Mod 5

Question 1

Static Equilibrium

Answer 1

rates of forward and reverse rxn are zero
forward and reverse rxn both have high activation energy
no rxn in either direction once equilibrium is reached

Question 2

Dynamic Equilibrium

Answer 2

rate of forward and reverse rxn are equal and greater than zero
ratio of reactants to products is constant

Question 3

Closed System

Answer 3

constant number of particles in system (no matter exchange)
energy can be exchanged
e.g. saucepan with lid
static and dynamic equilibrium require closed

Question 4

Open System

Answer 4

Allows for exchange of matter and energy
e.g. room with door open

Question 5

Isolated System

Answer 5

no exchange of matter or energy
pretty much theoretical
could be a black hole or the universe

Question 6

cobalt 2 hydrated and dehydrated

Answer 6

hexahydrate cocl2 6h2o is dark pink
dihydrate cocl2 2h2o is purple
anhydrous cocl2 is sky blue
when water is evaporated equilibrium shifts to change colour
reversible as when water is added, colour changes again
cocl(h2o)6(2+) + 4cl- <=> cocl4(2-) + 6h2o –> pink to blue (all things are aqueous except water)

Question 7

burning magnesium

Answer 7

2mg + o2 –> 2mgo
irreversible
white solid formed as precipitate
when solid placed back in water nothing changes and it is irreversible

Question 8

combustion of steel wool

Answer 8

reddish-brown solid when heated
when placed in ice bath no change occurs so non reversible
4Fe + 3O2 –> 2Fe2O3

Question 9

Enthalpy

Answer 9

defined as internal energy of a system, it is used interchangeably with heat in chemistry
standard enthalphy of formation = enthalpy products - enthalpy reactants
∆H < 0, rxn has forward enthalpy drive (exothermic) –> release energy to environment
∆H > 0, rxn had reverse enthalpy drive (endothermic) –> absorb energy from surroundings

Question 10

Entropy

Answer 10

measure of state of disorder in a chemical system
∆ Entropy–> entropy products - entropy reactants

Question 11

zeroth law of thermodynamics

Answer 11

if 2 systems are on thermal equilibrium with a 3rd system, they are in thermal equilibrium with each other
A <=> C, B <=> C, then A <=> B

Question 12

first law of thermodynamics

Answer 12

energy movement in and out of system is in accordance with the Law of Conservation of Energy
Energy initial = Energy final

Question 13

second law of thermodynamics

Answer 13

the entropy of an isolated system not at equilibrium will increase over time, approaching a maximum value at equilibrium
entropy of the universe is always increasing

Question 14

third law of thermodynamics

Answer 14

entropy of a system approaches a minimum as temperature approaches zero

Question 15

combustion, photosynthesis, respiration

Answer 15

combustion and photosynthesis both go to completion and therefore enthalpy and entropy can be analysed
respiration has same products as photosynthesis and proceeds in reverse, but its not a reversible reaction because the conditions required are vastly different
photosynthesis –> endothermic, positive enthalpy change, negative entropy change
combustion –> exothermic, negative enthalpy change, positive entropy change

Question 16

LCP

Answer 16

if a system at dynamic equilibrium is disturbed, the position of equilibrium is shifted to counterract/ minimise the change until a new equilibrium is reached
conc
conc increases, system shifts to opposite side (decrease the addition)
conc decreases, system shifts to same side (increase the loss)
pure solid and liquid constant conc
if solvent is pure liquid, + or - amount of solvent changed conc of solute
Temp
exothermic forward
increase temp favours reverse endo to lower temp and vice versa
endothermic forward
increase temp favours forward endo to lower temp and vice versa
Pressure/Volume
increase pressure = decrease volume
increase pressure shifts equilibrium to side with less moles of gas to produce more moles of that gas
decrease pressure shifts equilibrium to side with more moles of gas to reduce moles of gas
Catalyst
increases ROR –> provides and alternate rnx pathway by reducing activation energy
reactants depleted more rapidly
products created more rapidly

