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

law of conservation of mass

matter cannot be created or destroyed in a chemical reaction

activation energy

min energy required by reactants in order to react

• if min energy requirement is not reached, no reaction will occur

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

• electrode

• gas pressures

• temperature

• current

• voltage

• impurities

the down cell

production of sodium & chlorine

• electrolysis of molten sodium chloride (NaCl)

• 600°C to maintain NaCl_(l) in molten state

• NaCl _(l) melts at 801°C, CaCl₂ added to reduce melting point (flux)

• Cl_{2 (g)} by-product

the hall heroult cell

production of alumminium

• electrolysis of alumina (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆)

  → Na₃AlF₆ has much lower b.p

  → cannot add too much or else doesn’t dissolve

• maintained at 980°C

• operate low voltage but high current

membrane cells

• electrolysis of brine (concentrated NaCl) solution

• production of Cl_{2 (g)} , H_{2 (g)} , NaOH _(aq)

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 charge must be available for recharge

  → must not be lost

green hydrogen from electrolysis

• alkaline electrolysis cell

• polymer electrolyte membrane elctrolysis cell (PEMECs)

• solid oxide electrolysis cells

polymer electrolyte membrane elctrolysis 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

weaknesses of polymer electrolyte membrane elctrolysis 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 CO₂ from atmosphere

• produces O₂

• can create green ammonia