unit 3 aos 2 chem

collision theory — for a successful reaction particles must…

• collide each other

• have enough energy to pass their activation energy barrier

• collide with correct orientation for bond breaking

not all particle collisions cause reaction

particles might just bounce off each other if not enough energy or not correct orientation — only changes velocity

activation energy def

• minimum amount energy needed for reaction to occur by breaking reactant bonds

• symbol = Ea

• in energy profile diagram this is diff between reactant energy line and peak

e.g. ways to measure reaction rates

• volume of gas evolved

• mass solid formed

• decrease in mass bc of gas evolved

• colour intensity of solution

• precipitate formation

• pH

• temperature

— recorded as changes over time

the steeper the gradient in reaction rate graphs

the faster the rate of reaction

factors affecting reaction rate

• surface area

• concentration

• pressure

• temperature

• catalysts

some cars park too close?

factors affecting reaction rate — concentration

as conc increases → number of particles in a given volume increases → frequency of collisions increase → number of successful collisions per sec increase → reaction rate increase

factors affecting reaction rate — pressure

• can be increased by 1) adding more reactants to same volume or 2) decreasing volume

• increased pressure → increase partial pressures/partial concs of all subs → increase frequency collisions → increase number of successful collisions per sec → increase reaction rate

factors affecting reaction rate — temperature

increased temp → increased kinetic energy particles hence increased speed of particles →increased frequency collisions → also increased proportion particles that have enough energy to pass activation energy barrier → increased proportion successful collisions per sec → increased reaction rate

why adding inert gas doesn’t increase rate of reaction

increases total pressure but not partial pressures of reactants → frequency of collisions increase but not frequency of successful collisions → no increase reaction rate

factors affecting reaction rate — surface area

• for heterogeneous reactions (subs in reactions at diff phases)

• increase surface area → increase particles exposure and contact area → increased frequency collisions → increased number successful collisions per sec→ increased reaction rate

why is increasing surface area sometimes dangerous

in combustion reactions → if powder instead of large solids just need spark → cause very fast reaction → explosion

catalysts def

subs that lower activation energy to increase reaction rate w/o being consumed

factors affecting reaction rates — catalysts

catalysts added → make reactant bonds weaker hence lowers activation energy needed to break them → increased proportion particles that pass activation energy barrier via alternative pathway → increased frequency successful collisions → increased reaction rate

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catalysts added → provide alternative pathway with lower activation energy → increased proportion of particles with enough energy to pass activation energy barrier → increased frequency proportion successful collisions → increased reaction rate

closed system def

system allows transfer of energy but not matter with surroundings — reactants and products contained

open system def

system allows transfer of energy and matter with surroundings — reactants and products not contained

isolated system def

system doesn’t allow transfer of energy and matter with surroundings — e.g. well insulated calorimeter

why rate of evaporation in a closed system eventually reach 0

gas molecules colliding randomly everywhere in flask → some collide liquid surface and re-absorbed back into the liquid → as more evaporation occurs and more gas particles form → more chance of gas particles hitting liquid and re-absorbed → eventually reach when 2 processes have same rate → rate of evaporation measured is 0 (as what being produced, same amount being consumed)

in reversible reactions the yield of products…

is always less than the stoichiometric prediction

are all chem reactions equilibrium reactions

yes

but some of them the degree of reverse reaction is so small it can be ignored/negligible

equilibrium reaction def

reaction where forward and reverse reactions are significant

reversible reactions def

equilibrium reactions where products re-form into reactants to significant extent

• eqs have double arrow

• yield of products always less than stoichiometric prediction

irreversible reaction def

only occur in forward direction where products do not re-form into reactants

• e.g. combustion of fuel

rate vs extent of reactions

• rate: how fast it occurs (how long to establish position of equilibrium)

• extent: the degree which reactants convert into products (the position of equilibrium/how far to the right)

equilibrium constant def

value indicates extent conversion of reactants to products at equilibrium

• Kc

• if high value = large amount reactants conv into products

• if low value = less amount reactants conv into products

does catalyst affect equilibrium position (degree of conv of reactants into products)

no

it only affects rate of forward and reverse reactions (equally) and time taken to get to equilibrium

homogenous reaction def

reaction where all substances in same phase (physical state)

homogenous equilibrium def

all species in equilibrium mixture same physical state

heterogenous reaction def

reaction where substances involved in diff phases (physical states)