Question 17

Collision Theory

Answer 17

collision theory states that for a rxn to occur, the reactants must hit each other with sufficient activation energy and in the correct orientation
conc increase means more molecules and more likely for collisions to occur
increase temp means molecules gain KE cuz temp is avg KE so they are more excited and more likely to collide with enough energy
increase pressure brings molecules closer together making collisions more likely
addition of inert gas slows down rxn as collisions between reactants and inert gas do not react and will hinder other potential successful collisions, reducing RoR

Question 18

Whats is Keq and Q

Answer 18

aA + bB <=> cD + dD
Keq= ([C]^c x [D]^d)/ ([A]^a x [B]^b)
conc of products / conc of reactants
If K is large, equilibrium lies to the right products side
If K is small, equilibrium lies to the right products side
If K is middling value, equilibrium lies to the middle and reactants and products equal (usually between .1 and 10)
Q is the reaction quotient when the reaction isnt at equilibrium and can be found at any point
If Q < K system favours forward to increase conc of products
If Q > K system favours reverse to increase conc of reactants
If Q = K system is at equilibrium

Question 19

how is keq changed

Answer 19

Temp is the only factor that affects K, (K is same if pressure, conc, volume is changed)
if rxn is endothermic
increase in temp favours endothermic forward rxn, increase conc of products and keq
decrease in temp favours reverse exothermic rxn, increasing conc of reactants decreasing Keq
temp is only thing that can change K as the ratio between r and p cannot be preserved while equilibrium is re-established (because there is no immediate change to conc of species)

Question 20

effect of activation energy on position of equilibrium

Answer 20

changes to activation energy due to presence of catalyst don’t affect the position of equilibrium as catalyst lowers activation energy for both forward and reverse reactions equally
it just makes both reactions reach equilibrium quicker
lower activation energy of only forward or higher activation energy of only reverse, equilibrium will lie to right and keq will be larger, vice versa

Question 21

how does dissolution occur

Answer 21

ions from edge of lattice interact with water to form strong ion-dipole forces and dissolve in water
if small sample of salt added to water it is unsaturated and it is static equilibrium as there is no reverse rxn happening
solid always written on left of arrow (right of arrow is precipitation)
if enough salt is added and solution becomes saturated, dynamic equil such that ions are in constant motion

Question 22

dissolution as an equilibrium

Answer 22

dissolution is temp dependent
supersaturated solution is heated or cooled to dissolve more ions
becomes unstable, crystallisation occurs
> 10g/L –> soluble
1-10g/L –> sparingly/ partially soluble
< 1g/L –> insoluble

Question 23

removing toxins in cycads (aboriginal chem)

Answer 23

cycasin (c8h16n2o7) is carginogenic (highly soluble 56.6g/L)
stripped out flesh of cycad to expose kernel
seeds crushed to increase surface areas and submerged in boiling water to leach out water-soluble toxins or fermented until no longer toxic
either roast seeds before leaching in boiling water for a short period –> resulting in cycads perishing quick
didnt roast seeds and leached in boiling water for a long time –> did not perish quick
after leaching, water is drained and resultant extracted starch is pounded into fine powder and leached again and process is repeated to reduce cycasin
solid cycasin dissolves in water creating an equilibrium s <=> aq (endothermic)

Question 24

ksp

Answer 24

if Qsp < Ksp, solution is unsaturated and more ionic solid will dissolve if added
if Qsp > Ksp, solution is supersaturated and ionic solid will precipitate
if Qsp = Ksp, solution is saturated and at equilibrium

Question 25

potassium thiocyanate

Answer 25

fe3+ + SCN- <=> FeSCN2+
exothermic
yellow –> fe3+
blood red –> fescn2+
full rxn fe(no3)3 + KSCN <=> Fe(SCN)3 + KNO3