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A-Level Chemistry > Chapter 10 Reaction Rates and Equilibrium > Flashcards

Chapter 10 Reaction Rates and Equilibrium Flashcards

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1
Q

What is the rate of a chemical reaction?

A

How fast a reactant is used up or how fast a product is formed
Can be defined as the change in concentration of a reactant or product in a given time (mol dm^-3 s^-1)

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2
Q

Concentration time graph

A

Rate of reaction is fastest at the start of the reaction as each reactant is at its highest concentration (steepest)
Rate of reaction slows down as the reaction proceeds, because the reactants are being used up and their concentrations decrease (less steep) - but concentration of products continues to increase at a slower rate
Once one of the reactants is completely used up (limiting reactant)m concentration stops changing and rate of reaction is zero (horizontal line)

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3
Q

What factors can change the rate of a chemical reaction?

A

Concentration (or pressure - when reactants are gases)
Temperature
Use of a catalyst
Surface area of solid reactants

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4
Q

Collision theory

A

Two reacting particles must collide for a reaction to occur - only a small number of collisions result in a chemical reaction, most of the time they just bounce off each other

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5
Q

When is a collision effective?

A

When the particles collide with the correct orientation and when particles have sufficient energy to overcome the activation energy barrier of the reaction

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6
Q

What does increasing concentration do?

A

Rate of reaction increases as there is an increase in the number of particles in the same volume - thus the particles are closer together and collide more frequently ; therefore there will be more effective collisions leading to an increased rate of reaction

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7
Q

How does increasing pressure affect rate of reaction?

A

Concentration of gas molecules increases as they are more crowded - thus more collisions leading to more effective collisions

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8
Q

How to follow the progress of a reaction?

A

Monitor decrease in concentration of reactant

Follow formation of a product (increase in concentration)

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9
Q

Reactions that produce gases?

A

Monitor volume of gas produced at regular time intervals
Monitor loss of mass of reactants using a balance
THESE ARE BOTH PROPORTIONAL TO THE CHANGE IN CONCENTRATION of a reactant or product ; both give a measure of rate of reaction

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10
Q

Practical to measure gas production?

A

Reactant put in a conical flask and bung is taken off
Initial volume of gas in measuring cylinder is recorded (MAKE SURE THERE ARE NO AIR BUBBLES BEFORE YOU START - water level should be close to zero)
Add catalyst or second reactant and then put on bung - which should have a delivery tube attached coming to the cylinder
Gas produced, water displaced, measure volume of gas produced at regular intervals until no more gas produced
OR YOU COULD USE A GAS SYRINGE (more precise scale)

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11
Q

What is done after collecting the gas?

A

Graph plotted to show volume of gas produced against time - to find initial rate of reaction a tangent is drawn at t=0 ; gradient gives reaction rate (cm^3/s)

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12
Q

Monitoring the loss of mass of reactants using a balance?

A

Rate of reaction can also be determined by monitoring the loss in mass of reactants over a period of time ; carbonate and the acid are added to a flask on a balance and then the mass is recorded initially and at regular time intervals - complete when no more mass is lost ; graph of mass lost against time is plotted (opposite to volume of gas produced and time)

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13
Q

What is a catalyst?

A

A substance that changes the rate of a chemical reaction without undergoing any permanent change itself - it is not used up in the reaction
Offers an alternative reaction pathway with a lower activation energy

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14
Q

DRAW EXO AND ENDO DISGRAMS WITH Ea and Ec

A

Lower activation energy

No change to energy change

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15
Q

Homogenous catalysts?

A

Catalysts that have the same physical state as the reactants - the catalyst reacts with the reactants to form an intermediate ; this then breaks down to give the product and regenerates the catalyst

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16
Q

Example of homogenous catalyst?

A

Ozone depletion - Cl. Radicals act as catalyst with the depletion of ozone (O3) into O2
Both gases

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17
Q

What happens if the reactants are different states?

A

BOTH reactants have to be the same state as the catalyst

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18
Q

Heterogenous catalyst?

A

Different physical state from the reactants - they are usually solids in contact with gaseous reactants or reactants in solution ; reactants are firstly adsorbed (weakly bonded) to the surface of the catalyst and after the reaction takes place, they leave the surface of the catalyst through desorption

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19
Q

2 reactions that involve heterogenous catalysts?

A

Haber process - make ammonia iron (s) is catalyst and the reactants are N2 and 3H2
Hydrogenation of Alkenes - reactants are gases with a solid nickel catalyst

20
Q

What are catalytic converters?

A

Contain a catalyst like platinum/palladium where harmful gases are turned into less harmful ones (carbon monoxide oxidised to carbon dioxide and nitrogen monoxide to nitrogen) - they are non-toxic
EXPENSIVE HOWEVER
HONEYCOMB STRUCTURE TO INCREASE SURFACE AREA FOR HIGH RATE OF REACTION

21
Q

Catalyst effects on industry?

A

Increases rate of reaction by lowering activation energy - reduces temperature needed for the process and energy requirements
Less electricity/fossil fuels used and this cuts costs - increasing profitability ; outweigh the costs in developing a catalytic process
Sustainability now means high atom economy and few pollutants - less fossil fuels = less emissions of CO2 (global warming)

22
Q

Boltzmann distribution?

