U3 AOS2: Optimising Chemical Reactions

Question 1

Collision theory

For a reaction to occur, reactants must…

Answer 1

• Collide with each other
• Have sufficient energy to break the reactants’ bonds
• Collide with the correct orientation
  
  • Allows bonds to break & new bonds to form
  • If the orientation is incorrect, particles bounce off each other (reaction does not occur)

NOTE: Most collisions do not result in chemical reactions.

Question 2

Transition state

Answer 2

• New arrangement of atoms when Ea is absorbed
• Occurs at the state of maximum energy potential
• Bonds are both breaking and forming at this stage
• Atoms in this state are highly unstable and rearrange into products as the reaction progresses

Question 3

Activation energy (Ea)

Answer 3

• Minimum amount of energy that must be absorbed to break bonds in reactants so a chemical reaction can occur
• Reactions only occur when the energy of the collision is equal to or greater than the activation energy
  
  • Low activation energy = fast reaction rate
  • High activation energy = slow reaction rate

Question 4

Rate of reaction

Answer 4

= change in concentration / time

• Affected by
  
  • Surface area of solid reactants
  • Concentration of reactants in solution
  • Pressure of gaseous reactants
  • Temperature
  • Catalyst

Question 5

How does increasing surface area increase reaction rate?

Answer 5

• More particles are exposed on the solid’s surface
• More reactants collide with these exposed particles
• This increases the frequency of successful collisions

NOTE: Only the particles at the surface of the solid participate in the reaction.

Question 6

How does increasing concentration and pressure increase reaction rate?

Answer 6

• More reactant particles in the system per unit volume
• This increases the frequency of successful collisions

Question 7

How does increasing temperature increase reaction rate?

Answer 7

• Particles are moving faster and have higher kinetic energy
• This increases the proportion of collisions with E ≥ Ea (more collisions overcome the Ea barrier)

Question 8

How do catalysts increase reaction rate?

Answer 8

• Provide an alternative reaction pathway w/ a ↓ Ea
• Decrease the amount of energy required to break bonds in the reactants by temporarily binding to them
• This increases the proportion of collisions with E ≥ Ea
• Remember that catalysts
  
  • Don’t alter the extent of reaction, therefore the △H value stays the same
  • ↓ Ea of the forward and reverse reaction to the same extent, therefore percentage yield stays the same
  • Aren’t used up in the reaction (can be collected/reused)

Question 9

Homogenous vs heterogenous catalysts

Answer 9

• Homogenous – same state as reactants & products
• Heterogeneous – diff state to reactants & products
  
  • Generally easier to use
  • Can be easily separated from the products
  • Can be used at high temperatures

Question 10

Extent of reactions

Answer 10

• The proportion of reactants that have been converted into products 
• No information about how fast a reaction will happen

Question 11

Open vs closed systems

Answer 11

• Open (e.g. bushfires)
  
  • Matter & energy can be exchanged w/ the surroundings
• Closed (e.g. a submarine under water)
  
  • Only energy is exchanged w/ the surroundings
  • Equilibrium can only be reached in closed systems

Question 12

Reversible vs irreversible reactions

Answer 12

• Reversible – can only proceed in one direction
  
  • Products can be converted back to reactants
  • Changes in states are also reversible
  • Do not proceed to completion, thus, yield of products is never equal to the theoretical yield
  • Can reach dynamic equilibrium
• Irreversible – can proceed both forward & backward
  
  • Products can’t be converted back to reactants
  • Cannot reach dynamic equilibrium

Question 13

Dynamic equilibrium

Answer 13

• Concentration of products & reactants remain constant
• Forward and backward reactions are occurring simultaneously at the same rate (no observable net change)
• Constant temperature, pressure and amount of Ps & Rs
• Reaction is ‘incomplete’
• At a molecular level
  
  • Bonds are constantly being broken and formed
  • Ps & Rs are constantly converted from one to the other

Question 14

Position of equilibrium

Answer 14

• Relative amounts of reactants & products at equilibrium
• Can be changed by:
  
  • Adding/removing a reactant or product at constant V & T
  • Changing volume at constant temperature
  • Changing pressure at constant volume & temperature
  • Changing temperature at constant volume & pressure

Question 15

Le Chatelier’s Principle

Answer 15

• If an equilibrium system is subject to change, the system will adjust itself to partially oppose the effect of the change

Question 16

Effect of changing reactants or products on the position of equilibrium

Answer 16

• Adding reactants / removing products
  
  • Forward reaction is favoured to form more products
  • Position of equilibrium shifts to the right
• Adding products / removing reactants
  
  • Backward reaction is favoured to form more reactants
  • Position of equilibrium shifts to the left

Question 17

Effect of changing pressure or volume on the position of equilibrium 

Answer 17

• Increasing pressure by decreasing volume
  
  • System shifts in the direction that lowers pressure
  • Side with less gas particles is favoured
  • Position of equilibrium shifts to the side with less particles
• Decreasing pressure by increased volume
  
  • System shifts in the direction that increases pressure
  • Side with more gas particles is favoured
  • Position of equilibrium shifts to the side with more particles
• Position of equilibrium is not affected if the number of reactant and product particles are equal

NOTE: This only applies to gaseous systems. In liquids and solids, particles are too tightly for an increase in pressure to have a noticeable effect on volume.

