Chap 8A - Chemical equilibria

Define irreversible reaction

(Def.): Irreversible reactions are chemical reactions that proceed to completion. These chemical reactions take place in one direction almost exclusively and are denoted by a single–headed arrow (→). The limiting reagent is completely used up in an irreversible reaction

Draw concentration time and rate time graph for irreversible reactions (aA + bB → cC + dD)

Define reversible reaction

• Definition : A reversible reaction is one which can occur in both direction, denoted by a double-headed arrow ‘⇌’ is used to denote a reversible reaction, they are incomplete and reach a state of dynamic equilibrium containing a mixture of both reactants and products

• There is no excess or limiting reagents as they are never completed

Draw conc time and rate time graph for reversible reactions

Describe and define dynamic equilibrium

• (Def.) : A state of dynamic equilibrium is said to have reached when the rate of the forward reaction is equal to the rate of the backward reaction in a reversible reaction and the concentrations of both reactants and products are constant

• System is closed (does not allow matter to enter or leave but allow free transfer of energy)

• Forward and backward reactions are still taking place!

• Things still happening but cannot be seen as all macroscopic properties remain constant 

Describe and define static equilibrium

• Static equilibrium (Def.): state in which there is no change in both macroscopic and microscopic (molecular scale) properties in a system

• Equilibrium can be achieved “from either direction” : Equilibrium can be established starting with either the reactants or products

Describe A + B ⇌ C + D reaction

1. At time = 0 (start of reaction), forward reaction occurs to form C and D 

2. Since C and D commences, backward reaction commences to form A and B 

3. At teqm, the forward and backward reactions are still occurring

• A state of dynamic equilibrium is attained where Rf = Rb = constant (NOT 0)

• At this point, there is no net change in the amount of reactants and products

• At equilibrium, the reaction is incomplete (reactants are still present in the system at equilibrium)

• Once equilibrium is reached, the system will remain in this state, until conditions affecting the rates are introduced

Define and describe reaction quotient

• (Def.): For a reversible reaction, it is the ratio of the concentrations of products to reactants raised to their stoichiometric coefficients in the balanced equation

• Qc is not constant (Changes with time until the reaction reaches equilibrium)

State Qc equation and its units

Explain Qc < Kc and vice versa

Qc < Kc	Qc > Kc
Reaction will proceed in the forward direction to consume more reactants and form more products until Qc = Kc	reaction will proceed in the backward direction to form more reactants and consume more products until Qc = Kc

Define and describe Kc (words)

• Kc (Def.): Measures the extent to which reactants are converted to products at equilibrium at a given temperature

• At equilibrium: forward = backward rate 

• Equilibrium law: When reaction is at equilibrium, Qc = Kc

• Constant at constant temperature 

Describe units of K

• Units dependent on chemical equation (ALWAYS state it) (possible to have no units) 

  • K can be expressed in terms of:

    • Concentration (in mol dm–3), denoted by Kc

    • For gaseous reactions, partial pressure (usually in Pa or atm), denoted by Kp

• The value of Kc or Kp is unaffected by changes in the conc or pressure of reactants or products, or the presence of a catalyst (affected by temperature only) 

Define Kc (the equation one)

Define Kp (the equation one)

Describe homo and hete mixtures

Homogeneous	Heterogeneous
The reaction mixtures contain reactants and products in the same phase or physical states	The reaction mixtures contain reactants and product in different phases or physical states

Describe 3 species to exclude from Kc/Kp expressions

1. [Pure solids] 

• Exclude solids in all equilibrium expressions for all mixtures because concentration of a pure solid = its density is constant and independent of the mass of the solid present.

2. [Pure liquids] 

• Exclude pure liquids in expressions for heterogeneous equilibria because concentration of a pure liquid calculated from its density is a constant in heterogeneous mixtures and independent of the mass of liquid present

3. [Solvents] 

• Exclude solvents in all equilibrium expressions as they are present in excess

• Being pure liquids, their concentrations calculated from their densities is a constant

Describe position of equilibrium

• Refers to the composition of an equilibrium mixture (the relative proportion of products to reactants in an equilibrium mixture) 

• May lie on the right (towards the products) or lie on the left (towards the reactants)

• Magnitude of Kc or Kp indicates the extent of the forward reaction at equilibrium at a given temperature

  • Magnitude of Kc or Kp gives no information about the ROR

Describe small VS large K

Small K	Large K
Means the position of equilibrium lies on the left The extent of conversion of reactants into products is small, so the proportion of products to reactants is low	Means the position of equilibrium lies on the right The extent of conversion of reactants into products is large, so the proportion of products to reactants is high