• occurs at boundary between 2 physical states

• can also happen between immiscible liquids (don’t mix and form homogenous mixture)

equilibrium def

when forward and reverse reactions occurring at same rate

when reaction at equilibrium…

• reactants being formed and used at same rate

• products being formed and used at same rate

  — their concs don’t change

• dynamic state not static

when considering getting to reaction equilibrium

consider:

• starting point (initial concs of reactants and products) → what this means for net forward or net backward reaction → as more reactants or products consumed and more reactants or products produced eventually reaction rstes equal → equilibrium reached → no net change on conc of substances anymore

equilibrium and successful collisions between particles

at equilibrium frequency of successful collisions for forward reaction is equal to frequency successful collisions for backward reaction

representing equilibrium reactions with chem eqs

• use reversible arrow

• thermochem eqs with delta H value convey more info about eqs as equilibrium can be diff for endo or exo reactions

representing equilibrium reactions with rate vs time graphs

in this e.g. where we start with reactant high conc (can diff according to example):

• as reactants used up → conc drops → hence rate forward reaction drops

• as products produced → conc increases → hence rate backward reaction increases

• this continues until reach a time where same rates of reactions → equilibrium reached

• but note still in above e.g. there is a net forward reaction as still more forward than reverse happening even as forward reaction rate decreases

• if situation flipped and more product conc at start then the graph labels would swap and reverse reaction would be on top instead (net reverse reaction)

representing equilibrium reactions with conc vs time graphs

in this e.g. where we start with reactant high conc and net forward reaction (can diff according to example):

• as reactants used up → conc drops until equilibrium point

• as products produced → conc increases until equilibrium point

• final conc at equilibrium depends on initial conc, equation stoich, value of equilibrium constant etc

• the substances change according to their stoich molar ratios from equation (e.g. from graph = C increases twice amount than B does bc their molar ratios is 2:1)

• note if catalyst added only change is that equilibrium (at t) reached at lower time → reached faster — final subs concs not changed

concentration fraction def

• fraction from concentrations of substances in equilibrium reaction that have a constant value (constant: unchanging over time at eq?)

• the concs of products divided by concs of reactants all to the power of their coefficients respectively

• Q

equilibrium law def

relationship between concs of products and reactants, also considering their stoic ratios

equilibrium constant def

value of conc fraction at equilibrium

• Kc or K

• also gives extent which reactants conv into products

equilibrium constant formula

fraction

where numerator is product concs to the power of their corresponding stoic equation coefficients

over the denominator of reactant concs to the power of their corresponding stoic equation coefficients

using equilibrium constant to indicate reaction extent (whether net F or net R)

• if K > 10⁴ = net F reaction rate → has been more conv of reactants into products to get to equilibrium

• if K is between K = 10⁴ and K = 10⁻⁴ then extent of conv reactants into products moderate

• if K < 10⁻⁴ = net R reaction rate → little conv of reactants into products to get to equilibrium

— to make this make sense rmbr that K is from a fraction where numerator is product conc → hence makes sense that if more forward reaction then more products conc hence larger K value

describing equilibrium qualitatively

• use describing via ‘position of equilibrium’

• if K is large then net F reaction rate to reach equilibrium → equilibrium lies to the right

• if K is small then net R reaction rate to reach equilibrium → equilibrium lies to the left

what can affect equilibrium constant for a reaction

• temp

• how the equation for it is written (e.g. coefficients or direction)

what is the unit of K

• depends on amount of M in fraction formula

• if all M units cancel out K can be without units!

how K can change according to equation changes for same reaction

• if equation coefficients double = square K value

• if equations coefficients halve = square root K value

• if equation is reversed = reciprocal K value

using stoich in equilibrium law calculations

ICE method must be used sometimes (e.g. if initial concs given and only one equilibrium conc known)

Initial amount - note usually if not given product amount it is zero as net F is occurring and at start it is not there yet duh

Change - can either be increase in amount from initial to equilibrium point (positive) or decrease in amount (negative) → note when product conc increases, the reactant conc correspondingly decreases according to their mol ratios!!

