Chemistry: Mod 5 - Equilibrium and Acid Reactions

what is an irreversible reaction?

Only occur in the forward direction

how long does an irreversible reaction go for?

Proceeds until the limiting reagent is completely used up (reaches completion)

what is an irreversible reaction?

Only occur in the forward direction

how long does an irreversible reaction go for?

Proceeds until the limiting reagent is completely used up (reaches completion)

what is used to represent irreversible reactions?

—>

what is a reversible reaction?

Can occur in both: forward and reverse

what is the duration of a reversible reaction?

• Doesn’t reach completion

• There will always be all reactants and products in the reaction mixture

what is used to represent a reversible reaction? give an example?

3H_{2 (g)}+ N_{2 (g)}  ⇌ 2NH_{3 (g)}

• Usually physical changes are reversible

• Some chemical changes are reversible

what is equilibrium?

Refers to the state of a chemical system in which

• The concentrations of both reactants and products do not change with time

• Rate of the forward reaction = rate of the reverse reaction

• All reactions proceed until either equilibrium is reached

what are phjysical observations of a system at equilibrium?

1. Constant colour

2. Constant pressure

3. Constant temperature

what is the rate of reaction ?

Rate of Reaction: how rapidly a reaction proceeds

• Depends on the frequency of effective collisions

factors that affect the rate of reaction

• Temperature

• Concentration (only for (aq) or (g) species)

• Pressure (only for (g) species)

• Catalyst

what are the two types of equilibrium?

1. static

2. dynamic

what type of reaction is in static equilibrium ?

irreversible reactions

what type of reaction is in dynamic equilibrium ?

reversible

what is the rates of reaction in static equilibrium ?

Rate of forward reaction = rate of reverse reaction = 0

what is the rates of reaction in dynamic equilibrium ?

Rate of forward reaction = rate of reverse reaction = non-zero

are there changes in static equilibrium?

Microscopic: no 

• Because all reactions are = 0

Macroscopic: no

• The concentrations of reactants and products are constant (doesn’t mean equal)

are there changes in dynamic equilibrium?

Microscopic: yes

• Both the forward and reverse reactions continue to occur

Macroscopic: no

The concentrations of reactants and products are constant (doesn’t mean equal)

what species are present in static equilibrium?

just the products

what species are present in dynamic equilibrium

all - products and reactants

what system can static equilibrium occur in?

open and closed systems

what system can dynamic equilibrium occur in?

closed

what are the types of systems?

Open: matter and energy can enter and leave

Closed: energy can enter and leave, NOT matter

Isolated: neither energy nor matter can enter or leave

how do you model dynamic equilibrium?

water tank model: Aspect	How is it modelled?
Forward and Reverse reactions occur simultaneously	Transfer of water in both directions simultaneously
Concentrations remain constant at equilibrium	Constant water levels after transfers
Forward and reverse reaction rates are equal at equilibrium	Equal amounts of water are transferred in both directions
Reactants and Products	Water in different containers
The reaction rates are proportional to concentration	The size of the beakers

what is the limitation and benefits of the water tank model?

Benefit of Model	Limitation of Model
Representations of complex concepts Identifies consistent trends to explain phenomena Based on a simplified set of assumptions	Can only model a mole ratio of 1:1 Only depicts one reactant to one product Doesn’t model observable properties like temperature, pressure, colour (only concentration)

what are spontaneous reactions?

proceeds without needing a continual input of energy from an external source

what are non-spontaneous reactions?

requires a continual input of energy from an external source

what is enthalpy?

1. Enthalpy (H): the heat content of a system

• Absolute enthalpy cannot be measured - instead the change in enthalpy (ΔH) is used

• ΔH: measure of how much heat energy is released or absorbed in kj/mol

• ΔH > 0 = exothermic reaction (favourable to spontaneity)

• ΔH < 0 = endothermic reaction

what is entropy?

1. Entropy (S): measure of the dispersion of available energy

• A greater dispersal of energy will increase entropy

• Increase in entropy:

  • Increase in temperature

  • Increase in freedom of movement (gas > liquid > solid)

  • Increase in no. of particles

  • Mixing different particles together

• Second law of thermodynamics: entropy of the universe must remain the same or increase

• ΔS > 0 = favourable to spontaneity

what is gibbs free energy?

