module 5 HSC Flashcards
what happens when chemical reactions do not go through to completion
irreversible reaction + an example
reactant forms products which cannot be reverted back to reactants
eg. combustion reactions eg. magnesium and steel wool
reversible reactions + example
products that were once formed hat can react again to form reactants
eg. cobalt (II) chloride hexahydrate (dark pink) –> chloride dihydrate (purple)
combustion of magnesium and stee wool
do with steel wool and repeat for magnesium strip
- white solid forms when magnesium heated –> ice bath no changes
- reddish brown solid forms when wool is heated –> ice bath no changes
- shows irreversibility
closed system
only energy can be exchanged with the surroundings where there is no matter transfer
open system
both matter and energy can be exchanged with the surroundings
static equilibrium
rates of forward and reverse reaction are equal and zero
eg. at completion: dissolution of unsaturated solution
eg. before initiation: combustion without initial spark
eg. reversible but with insurmountable activation energy: diamond to graphite
dynamic equilibrium
rate of forward reaction are equal and non zero
- concentration is constant
advantages + disadvantages of molecular modelling kits
ad:
-helps visualise complex phenomenon
- demonstrated both static and dynamic equilibrium well
dis:
-bond are rigid so we cant see electrons moving
- doesnt show transfer of energy
advantages of modelling dynamic equilibrium
- check
enthalpy
internal energy of a system
- only measures the change
delta H = sum of products - sum of reactants
eg. combustion of fuel
exothermic = forward enthalpy drive
endo thermic = reverse enthalpy drive
exothermic reaction is what type of drive
forward enthalpy drive
endothermic reaction is what type of drive
reverse enthalpy drive
entropy
measure of the state of disorder within a chemical system
- absolute can be measured
s> 0 = forward entropy drive
s< 0 = reverse entropy drive
difference between enthalpy and entropy
enthalpy can only measure change in enthalpy whereas entropy can measure absolute entropy
entropy and enthalpy for photosynthesis
6CO2 + 6H2O –> C6H12O6 + 6O2
s < 0 as ordered glucose molecules are created (reverse entropy drive)
h>0 = reverse entropy drive as energy is absorbed from surrounding
reversible reactions and delta G
reversible reactions tend to have completing enthalpy and entropy drives
G>0 = non-spontaneous
h>0, s<0 eg. photosynthesis
G<0 = spntaneous
h<0, s> 0 eg. combustion
combustion entropy and enthalpy
s>0 –> more heat = more disorder –> forward entropy drive
H<0 –> energy released –> forward enthalpy drive
collision theory
chemical reactions take place when molecules with sufficient energy collide with correct orientation
- increases rate of reaction due to the increase in the frequency of collisions
- initially concentration of reactants
- reactant particles collide at high frequency
- rate of reactants converted to products is high
- concentration of reactants decrease as they are converted to products
- rate of forward reaction decreases as products are formed the product concentration increases
- increased frequency of collisions of products
- rate of reverse reaction increases
- continues until forward and reverse are equal
le chateliers principle
if a system at dynamic equilibrium is disturbed, then the system will shift as to minimise the change until a new equilibrium is reached
temperature lcp
increase temp -> increase rate of reaction -> proportion of molecules with enough energy to overcome the activation energy barrier increases –. increases collision frequency
- has a greater effect on reaction rate when reaction rate is high as a greater relative proportion of molecules have enough energy to overcome barrier
- since forward reaction has a higher activation energy -> teh rate of the forward reaction is increased where the endo is favoured
lines for graphs
straight drop = concentration
gradual = temperature
volume change = straight line
catalyst = increased rate of reaction
equilibrium constant
products / reactants
Q vs K
Q>K = proceed left
Q=K dynamic equilibrium
Q<K proceed right
only factor that can change K
temperature
manipulating equilibrium constant
reciprocal (k) = reversing reaction
doubled (k) = coeffs doubled
multiply (k’s) = two reactions added together
catalyst
provides an alternate pathway with a lower activation energy for both forward and reverse reactions
- does not change concentrations or equilibrium just hastens the attainment of equilibrium
- does not impact the keq
practical for keq
iron (III) thiocynate
fe3+ + SCN- ->-< [FeSCN] 2+
H<0
Fe3+ = yellow
SCN - = colourless
[FeSCN]2+ = blood red
adding sodium hydroxide –> make the solution lighter in colour as the Fe3+ is precipitate out
look at colourmetry
gibbs free energy
delta G = delta H - Temperature (delta S)
- T in kelvin
G>0 non-spontaneous
G<0 spontaneous
large negative H = keq larger
enthalpy and keq relationship
larger H keq = smaller
smaller H keq = larger
when does a solute dissolve
when the formation of intermolecular forces between solvent and solutes are more favourable than the existing intermolecular forces
ionic compounds dissociation
individual ions form ion-dipole forces with water
- solute-solute interactions are overcome endothermic (ionic bonds in salts, ionic lattice held together by ionic bonds is broken)
- solvent-solvent interactions overcome endothermic (dipole-dipole interactions, hydrogen bonding, dispersion forces)
ion-dipole forces formed exothermic
- formed during solvation
- solvent molecules form concentric rings called hydration spheres around individual ions
- ion-dipole forces are formed between ions and water molecules
salt that dissolves exothermically
NaOH
salt that dissolves endothermically
KCl
dissolution is vs dissociation
process of solute dissolving in solvent and it is a physical change
separation of ions that occurs when a solid ionic compound dissolves
what does dis
what
what does dissolution lead to
increased entropy as hydrated ions move freely in contrast to fixed ionic lattice
dissolution as an equilibrium
static equilibrium : unsaturated
dynamic equilibrium: saturated
unsaturated solution: more can be dissolved
saturated solution: no more can be dissolved
supersaturated solution:
- allows for more solute to be dissolved
concentration of saturated solution
> 10 soluble
1-10 partially soluble
<10 insoluble
cycad
- contains toxin cycasin C8H16N2O7
- carcinogenic and neurotoxic
how to detoxify cycad
- strip off outer layer
- seed crushed to increase surface area
two methods:
roast then
- submerge in boiling water to leach out to remove water-soluble toxins + ferments till no longer toxic
leach for longer periods time
then:
- water drained
-resultant extracted
- starch is dried and pounded into a fine powder again
- leached again to remove more toxins
explain cycads with solubility equilibrium
- cycasin is readily soluble in water 56.6g
- when leeched in water the solid toxin dissolves and reaches dynamic equilibrium
- boiling increases reaction rate due to the increased temperature
C8H16N2O7 (s) –> (equilibrium) C8H16N2O7 (aq)
cobalt chloride
cobalt chloride hexahydrate is reversible
- endothermic
Co(H2O)6 2+ (aq) + HCL - (aq) -><. CoCl4 2- (aq) + 6H2O (l)
precipitation
involves two solutions mixing together that results in the formation of an insoluble solid which is the precipitate
mass of solute (units)
g/100ml
molar concentration of salt (units)
mol L-1
ionic product
QSP : concentrations that are not necessarily at equilibrium
QSP < KSP : unsaturated
QSP = KSP : saturated
QSP > KSP : supersaturated
NAGSAG
nitrate
acetate
group 1
sulfate except CASTROBAR
ammonium
group 7 except toxic trio