A

Exemplifies the spread of molecular energies in gases - some molecules move slowly with low energy and some molecules move fast with high energy ; most molecules move close to the average speed and have the average energy
LINE IS MARKED WITH Ea SHOWING THE ACTIVATION ENERGY AND THAT ONLY A SMALL PROPORTION OF THE MOLECULES HAVE MORE ENERGY THAN Ea TO REACT

23
Q

Features of Boltzmann Distribution?

A

No molecules have zero energy (starts at origin)
X axis = energy
Y axis = number of particles with given energy
Area under graph = number of total particles
No maximum energy for a molecule - does not meet the x-axis at high energy and the curve would need to reach infinite energy to meet the x-axis

24
Q

Effect of temperature on Boltzmann?

A

Average energy of molecules also increases - more molecules have higher energy ; graph is now stretched over a greater range of energy values and the peak of the graph is lower on y-axis but further along on x-axis
Number of molecules is the same so area is the same
Peak is at higher energy

25
Q

At T2 (higher energy on Boltzmann)?

A

More molecules have energy greater than or equal to activation energy
Greater proportion of collisions lead to increased rate of reaction (effective)
Collisions are also more frequent because more KE but increased energy of molecules is more improtant

26
Q

Boltzmann distribution and catalysts?

A

Move Ea back to Ec - greater proportion of particles exceed the lower activation energy
On collision, more molecules will collide effectively to form products - increasing rate of reaction

27
Q

Reversible reactions

A

Reactions that take place in both the forward and reverse direction (Haber process) - ->

28
Q

Dynamic equilibrium

A

Rate of forward reaction = rate of reverse reaction
Concentrations of reactants and products do not change
They are dynamic as both reactions are taking place - as fast as reactants are becoming products, the products are becoming reactants thus concentrations stay the same

29
Q

What must a system be for dynamic equilibrium?

A

MUST BE A CLOSED SYSTEM - isolated from its surroundings so temperature, pressure and concentrations of reactants and products are unaffected by outside influences

30
Q

Le chatelier’s principle

A

When a system in equilibrium is subject3 to an external change the system readjusts itself to minimise the effect of that change

31
Q

Effect of concentration changes on equilibrium

A

Changes the rate of forward or reverse reaction which changes the position of the equilibrium
If more products formed then position has been shifted to the right
If more reactants form then position has shifted to the left
AFTER READJUSTMENT

32
Q

If reactants and products are different colours

A

Then you can exemplify this with an experiment
Aqueous chromate ions + H+ (yellow) -> CrO72- + H2O (orange)
Add solution of yellow potassium chromate and sulfuric acid (forward reaction) - turns orange (more H+ ions - increases forward reaction rate - causes equilibrium position to shift to minimise the change in H+ conc ; decreases the conc of H+ ions by making more products and shifting to the right)
ADD IN NAOH (H2O) MEANS MORE PRODUCTS THEREFORE EQUILIBRIUM SHIFTS TO LEFT AND TURNS YELLOW AGAIN (decreases conc of H+ ions - decreases rate of forward reaction and causes equilibrium to shift to minimise the change ; increases concentration of reactant by moving to the left making more H+ and turning it yellow)

33
Q

Effect of temperature on equilibrium?

A

Forward reaction has opposite Enthalpy change to reverse reaction
INCREASE IN TEMPERATURE CAUSES SHIFT IN ENDOTHERMIC DIRECTION (+)
DECREASE IN TEMPERATURE CAUSES SHIFT IN EXOTHERMIC DIRECTION (-)

34
Q

Effect of pressure changes on equilibrium

A

Results in equilibrium position changing
If increase in pressure - equilibrium shifts to the side with the least number of gaseous moles
As there are fewer gaseous moles on one side - it minimises the increase in pressure
VICE VERSA

35
Q

Effect of a catalyst on equilibrium?

A

Does not change the position of equilibrium and only speeds up rate of forward and backwards reaction equally - increases rate at which equilibrium is established

36
Q

Compromise low temp

A

Produce high yield by slow rate of reaction - may be too slow so no equilibrium/not profitable

37
Q

Compromise high pressure

A

Increases conc and rate of reaction but requires a lot of energy and very expensive - also safety is a concern as it could cause explosions/leak out and be toxic
Reactants are also recycled repeatedly so all is used

38
Q

Equilibrium constants?

A

Provide the actual position of equilibrium - magnitude of equilibrium indicates whether there are more reactants or more products in an equilibrium

39
Q

Reaction for Kc

A

aA +bB->

40
Q

Kc =

A

([C]^c[D]^d)/([A]^a[B]^b)
Products/reactants
[] = equilibrium concentrations of the reactants and products of this equilibrium

41
Q

How to deal with units

A

CANCEL THEM OUT FIRST

42
Q

Kc = 1

A

Position of equilibrium is halfway between reactants and products

43
Q

Kc>1

A

Position of equilibrium that is towards the products

44
Q

Kc<1

A

Position of equilibrium towards the reactants

45
Q

Larger the value of Kc

A

Further the position lies to the right and the greater the concentration of the products compared with the reactants

A-Level Chemistry (28 decks)

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