Question 18

Effect of changing concentration on the position of equilibrium 

Answer 18

• Decreased concentration via dilution (+ water)
  
  • System will favour the side with more particles
• Increased concentration
  
  • System will favour the side with less particles
• Position of equilibrium is not affected if the number of reactant and product particles are equal

Question 19

Effect of changing temperature on the position of equilibrium and equilibrium constant (K)

Answer 19

• Exothermic reactions (– △H) release heat
  
  • Increasing temp adds heat to the RHS
    → System wants to decrease heat by favouring the reverse reaction, (equilibrium shifts to the left)
    → Decreased K
  • Decreasing temp removes heat from the RHS
    → system wants to increase heat by favouring the forward reaction, (equilibrium shifts to the right)
    → Increased K
• Endothermic reactions (+ △H) absorb heat
  
  • Increasing temp adds heat to the LHS
    → system wants to consume heat by favouring the forward reaction, (equilibrium shifts to the right)
    → Increased K
  • Decreasing temp removes heat from the LHS
    → system wants to increase heat by favouring the reverse reaction, (equilibrium shifts to the left)
    → Decreased K

TIP: Think of heat as a product (exothermic) or reactant (endothermic).

Question 20

Effect of catalyst on equilibrium

Answer 20

• Decrease Ea of the forward & reverse reaction by same amount
• This will increase the rate in which equilibrium is reached (system can reach equilibrium faster)
• This has no effect on the position of equilibrium or K

Question 21

Equilibrium constant (K)

Answer 21

• Value indicates the extent of reaction at equilibrium
  
  • Affected by temperature
  • NOT affected by concentration, pressure or catalysts
• Small K value – more reactants than products
• Large K value – more products than reactants
• If K is close to 1 – concentrations of Rs & Ps are about equal

Question 22

Reaction quotient (Q)

Answer 22

• Extent of a reaction that is not at equilibrium
• Value of Q changes as the reaction progresses
• Q = K when the system at a particular temp reaches equilibrium
• If Q > K, the system will shift to the left to reach equilibrium
• If Q < K, the system will shift to the right to reach equilibrium

Question 23

How can equilibrium systems be designed to be more energy efficient?

Answer 23

• Catalysts increase reaction rate w/o the need for high temp
  
  • This decreases energy input
  • Lower temp is less expensive and needs less fuel
• Heat exchangers recover wasted heat from exo reactions
  
  • Recover and reuse heat energy
  • Reduces energy input requirements

Question 24

Electrolysis

Answer 24

• Passage of electrical energy from a power supply (e.g. a battery) through a conducting liquid
• Electrical energy is converted into chemical energy
• Allow non-spontaneous reactions to occur

Question 25

Galvanic vs electrolytic cells

Answer 25

• Galvanic cells
  
  • AN OIL RIG CAT
  • Anode = negative, cathode = positive
  • Spontaneous reactions
  • Chemical → electrical energy
  • Negative gradient
• Electrolytic cells
  
  • AN OIL RIG CAT
  • Anode = positive, cathode = negative
  • Non-spontaneous reactions
  • Electrical → chemical energy
  • Positive gradient

Question 26

Using the electrochemical series to predict electrolytic half-cell reactions

Answer 26

• Oxidation reaction is above the reduction reaction on the electrochemical series (positive gradient)

Question 27

Competition at electrodes in electrolytic cells

Answer 27

• Water is a potential reactant when aqueous electrolytes are used (look for the word ‘solution’)
• Reactive electrodes may also participate in the reaction

Question 28

Voltage required to operate an electrolytic cell

Answer 28

= higher half-cell E⁰ – lower half-cell E⁰

• GIVEN WITH A GREATER THAN SIGN e.g. >+1.20V

Question 29

Limitation of the electrochemical series in predicting electrolytic half-cell reactions

Electrolytic cells

Answer 29

• It is based on standard conditions and most electrolysis reactions are not performed at standard conditions
• Reactions are affected by electrolyte concentration, gas pressures, current, voltage and electrode types

Question 30

Separation and continuous removal of products in electrolytic cells

Answer 30

• Electrolytic reactions form products that are reactants of a spontaneous redox reaction
• Must be separated to prevent them spontaneously reacting (undesirable and potentially dangerous)
• This can be done by…
  
  • Using a semipermeable membrane to separate products
  • Using a mesh to separate the electrodes
  • Removing the products as they form

Question 31

Aqueous vs molten electrolytes in electrolytic cells

Answer 31

• Aqueous (aq)
  
  • Used in favour of a molten electrolyte when possible
  • Contains water which can be a potential reactant
  • Can easily operate at SLC
• Molten (l)
  
  • An ionic compound heated to become a liquid
  • No water present
  • Requires much more energy
  • Must operate at high temperatures (at its melting point)
  • Expensive and often hazardous (due to high temp)