Equilibrium - the final amount of substances at equilibrium based on change

— then conv this amount into concs and do further calcs as needed

reaction quotient def

concentration fraction at any other stage apart from equilibrium during reaction

Q

reaction quotient vs concentration factor

same thing? it’s just diff to K though

if Q = K

reaction is at equilibrium

if value of Q is diff to K

reaction has yet to reach equilibrium

if Q > K

• net backward reaction is occurring (because K requires smaller value hence decrease in numerator hence decrease in product conc)

  — aka backward reaction rate greater than forward reaction rate as reaction moves towards equilibrium

if Q < K

• net forward reaction is occurring (because K requires bigger value hence increase in numerator hence increase in product conc)

  — aka forward reaction rate is greater than backward reaction rate as reaction moves towards equilibrium

dynamic state of equilibrium def

reaction still taking place (hasn’t stopped) but substances concs kept constant as forward reaction rate equals reverse reaction rate

at equilibrium…

the forwards and backwards reactions are occurring at same rate not same extent

yield def

amount of product

le chatelier’s principle def

when change made to equilibrium system this system opposes this to restore an equilibrium state

le chatelier’s principle

any change that affects equilibrium position causes that equilibrium to shift in a way to partially oppose that change

ways a system at equilibrium may be disturbed

• adding or removing subs involved (can be physical or chemical)

• changing volume at constant temp

• changing temp

le chatelier’s principle — adding substance to system

• change: adding subs

• effect: increased conc of reactant or product

• hence system try to oppose change and re-establish equilibrium → reduce conc of either reactant or product (depend on what was added initially) → one direction favoured → equilibrium shifted to right or left

le chatelier’s principle — removing substance from system

• change: removing subs

• effect: reduced conc of reactant or product

• hence system try to oppose change and re-establish equilibrium → increase conc either reactant or product (depend which was removed initially) → one direction favoured → equilibrium shifted to right or left

adding or removing subs at equilibrium — collision theory and equilibrium law

• collision theory: subs added or removed → conc increase or decrease of subs → subs reaction rates affected accordingly → as reactions continue conc changes as usual and reaction rate changes until equilibrium reached

• equilibrium law: subs added or removed → conc increase or decrease → affects Q formula accordingly → Q needs to increase or decrease to meet Kc → hence either net F or net R reaction rates until new equilibrium reached where Q = Kc

le chatelier’s principle — volume increase

• change: volume increase

• effect: all partial concs decrease hence ttl conc decrease

• hence system try to oppose change by favouring direction that produces more particles → net F or net R reaction rates until equilibrium → equilibrium shifted to right or left

le chatelier’s principle — volume decrease

• change: volume decrease

• effect: all partial concs increase hence ttl conc increase

• hence system try to oppose change by favouring direction produce less particles → net F or net R reaction rates until equilibrium → equilibrium shifted to right or left

increasing or decreasing volume at equilibrium — collision theory and equilibrium law

• collision theory: volume increase or decrease → all partial concs increase or decrease → ttl conc increase or decrease → direction with reactants of more particles have increased collision → hence this direction has net rate → as reactions continue concs change accordingly and reaction rates change until equilibrium reached

• equilibrium law: volume increase or decrease → all partial concs increase or decrease → affects either numerator or denominator more in Q formula → hence Q increase or decreased → Q must then increase or decrease to meet Kc value → reaction rates change accordingly until Q = Kc at equilibrium

when can volume not be used to change position of equilibrium

when both reaction sides same no. of moles → system cannot change conc of particles (and vol change didn’t disturb eq at all?)

how to maximise yield of equilibrium reactions

by disturbing existing equilibrium so system opposes change by favouring one direction to produce more product amount while re-establishing equilibrium

le chatelier’s principle — increasing temp

• change: giving heat

• effect: increasing thermal energy / temp

• hence system try to oppose change by decreasing thermal energy → favours endothermic reaction direction as converts thermal to chem energy → new equilibrium position and changed equilibrium constant

le chatelier’s principle — decreasing temp

• change: removing heat

• effect: decreasing thermal energy / temp

• hence system try to oppose change by increasing thermal energy → favours exothermic reaction direction as converts chem to thermal energy → new equilibrium position and new equilibrium constant

ways system equilibrium can be disturbed — which cause sudden conc change

• causes sudden conc change: adding/removing substances, volume changes

• doesn’t cause sudden conc change: temp changes

ways system equilibrium can be disturbed — which cause diff equilibrium constant

• cause diff equilibrium constant: temp

• doesn’t cause diff equilibrium constant: adding/removing substances, volume changes

increasing or decreasing temp — collision theory

when temp increases → reaction rates affected → however endothermic reaction increases more as it absorbs thermal energy → hence endo net rate > exo net rate until equilibrium reached — image for this