1. Gibbs free energy change (∆G)

• Allows us to compare the relative contributions of enthalpy and entropy to predict overall spontaneity 

ΔG = ΔH - TΔS ∆G˚ = ∑G˚(product)  - ∑G˚(reactant)

• If ΔG < 0: spontaneous process

  • T = temperature

• Note: a non-spontaneous reaction can still occur if it is driven by a spontaneous process (the overall entropy of the universe increases)

what entropy and enthalpy combinations cause spontaneity and non-spontaneity?

		ΔH
		Endothermic (+)	Exothermic (-)
ΔS	+	Spontaneous at high temperatures	Spontaneous at all temperatures (ΔG < 0)
	-	Non-spontaneous at all temperatures (ΔG >0)	Spontaneous at low temperatures

what is the entropy of mixing?

• Having a mixture of both reactants and products increases the entropy and has more negative free energy 

  • Because it increases the number of possible arrangements by having a mixture of molecules

• Increasing entropy results in a decreasing ΔG → leading to spontaneity

• Due to entropy of mixing:

  • The position with the lowest Gibbs free energy is between pure reactants and pure products

  • The reaction proceeds until this position is reached (the point of equilibrium)

what state do thermodynamic systems prefer?

• Thermodynamic systems tend to move towards a state with lower Gibbs free energy

write the chemical reaction for cobalt (ii) chloride with its colour change

Co(H₂O)₆²⁺_(aq) + 4Cl^-_(aq) ⇌ CoCl₄^2-_(aq) + 6H₂O_(l)    

∆H>0: cobalt (II) chloride hydrate (pink to blue)

write the chemical reaction for iron (iii) nitrate and potassium thiocynate with its colour change

Fe³⁺_(aq) + SCN^-_(aq) ⇌ [Fe(SCN)]²⁺_(aq)   

∆H< 0: iron(III) thiocyanate (pale yellow to blood red)

write the chemical reaction for burning magnesium

2Mg_(s) + O_{2 (g)} → MgO_(s)   

∆H<0: burning magnesium (NOT reversible - oxidation

write the chemical reaction for burning steel wool

4Fe_(s) + 3O_{2 (g)} → 2Fe₂O_{3 (s)}   

∆H<0: burning steel wool (NOT reversible - oxidation)

depict equilibrium on a concentration profile diagram

graph dynamic equilibrium on a rate vs time graph

what is le chatelier’s principle?

Le Chatelier’s Principle

“If a system is at dynamic equilibrium is disturbed (by changing the conditions), then the system will adjust itself to minimise the effect of the disturbance to attain a new equilibrium”

a shift to the right =

forward reaction favoured

a shift to the left =

reverse reaction favoured

how does changing concentration affect equilibrium?

• ↑ [reactants] → equilibrium favours forward reaction → ↑ [products] and ↓ [reactants]

  • The original concentration won’t be the same as there is a new equilibrium formed

  • Will only partly counteract the change

• FOR (aq): if water is added, then ALL aqueous solutions will be diluted = ↓ conc.

• Can also be removed so [species] ↓

explain concentration’s effect on equilbrium through rates of reaction

• ↑ [species] = more particle → higher chance of successful collisions 

• Collision of reactants result in products formed, so less reactant particles, so less chance of successful collisions of reactants

• Equilibrium reduces to rate_(forward) = rate_(reverse)

how does a change in concentration look on a graph?

how does changing pressure and volume affect equilibrium?

• As ↑ V = P↓ →  [all species] ↓

  • Same no. of moles of each species in a larger volume 

  • C = n/v

• System will respond by doing the opposite of the change

NOTE: change in pressure only occurs in gaseous systems

• More moles of gas on one side = more pressure

what is the relationship between pressure and volume?

Relationship between Volume and Pressure: ↑V = ↓P

• Doubling volume = halving pressure

explain the change of pressure/volume on equilibrium through rates of reaction

• ↓ conc has less chances of successful collisions = rate ↓

• Reverse reaction favoured to increase the lost concentration

how does the pressure/volume change look on a graph?

