chemistry u3 aos1 (redox reactions and cell chemistry)

short hand convention

[O.A./ R.A.]

conjugate redox pair

[reactant/ product]

each overall redox equation will have 2 pairs;

whereas half-equation will only have 1 pair

how does redox reactions occur

both oxidation and reduction reactions occur simultaneously; involve transfer of electrons

oxidation state/ number

= numbers used to represent how oxidised/ reduced particles of a specific element are

oxidation state of O in F₂O

+2

oxidation state of H in metal hydrides, list the examples

-1;

NaH, LiH, CaH₂

oxidation state of O in H₂O₂

-1

is combustion reaction a redox reaction?

yes;

O.A.: oxygen

R.A.: any fuel

when do we separate/ not separate the species when we write OA/ RA?

(to separate)

• ionic aqueous → separate the ions

  e.g. HCl (aq)

(not to separate)

• ionic solid

  e.g. Ag₂O (s)

• molecular

oxidation state of H in CaH₂

-2

oxidation of N in NH₄NO₃

for NH₄⁺ → -3

for NO₃^- → +5

is displacement reaction exo or endo?

(almost) all displacement reactions are exothermic

this is because the bonds in the products are always more stable in those in reactants (lower chemical potential energy; higher kinetic thermal energy); therefore all single displacement reactions are exo

energy conversion in galvanic cell

chemical to electrical energy

general structure of a galvanic cell

• composed of 2 half-cells

• an electrode (can be reactive/ inert/ within a test tube → for gas reactants)

  → allows delocalised electrons to move through it

  → OR transfer electrons

• an electrolyte (solution/ liquid)

  → conducts electricity, due to presence of ions

purpose of voltmeter

to measure the voltage through the wire/ to show the flow of electron (NOT CURRENT!!)

why are electrodes often metals?

good conductor of electricity

purpose of external circuit

allows electrons to flow through this wire from one electrode (anode) to another (cathode)

what is the internal circuit, purpose?

salt bridge

to complete the circuit and balance the charge

what is a salt bridge made up of, give example of the electrolyte

salt bridge contains electrolyte with spectator ions that do not participate in the half-cell reactions

possible solution for salt bridge: NaNO₃, KNO₃

what are the criteria for the electrolyte for salt bridge?

• do not participate in the half-cell reactions

• do not react to form precipitate

• needs to be soluble in water to produce free moving electrons

types of half cells

** beware of the hydrogen half cell

→ you need a test tube and a Pt electrode

standard lab conditions SLC

• 298K

• 100kPa

• 1M concentration

how do you explain why there is a spontaneous reaction occurring?

when the OA is stronger the the conjugate OA of the RA (or vice versa)

how do we determine the polarity of electrodes when calculating the voltage of a galvanic cell?

note which terminal is each electrode connected to when a positive reading on voltmeter is obtained

how to calculate emf/ cell potential difference?

**emf for a spontaneous reaction will always be POSITIVE

what does a E⁰ imply?

• it DOES NOT indicate the rate of reaction

• it DOES indicate whether a spontaneous reaction occur

  → the greater the difference in emf, more likely the spontaneous reaction will occur

  → the smaller the difference, the slower the reaction

why is aluminium sometimes unreactive?

it reacts on exposure to air to form aluminium oxide, which prevent them from reacting

for a Daniel cell, it does not have salt bridge, what is the purpose of having a porous pot separating the electrodes?

to allow migration of ions (cations and anions) to maintain neutrality

what ions will not be passing through the porous container, why?

Cu²⁺ , or else there will be a direct reaction between this ion and Zn, no current will then produce

do we need to replace salt bridge? why?

yes, a new salt bridge is needed every time a cell is constructed, because the ions are used up as they migrate into cells

what are the types of galvanic cell?

• primary cell

  → alkaline cell

  → button cell

• fuel cell

properties for primary cell

• non-rechargeable

• go flat when reaction reaches equilibrium

• must be discarded

• products migrate away from electrodes and consumed by side reactions in cell

  (unlike secondary that remains around electrodes)

• products remain in cell

• reactants are solid/ liquid

• store energy

properties of fuel cell

• reactants are not stored but supplied continuously

• constant production of energy

• continuous removal of products

• combustion reaction

  (heat energy is converted directly to electrical energy)

• high efficiency

• gaseous reactants

• electrodes are catalyst

• porous electrodes allow reactants to diffuse through to react with ions in electrolyte

• do not store energy

compare fuel cell and primary cell

fuel cell

• do not store energy

• constant supply of reactants

• continuous production of electrical energy

• constant removal of products

• gaseous products

• electrodes are catalyst

• porous electrodes to allow electrons to pass through to react with electrolyte

primary cell

• store energy

• solid/ liquid reactants

• products remain in cell

similarities

• both are galvanic cell

• both involve spontaneous redox reaction

• both convert chemical energy to electrical energy

hydrogen fuel cell

purpose of porous electrode

• keep the reactants apart

• allows the reactants to diffuse through, react with the ions in electrolyte

why is catalyst incorporated into electrodes in fuel cell?

• increase the rate of oxidation and reduction half reactions

do electrodes get consume in fuel cell (think abt how catalyst is incorporated)

no

what would be a possible electrolyte in both an acidic/ alkaline fuel cell?

• (acidic) phosphoric acid

• (alkaline) potassium hydroxide

what is the state of a methanol as a fuel in combustion reaction?

liquid

why does electrolyte flow continuously throughout the fuel cell?

• allows the ions to move

• to balance the charge (similar to salt bridge; e.g. anions move towards anode, vice versa)

advantage of fuel cell

• high energy efficiency

  (due to direct conversion of chemical energy to electrical energy)

• continuous electricity production

• water is the only product (in hydrogen fuel cell), no CO₂ emitted

• a variety of fuels can be used

• quiet

• does not need to be recharged as long as fuel is supplied

• does not need to be connected to the electricity grid

disadvantages of fuel cell

• continuous supply of fuel is needed

• expensive electrodes (catalyst) and electrolyte

• some electrodes and electrolytes are harmful

• fuel must be very pure or else catalyst will be poisoned

• hydrogen is a dangerous gas

• hydrogen is mostly sourced from fossil fuels, which its production involves greenhouse gas and requires significant input of energy

• few refuelling stations

• inverter is required for alternating current

green chemistry principles for designing fuel cell

• reducing waste

  → utilise the maximum amount of reactants into desired products

  → utilise energy efficient process

• using renewable feedstocks

  → maximise the use of biofuels (e.g. Hydrogen production)

• minimising hazardous effects on human health and environment

  → design safer materials/ reduce the use and production of harmful substances

  → chemicals are designed to eventually degrade into harmless products that don’t build up in environment

circular VS linear economy

(circular)

→ optimal use of resources and reuse them

(linear)

→ products are discarded

two ways of producing hydrogen sustainably

• electrolysis of water powered by solar/ wind energy/ energy generated from biofuels

• steam reforming (methane in biogas sourced from landfill, reacts with steam, in presence of Ni catalyst, forming H₂ and CO)

properties of hydrogen

• highly flammable

• potential explosive

storage of hydrogen

• compressed into a high-pressured gaseous form/ liquid hydrogen

does hydrogen powered vehicles contribute to greenhouse gas levels?

yes, water vapour produced is a greenhouse gas

same as fuel cell, as heat as produced during electricity production, which would turn the water into water vapour

compare the amount of hydrogen required by a hydrogen fuel cell and a hydrogen-powered combustion engine to generate the same amount of energy

hydrogen fuel cell would use less hydrogen as it is more efficient (high energy efficiency) than a combustion engine