unsure abt temp decrease? maybe temp decreases → reaction rates affected → however exothermic reaction increases more as produces thermal energy → exo net rate > endo net rate until equilibrium reached

equilibrium shifting

• to the right: forward reaction favoured

• to the left: reverse reaction favoured

why can change in gases vol at constant temp also be interpreted using pressure not just conc

because pressure is proportional to conc → hence partial pressures may be referenced when vol changes

why does changing temp in system change equilibrium constant value

bc change in temp = change in energy available to system

changing conc by adding/removing or vol changes only causes redistribution of available energy and diff eq position but K value stays same

notes on conc vs time graphs

• conc changes reflect stoic equation ratios

• at equilibrium all concs become constant horizontal line at same time for all

• sudden dip or spike in one conc means one subs added or removed

• sudden dip or spike in all concs mean volume change

• change in conc values without obvious spike or dip indicate temp change

• one side equation all concs will increase or decrease and other side equation all concs vice versa

can Le Chatelier’s principle apply to open systems

yes

e.g. if open system and has a gas product in reaction the gas will keep escaping (subs removal) and equilibrium continually disrupted hence system oppose and more reactants consumed to replace it until all run out

how can extent and rate conflict for reaction from temp industrially?

e.g. in exothermic reaction

might want high temp so high rate of reaction occurs and more efficient product formation

however high temp also disturbs equilibrium → hence favouring reverse endothermic reaction and product conc actually decreases too → lowers equilibrium constant value

electrolysis def

non-spont chem reaction occurs by passing electrical current thru subs in solution or molten state

electrolytic cell def

electric cell where non-spont redox reaction occurs by applying external voltage across electrodes

electrolytic cell diagram

3 features of electrolytic cell

• freely moving ions

  — can donate/accept electrons and allow them flow thru external circuit → circuit completed and solution (?) electrically conductive

  — cations attracted to cathode and anions to anode

• 2 electrodes where redox reactions at

  — positive ions reduce at cathode (-ve electrode)

  — negative ions oxidise at anode (+ve electrode)

• external electrons source

  — electrons flow in one direction to cathode (negative electrode) so reduction can occur — from cations

  — electrons given to power source from anode (positive electrode) so oxidation can occur — from anions

electrolysis of ionic compounds: molten NaCl (l)

heating solid NaCl cause ions to separate and freely move as pure liquid subs → molten liquid = called a melt → electric current passed thru → electrons flow to cathode (inert electrode) → becomes negative → sodium ions attracted to cathode and reduce here → positive anode forms → chloride ions attracted to anode and oxidise here

why does applying current with two electrodes in pure water cause no electrolysis?

not enough freely moving ions in pure water to carry electric current so no current flow→ no electrolysis

electrolysis of water

low conc of electrolyte like KNO3 added → freely moving ions available for current flow → cathode is negative with available electrons → water reduced here to form hydrogen gas and hydroxide ions (turns area more alkaline) → anode is positive and no available electrons → so water oxidise here to form oxygen gas and protons (turns are more acidic) → note if allowed to mix freely the protons and hydroxide can form water again (neutralisation reaction)

— reaction: 6 H2O (l) → 2 H2 (g) + O2 (g) + 4H+ (aq) + 4 OH- (aq)

but protons and hydroxide ions form water so final equation:

2 H2O (l) → 2 H2 (g) + O2 (g)

using electrochem series for spontaneous vs non-spontaneous redox reactions

• if spontaneous = clockwise circle

• if non-spontaneous (forced by energy input) = anticlockwise circle

galvanic cells vs electrolytic cells

• g cells = spont redox reaction prod electrical energy, e cells = electrical energy needed to drive non-spont reaction

• g cells separation into half cells needed as spont reactions and electron travel needed but e cells no need separation (into diff containers) as non spont

• g cell chem energy converted to electrical, e cell electrical energy converted to chem

• g cell anode is negative but e cell anode is positive

• g cell cathode is positive but e cell cathode is negative

• g cell Eo is positive, e cell Eo is negative (g cell produce voltage, e cell consume it?)

min amount voltage needed for electrolysis to happen

more than what is produced for those subs if they were to react spontaneously

Eo cathode - Eo anode

direction of electron flow in electrolytic cell

— anode to cathode

note that it is always this way regardless of cell type

why are states in molten electrolytic cell equations diff to electrochem series states sometimes?