Note that this reaction has their molecular ratio represented in their concentration - volume decreased, pressure increased

how does temperature change equilibrium?

• ↑ temp = more heat in system =  equilibrium favours endothermic direction to absorb added heat

• ↓ temp = less heat in system = equilibrium favours exothermic direction to produce additional heat

explain the effect temperature has on equilibrium through rates of reaction?

• When temp ↑, all particles gain more kinetic energy

• Rate of endothermic reaction ↑ more than exothermic because the E_a of endothermic is higher = rate will increase in a greater proportion

• equilibrium shifts in endothermic reaction

  • There is a proportional increase in the number of collisions exceeding the E_a will be higher for endothermic reactions

Maxwell Boltzmann Distribution

what is maxwell boltzmann distribution?

• There is a proportional increase in the number of collisions exceeding the E_a will be higher for endothermic reactions

how does the temperature change look on a graph?

what changes DON”T affect the equilibrium

1. catalyst

2. addition of inert gas at a constant volume

why does adding a catalyst not affect the equilibrium?

• Catalyst ↑ rate of reaction without being consumed 

  • Offers an alternate pathway with lower E_a for both the forward and reverse reaction equally

  • Won’t disturb the equilibrium

• System will reach equilibrium faster

why does adding an inert gas at a constant volume not affect the equilibrium?

• Inert gas = noble gas

• The addition will ↑ pressure, but [species] remains same IF volume doesn’t change

NOTE: inert gas added at a constant pressure will disturb the equilibrium as the volume has to increase to maintain constant pressure

• [species] changes so equilibrium changes

will adding inert gas at a constant pressure disturb the equilibrium?

yes, inert gas added at a constant pressure will disturb the equilibrium as the volume has to increase to maintain constant pressure

• [species] changes so equilibrium changes

what is an inert gas

noble gas

how do you answer LCP questions?

1. State the change that disturbs the equilibrium

2. “According to Le Chatelier’s Principle → state equilibrium shifts to minimise disturbance

3. Where does the equilibrium shift?

4. Justify why did it shift there: “to replace/increase/remove etc. … and minimise the disturbance”

5. State effect of the shift: what happens to the concentrations

what is the reaction of nitrogen dioxide to dinitrogen tetroxide and its colour change?

2NO_{2 (g)} ⇌ N₂O_{4 (g)}   

∆H< 0 (brown to colourless)

what is K_eq?

K_eq: tells us whether there are more products or reactants in an equilibrium mixture

• Describes the position of equilibrium'

• Provides information about the ratio of products to reactants at a specific temperature

when does a K_eq change

only if the temperature changes

why does the change in concentration not change the K_eq?

• Changing [one species] will cause the equilibrium to counteract the change, but the ratio between the reactants and products remain the same

K_eq =

[products] / [reactants]

• ignore any species that are solid, liquid - they don’t change at a constant temperature (let it = 1)

how does K_eq change when temperature changes?

K_eq changes, based on the exo/endothermic nature of the reaction

• Exothermic: if ↑T, K_eq ↓

• Endothermic: if ↑T, K_eq ↑

if the original equation with (K_eq) is reversed what is K_eq now?

1/K_eq

if the original equation with (K_eq) is multiplied by a number (e.g 2) what is K_eq now?

(K_eq)²

if the original equation with (K_eq) is divided by a number (e.g 2) what is K_eq now?

√K_eq

if the original equation with (K_{eq 1}) is added with another equation with (K_{eq 2}) what is K_eq now?

K_eq = K_{eq 1} x K_{eq 2}

where does the concentration have to be from to calculate Keq?

equilibrium

where does the concentration have to be from to calculate Q (reaction quotient)

anywhere

what does it mean if Keq is large?

• If K is larger, the further the equilibrium lies to the right:[Products]> [Reactants] 

  • A very large K, proceeds almost to completion

what does it mean if Keq is small?

• If K is smaller, the equilibrium lies to the left:[Reactants] > [Products]

  • Very small K, hardly proceeds at all

what is Q. = K

•  system is at equilibrium

what if Q > K

• reverse reaction favoured (more reactants needed to reach equilibrium)

what if Q < K?