Question 32

Chemical additives in electrolytic cells

Answer 32

• Can lower the melting point of molten electrolytes
• This allows the cell to operate at lower temperatures, making it cheaper and safer to run
• E.g. adding CaCl₂ to NaCl (l) lowers its melting point

Question 33

Power supply and external circuit of electrolytic cells

Answer 33

• Power supply must be direct current (current that consistently flows in a single direction)
• Wire forms the external circuit, allowing electrons to flow from the anode to the cathode

Question 34

Primary vs secondary cells

Answer 34

• Primary cells cannot be recharged (disposable)
  
  • Go flat when the reaction reaches equilibrium
  • Products move away from electrodes or are consumed by side reactions, preventing them from being recharged
  • E.g. alkaline cells
• Secondary cells can be recharged
  
  • Aka rechargeable cells or accumulators
  • E.g. lithium-ion cells

Question 35

Secondary cells

Answer 35

• Act as galvanic cells during discharge
  
  • Chemical to electrical energy
  • Spontaneous reaction
  • Anode is negative
• Act as electrolytic cells during recharge
  
  • Electrical to chemical
  • Non-spontaneous reaction
  • Anode is positive

TIP: Recharge = electrical to chemical energy.

Question 36

Conditions required for the recharge of secondary cells

Answer 36

• Connection to a power supply with a voltage higher than that produced by the cell during discharge
• Undamaged electrodes
• Discharge products remain in contact with electrodes (so that they can be converted back into reactants via electrolysis)

Question 37

Over time, batteries cannot be adequately recharged and will need replacing. Why?

Answer 37

• Products may be unable to remain in contact with electrodes after numerous recharge/discharge cycles
• Unwanted side reactions can occur at high temperatures, consuming the cell’s reactants and products
• Species can crystalise (solidify) at low temperatures, preventing the flow of ions
• Electrodes can be damaged

Question 38

Electroplating

Answer 38

• A commercial application of electrolysis
• Results in a thin layer of metal over another surface
• Anode (+) has a metal that loses electrons (oxidation)
• Cathode (–) has an object that gains electrons (reduction)
  
  • Metal ions deposited on the object results in a thin metal layer over its surface

Question 39

Factors that determine the amount of products that form in electrolytic cells

Answer 39

• Charge on the ion in the electrode reaction
• Current flowing through the cell
• Length of time that the current flows

NOTE: Faraday’s laws describe the relationship between these factors.

Question 40

Green hydrogen

Answer 40

• Hydrogen gas produced using renewable energy sources
• Cleanest form hydrogen (no carbon emissions)
• E.g. through PEM electrolysis or artificial photosynthesis

Question 41

Advantages and disadvantages of green hydrogen

Answer 41

• Advantages
  
  • Only product of its combustion is water
  • High energy density
  • Abundant (present in H2O & most carbon compounds)
• Disadvantages
  
  • Limited infrastructure for production/storage/distribution
  • Electrolysis process is energy intensive
  • Need high pressures to efficiently store it as a gas (exp)
  • Not found as an element (energy needed to produce it)
  • Explosive (requires careful handling and storage)

Question 42

Polymer electrolyte membrane (PEM) electrolyser 

Answer 42

• Can be powered by solar/wind power, hydroelectricity, biomass
• The PEM separates e⁻ & gases produced during electrolysis
  
  • Allows protons (H+) to flow, completing the internal circuit, while blocking other ions & e⁻
  • Efficient as it ↓ the contamination/mixing of gases
• Electrodes are often covered with a platinum catalyst which increases the production rate of the gases
• Has an acidic electrolyte
• High rate of hydrogen production ✅
• Expensive due to expensive materials (e.g. catalysts) ❌

NOTE: Increasing the PEM thickness would decrease the efficiency due to added resistance to proton flow.

Question 43

Reactions during PEM electrolysis

Answer 43

• Water oxidation at the anode
  
  • 2H₂O(l) → 4H⁺(aq) + O₂(g) + 4e⁻
• Proton reduction at the cathode
  
  • 4H⁺(aq) + 4e⁻ → 2H₂(g)
• Overall equation
  
  • 2H₂O(l) → 2H₂(g) + O₂(g)
  • Electric current passes through water, causing it to split into hydrogen and oxygen (can then be collected)

NOTE: This is non-spontaneous.

Question 44

Artificial photosynthesis

Answer 44

• Human-made materials capture sunlight and split water molecules to create hydrogen and oxygen
• Electrodes are often covered with catalysts which increases the rate of production of the gases
• H₂(g) is produced instead of glucose
• Acidic

Question 45

Reactions during artificial photosynthesis

Answer 45

• Water oxidation in acid using catalysts (anode)
  
  • 2H₂O(l) → 4H⁺(aq) + O₂(g) + 4e⁻
• Proton reduction in the presence of catalysts (cathode)
  
  • 4H⁺(aq) + 4e⁻ → 2H₂(g)
• Overall equation
  
  • 2H₂O(l) → 2H₂(g) + O₂(g)
  • Electric current is generated by sunlight

Question 46

Advantages of artificial photosynthesis

Answer 46

• Does not create greenhouse gases (liquid water is produced)
• Does not require the use of fossil fuels
• Can remove carbon dioxide from the atmosphere