bc need to consider temp here for molten conditions (usually very high temp so some subs may be liquids, gases etc) — higher than the databook series temp of 25 degrees?

standard cell potential of galvanic cell vs electrolytic cell

• g cell = positive (makes sense as voltage produced)

• e cell = negative (makes sense as voltage consumed)

why is using electrochem series important for electrolysis

sometimes e cell has more than one possible reaction around an electrode as diff substances present → need electrochem series to predict redox reactions that occur among other competing possibilities based on their oxidising and reducing strength → to predict products of electrolysis

predicting the products of electrolysis

1. list species present (include electrode metals, potential oxidants and reductants, water if there, impurities)

2. write half-eqs of these species in descending order of Eo

3. circle species present in e cell that can participate

4. select oxidant (undergoing reduction) with highest Eo → bc needs less energy for reduction than an oxidant with lower Eo which ends up giving an overall higher voltage

5. select reductant (undergoing oxidation) with lowest Eo → bc needs less energy for oxidation than a reductant with higher Eo which ends up giving an overall higher voltage

6. write the reduction and oxidation half-eqs and combine to form overall full eq

7. find min voltage required by Eo cathode - Eo anode

— the two half eqs should make the smallest possible anticlockwise circle on electrochem series (as needs the least energy amount)

using pH to infer redox reactions occurring

sometimes questions present an indicator and the colour it turns around electrodes to indicate what is produced there from a half eq

• hydronium prod → pH decrease — more acidic

• hydrogen ions → pH decrease — more acidic

• hydroxide ions → pH increase — more alkaline

factors affecting electrolysis of solutions

• electrolyte conc

• electrolyte nature

• electrodes nature

factors affecting electrolysis of solutions — conc of electrolyte

• when conc changes and not 1 M → this changes their electrochem series order → bc in databook all species recorded in order at 1 M

• when orders are changed this means the stronger oxidants and reductants may now be diff to what shown in databook

factors affecting electrolysis of solutions — nature of electrolyte

when diff electrolytes present (e.g. dilute sodium nitrate solution vs dilute copper nitrate solution) → changes which products form in cell as subs to consider for strongest oxidant and reductant differs

factors affecting electrolysis of solutions — nature of electrodes

electrodes can be inert (graphite or plat usually) or metal that participates in reaction as strong reductant to undergo oxidation (doesn’t occur at cathode as solid metals don’t gain electrons) → changes products of e cell

how half eqs can change as e cell progresses

note sometimes half-equations are just initial reactions → however as reaction progresses the strongest oxidants and reductants may change → for example as solid copper oxidises → after some time copper ions replace previous subs undergoing reduction and it reduces instead at cathode → here note that copper mass is shifting from anode to cathode!

commercial electrolytic cells example — the downs cell

• electrolysis of molten sodium chloride

• must be molten rather than in aqueous solution form as here water would reduce instead of sodium ions

• products of electrolysis (sodium and chlorine gas) continually removed so they do not react and reform sodium chloride — electrodes also separated by iron mesh to keep them apart for this

• added chems reduce melting point of sodium chloride

• density diffs to collect liquid sodium

• current can flow thru molten NaCl bc it exists as separate sodium and chloride ions

• fresh NaCl can be added when required

• chlorine gas at anode collects in a compartment as valuable by-product

• molten sodium form at cathode → less dense than electrolyte → floats to surface → overflows into separate container

• operates at high temps to maintain salt in molten state

commercial electrolytic cells example — the hall-héroult cell

• electrolysis of alumina dissolved in molten cryolite to prod aluminium

• occurs in steel vessel

• occurs at very high temps (around 980 degrees celsius — to melt alumina)

• anode: carbon blocks above the cell partially immersed in electrolyte → oxide liquid ions form oxygen gas → reacts with carbon anode to form CO2 → hence consumption of anode material means it needs replacement every now and then

• cathode: carbon lining of cell → al liquid ions drawn here and reduce to form al liquid

• cannot occur with soluble al salt in water as water stronger oxidant that al ions hence preferentially reduced

• cryolite: solvent and electrolyte, much lower melting point than alumina hence letting aluminium be deposited

• alumina: fed into electrolyte at regular intervals where it dissolves into ions (al ions and oxygen ions — both liquid), current applied moves ions

• al liquid and cryolite have density diffs → allow separation as al settle at cell bottom and drained for collection

• v specific amount alumina needed

• operates at low voltage but needs high current