• forward reaction favoured (more products needed to reach equilibrium)

what does the relationship between Q and K tell us about?

which way the reaction will proceed to reach equilibrium

what is colourimetry?

• An analytical technique to find the concentration of coloured compounds in solution based on their absorbance of light

  • Coloured solution absorbs it complementary colour (directly opposite in colour wheel)

  • E.g a red solution absorbs green and transmits red light

what is absorbance?

Absorbance is directly proportional to concentration and length of the sample

what is beer-lambert law?

A = εlc

A = absorbance

ε = molar absorptivity: measure of how strong a chemical species absorbs light (L/(cm x mol))

l=path length of sample (cm)

c= concentration of analyte in sample (mol/L)

how does the colorimeter work?

• There is a coloured filter after the slit that allows a small range of wavelengths to pass through the sample to reduce interference

• The detector converts the amount of light to an electrical signal

• The calorimeter measures the intensity of light through A = log₁₀(I₀/I)

  • I₀ = intensity of light passing through a blank sample

  • I = intensity of light passing through the coloured sample

• As other factors can affect the absorbance readings (impurities in solvent, scratches on cuvette, orientation of cuvette, stray light, instrument accuracy), one uses a calibration curve

what is a calibration curve?

• Measures the absorbance of standard solutions  under the same conditions

• The unknown concentration can be found using the linear graph (has to be linear)

  

what is the benefit of a calibration curve?

• Reduces systematic errors = improves accuracy and validity

what is K_a?

Ka: Acid dissociation constant

Equilibrium constant for the dissociation (ionisation) of an acid into a hydrogen or hydronium ion and anion Indicates the extent of dissociation/strength of an acid The stronger the acid, the ↑ Ka (eqm lies on the right, more ionisation) Polyprotic acids have more than one Ka value (for each H+ they donate)

what is K_b?

Kb: Base dissociation constant

Indicates the extent of dissociation/strength of base The stronger the base, the ↑Kb

what is the dissolution process of ionic compounds?

NaCl_(s) ⇌ Na⁺_(aq) + Cl ^-_(aq)

• The ions at the surface get surrounded by water molecules that have stronger cohesive forces (hydrogen bonds) compared to NaCl (ionic bonds)

• So ions move from their positions to form adhesive forces with water: ion-dipole forces ∴ ionic compound is dissolved, ions are solvated

• The hydrated ions have a shell of water molecules = solvation layer → prevents anions to collide directly with cations = keeps ions in solution

are all ionic compounds soluble?

not all ionic compounds are soluble due to their strong ionic bonds

what does the enthalpy of a solution formed by?

The enthalpy of a solution (∆H_solution) forms from the difference between energy required to break cohesive forces and energy produced when adhesive forces form

if ∆H <0 what force is stronger? adhesive or cohesive?

• adhesive forces > cohesive forces

if ∆H >0 what force is stronger? adhesive or cohesive?

• adhesive forces < cohesive forces

what is the total change in entropy of solution?

• ∆S = ∆S_solute + ∆S_solvent

if ions dissolve, then what happens to the change in entropy?

• If ions dissolve, then ∆S_solute increases (more freedom to move ∴∆S_solute > 0)

• If ions dissolve, then ∆S_solvent decreases (water is trapped in solvation layer ∴∆S_solvent < 0)

• Mostly, ∆S> 0 overall

what is solubility?

• maximum amount of solute that can dissolve in a specific volume of solvent at a certain temp

what is a saturated solution?

• the maximum amount of solute has been dissolved in a solvent

  • If more solute is added, it won’t dissolve

  • System is at dynamic equilibrium (all species are present in final mixture)

what is the solubility product constant?

K_sp

• Is for the equation written in the direction of dissolution

  • Cannot use solids in the calculation of K_sp

• As K_sp ↑, solubility ↑

can you compare salts solubility based of K_sp?

no, solubility has to be calculated from K_sp (the conc. of the reactant)

what is molarity?

Moles of solute (n)

Volume of solution (L)

mol/L or M

what is w/w% ?

g

/100g

what is m/v?

Mass per volume

g

mL or L

what is ppm?

Parts per million

(mass of solute/mass of solution) x 106  - in grams

what is ppb?

Parts per billion

(mass of solute/mass of solution) x 109 - in grams