY PMT QUESTIONS, SO TREAT AS AN EXAM Q

2SO₂ + O₂ ⇋ 2SO₃ ΔH = -197.4kJ

a) explain the conditions of low temperature and high pressure would be used to obtain the maximum equilibrium yield of sulfur trioxide.

b) explain why actual conditions used in the industry may be different from the conditions needed for a maximum equilibrium yield

a) Using the FMWY method

Temperature:

Feature: the forward reaction is exothermic.
Movement: Low temperatures favour the exothermic reaction and shift the equilibrium position right.
Why: to counteract the temperature change
Yield: increasing the equilibrium yield of sulfur trioxide

Pressure:

Feature: the right side has fewer moles.
Movement: high pressures favour the side with fewer moles so the equilibrium positions shifts right.
Why: to counteract the temperature change
Yield: increasing the equilibrium yield of sulfur trioxide

[from the mark scheme]

Statement that: Forward reaction is exothermic OR reverse reaction is endothermic ✔

Right hand side has fewer (gaseous) moles OR left hand side has more gaseous moles ✔

Lower temperature / cooling AND increasing pressure shifts (equilibrium position) to the right ✔

b) [copied exact from mark scheme]

Low temperature gives a slow rate OR high temperatures needed to increase rate ✔

High pressure is expensive (to generate) OR high pressure provides a safety risk ALLOW high pressures are dangerous✔

explain why pressure and concentration do not affect Kc

increasing the concentration of reactants means the reaction is not at equilibrium, so no Kc. you wait til the system regains equilibrium, and it does this by producing more product, so Kc will be the same.

pressure… just trust me.

!! QUESTION IS BASED OFF MANY PMT QUESTIONS, SO TREAT AS AN EXAM Q

using a boltzmann distribution curve, explain why the rate of reaction increases with a catalyst [4]

Curve starts within two small squares of origin AND not touching the x-axis at high energy ✓

axes labels: y: (number of) molecules/particles AND x: (kinetic) energy ✓

• Catalyst provides a lower activation energy OR Ec shown below Ea on Boltzmann distribution ✓

• Greater proportion of molecules/particles/collisions have energy above activation energy (with catalyst) OR greater area under curve above activation energy ✓

TIP: when talking about catalyst, talk about proportion of molecules with Ea increases

DO NOT ALLOW two curves
DO NOT ALLOW ‘atoms’ as y-axis label

IGNORE (more) successful collisions
IGNORE response implying ‘more collisions’ (confusion with effect of greater temperature)

this graph shows the volume of gas produced over time.

draw the same graph at a higher and at a lower concentration

then do the same thing for pressure

pressure would be the exact same

explain how catalysts make reactions less harmful to the environment

lower temperatures (and pressures)

so less fossil fuels need to be burnt / less energy required / less CO₂ emissions

less waste or toxic solvent used

find the rate after 50s

giving units

true answer 0.68 cm3 s-1

if the question is about RATE, do NOT chat about le chat

okkok

this graph shows the volume of gas produced over time.

draw the same graph at a higher and at a lower temperature

this graph shows the volume of gas produced over time, of a reaction using 60cm3 of 0.100moldm-3 HCl solution (the limiting reagent)

sketch the graph if 40cm3 of 0.150mol/dm3 solution was used instead

because the same moles are produced, so the same moles of product are formed. the volume does NOT matter.

concentration increases rate of reaction

pros and cons of homo and heterogeneous catalysts

homo:

• very effective since same phase

• BUT same phase so hard to separate

hetero:

• different phases so easy separation

• limited SA; can get poisoned

auto

• cheap no catalyst required

• only works for some reactions. starts slow and once product made, reaction speeds

Using the boltzmann distribution model, explain how the rate of reaction is affected by temperature [4]

done

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