how can the rate and yield of chemical reactions be optimised?

measuring reaction rates

progress of reaction:

• monitored by either increase or decrease of reactant or formation of product

observe reaction rates by measuring change of:

• volume of gas

• mass of solid produced

• decrease in mass (gas evolved)

• intensity of colour of a solution

• formation of precipitate

• pH

• temp

electrolytic cell

electrical cell where non-spontaneous redox reaction occurs by using external potential difference across electrodes

electrolysis

non-spontaneous chemical reaction occurs by passing current through substance

• electrical → chemical energy

• one container

factors affecting electrolysis of solutions

• electrolyte

• concentration

  → Cl_{2 (g)} + e^-→ Cl^-_(aq) can move down if high concentration

• electrode

• gas pressures

• temperature

• current

• voltage

• impurities

electroplating

process of adding a thin metal coating by electrolysis

• anode: metal being plated onto article (gradually decreases & maintain metal ion’s concentration)

• cathode: article to be plated

• elctrolytic solution: salt of metal being plated

• low voltage electric current

  → metal atom lose e^- → go into solution as ions

  → metal ions gain e^- → deposit as metal coating on the cathode

factors that alter quality of metal coating formed

• type & concentration of electrolyte

• concentration of cations to be reduced

• shape of anode must be similar to cathode for an even metal coating

• compounds (making brighter/shinier)

design feature or operation principle of commercial electrolytic cells

• seperation and continuous removal of products

• inert or reactive electrode materials

• molten or aqueous electrolyte

• chemical additives to electrolyte

seperation and continuous removal of products

• ensure products do not react spontaneously

(ie. semi-permeable plastic membrane in membrane cell seperates chloride and hydrogen gas, which are continuously removed)

inert or reactive electrode materials

• cost of electrodes

• ability of electrodes to withstand cell operating conditions (electrodes must have high m.p for use in cells with molten electrolyte)

(ie. carbon electrodes are cheap and have high m.p)

molten or aqueous electrolyte

• whether presence of water will interfere with the electrolytic production of desired products

(ie. in the membrane cell, electrolysis of NaCl _(aq) results in production of H_{2 (g)} )

chemical additives to electrolyte

• lower m.p of molten electrolyte or are the solvent for the compound that is electrolysed

(ie. in down's cell, addition of CaCl₂ to molten NaCl _(l) lowers its m.p)

(ie. in the hall-heroult cell, molten cryolite is the solvent for alumina)

isuses with electroplating

• many toxic solutions used

• costly waste treatment

secondary cell (rechargeable batteries)

cell that can be recharged once its production of electric current drops

• galvanic cell (discharge) + electrolytic cell (recharge)

discharge

use spontaneous reaction to produce electricity

recharge

convert electrical energy back into chemical

• discharge products remain in contact with the electrodes at which they are produced

• connecting (-) terminal of charger to (-) battery, (+) to (+) to force e^- to travel in reverse

  → original reaction reversed and recharged

conditions required to make a battery rechargeable

discharge reaction can be reversed

• charger used to force e^- in the opposite direction

  → charger voltage > operating voltage

• products of discharge can be reversed by changing flow of e^-

• products of discharge must be available for recharge

  → must remain in contact with electrodes

  → must not be lost

green hydrogen from electrolysis

• alkaline electrolysis cell

• polymer electrolyte membrane elctrolysis cell (PEMECs)

• solid oxide electrolysis cells

polymer electrolyte membrane electrolysis cell (PEMECs)

• powered by renewable energy (photovoltaic (solar) or wind)

strengths of polymer electrolyte membrane elctrolysis cell (PEMECs)

• adaptability of size

• production of high purity hydrogen

• operating temp <100°C

  → less energy required

• potential for significant increase in efficiency

limitations to the polymer electrolyte membrane electrolysis cell (PEMECs)

• cost

  → expensive catalysts and membrane

green hydrogen from artificial photosynthesis

• light capture & e^- transport system

• water splitting

• CO₂ reduction

strengths of green hydrogen from artificial photosynthesis

• does not produce greenhouse gases

• does not require fossil fuels

• can remove CO_{2 (g)} from atmosphere

• produces O_{2 (g)}

• can create green ammonia

collision theory

for a chemical reaction to occur:

• particles must collide

• particles must collide with sufficient energy (break bonds)

  → activation energy

• particles must collide in the correct orientation

activation energy

minimum energy required by reactants in order to react

factors affecting rate of reaction

• surface area

• concentration/pressure

• gas pressure

• temperature

• use of catalysts

surface area

↑ surface area, ↑ particles available at the surface to react, ↑ frequency of successful collisions

concentration/pressure

↑ concentration/pressureti, ↑ particles, ↑ frequency of successful collisions as particles are closer & more are present

gas pressure

↑ gas pressure, ↓ space that particles can move, ↑ frequency of successful collisions as particles are closer

temperature

↑ temperature, ↑ kinetic energy, ↑ proportion of particles with energy to overcome activation energy, ↑ frequency of successful collisions (more effective)

OR

↑ temperature, ↑ kinetic energy, ↑ speed of particles, ↑ frequency of successful collisions

catalysts

substance that provides alternative pathway & decreases activation energy (is not consumed)

• ↓ activation energy pathway, ↑ proportion of particles with energy to overcome activation energy, ↑ frequency of successful collisions

open systems

system which both matter & energy can be transfered to & from its surroundings; reactants & products not contained

closed systems

system which energy, but not matter can be transfered to & from its surroundings; reactants & products contained

• no insulation

isolated systems

system which neither matter nor energy transfer to or from its surroundings

• insulation

heterogeneous catalysts

catalyst that have a different physical state to reactants/products

homogeneous catalysts

catalysts that have the same physical state to reactants/products

reversible reactions

products re-form into reactants

irreversible reactions

products do not re-form into reactants

dynamic equilibrium

• concentration of reactants & products do not change

• rate of forward reaction = rate of reverse reaction

• REACTION DOES NOT STOP!!

equilibrium constant (K)

value of concentration fraction at equilibrium

reaction quotient (Q)

value of concentration fraction at any stage of reaction

le chatelier’s principle

if an equilibrium system is subjected to a change, the system will adjust itself to partially oppose that change and restore equilibrium

• system will favour (forward/reverse) reaction to partially oppose to (change)

• position of equilibrium shifts to (left/right)

increasing reactants

response: decrease concentration of reactants
reaction favoured: forward reaction

equilibrium shift: →

increasing products

response: decrease concentration of products
reaction favoured: reverse reaction

equilibrium shift: ←

decreasing reactants

response: increase concentration of reactants
reaction favoured: reverse reaction

equilibrium shift: ←

decreasing products

response: increase concentration of products
reaction favoured: forward reaction

equilibrium shift: →

exothermic increase temperature

response: decrease temperature
reaction favoured: reverse reaction

equilibrium shift: ←

K value: decrease

exothermic decrease temperature

response: increase temperature
reaction favoured: forward reaction

equilibrium shift: →

K value: increase

endothermic increase temperature

response: decrease temperature
reaction favoured: forward reaction

equilibrium shift: →

K value: increase

endothermic decrease temperature

response: increase temperature
reaction favoured: reverse reaction

equilibrium shift: ←

K value: decrease

increasing volume

decreased concentration/pressure

response: increase concentration/pressure
reaction favoured: reaction that produces more particles

equilibrium shift: depends

decreasing volume

increased concentration/pressure

response: decrease concentration/pressure
reaction favoured: reaction that produces less particles

equilibrium shift: depends

yield

amount of product