Dynamic Equilibria, Esterification and Mass Transfer Limitations

dynamic equilibrium

both forward and reverse reactions are occurring simultaneously at the same rate. Concentration of Reactions and products remain constant.

Le Chatelier's Principle

States that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

equilibrium constant (law of mass action)

the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation

increase in concentration of A

Position of equilibrium will move to decrease the concentration of A by reacting increased A with B producing C + D and shifting position right

How can change in concentration be used in a reversible reaction?

Even if a desired product is not thermodynamically favoured, the end-product can be obtained if it is continuously removed from the solution

increase in pressure/ decreasing volume

The position of equilibrium will move so that the pressure is reduced. If reduce number of molecules, reduce pressure. If there are 3 molecules on left-hand side but only 2 on right By forming more C & D from A & B, you reduce the number of molecules and the pressure decreases (move to side with fewer molecules)

What if there are same number of molecules on both sides of equilibrium reaction?

Increasing pressure (decreasing volume) has no effect on the position of the equilibrium. Equilibrium can't move in any way that will reduce the pressure again.

What happens if temperature decreases?

Reaction will tend to heat itself up again to return to original temperature. Favours the exothermic reaction. Position of equilibrium will move to the right if forwards reaction is exothermic

other affects of temperature change

Produces a change in K (equilibrium constant). The dependence of K on temperature is determined by the sign of ΔH. The theoretical basis of this dependence is given by the Van 't Hoff equation

van 't Hoff equation

This equation is exact at any one temperature. Equation is integrated between two temperatures. Use to estimate a new equilibrium constant at a new absolute temperature assuming a constant standard enthalpy change over temperature range

Van 't Hoff Plot

For a reversible reaction, the equilibrium constant is measured at a variety of temperatures and plotted. Plot used to estimate the enthalpy and entropy of the chemical reaction within a process from the slope and intercept of the Van 't Hoff plot

mechanism studies

A chemical reaction may undergo different reaction mechanisms at different temperatures. VHP with two or more linear fits may be found. Each has different slope and intercept indicates different DH & DS changes for each mechanism.

addition of inert gas

the total pressure of the system increases but total pressure does not have any effect on Keq because addition of non-reactive gas does not alter relative partial pressures of other gases in container. However, if volume is allowed to increase - Partial pressures of all gases would be decreased. - Results in a shift towards side with greater number of moles of gas.

how does catalyst affect equilibrium

no effect on equilibrium as no change in Keq

solubility product constant (Ksp)

This is an expression for the chemical equations that represents the dissolution processes. A large Ksp (forward reaction favoured) means that the compound is very soluble. The reverse for a small Ksp.

Important - Solids are omitted from the equilibrium expression

what does the equilibrium constant describe?

how far a chemical reaction will go.

Fischer esterification

the formation of an ester from a carboxylic acid and an alcohol in acidic conditions

cheap, low Keq so requires either a product to be removed or an excess of 1 reactant to achieve equilibrium

Fischer esterification lab conditions

uses an excess of the alcohol. The alcohol can act as the solvent and as a reactant. The reaction usually uses an 4-10 fold excess. The carboxylic acid is a solid so cannot be used in excess because a heterogeneous reaction mixture would be formed. Reflux conditions

Fischer esterification catalyst

A very strong mineral acid is used as catalyst. The carboxylic acid is a weak electrophile. The mineral acid protonates the carbonyl carbon atom and makes it a little bit better electrophile. Rate is still low so mixture is refluxed

How is water removed in Fischer esterification?

Dry reagents prior to reaction, Add hydroscopic reagents like concentrated sulfuric acid, Solid absorbent such as molecular sieves may be added, A sparge gas may be used to increase liquid gas surface area and give motive force to move entrained water/vapour, Apply vacuum to remove lower boiling to remove water vapour from reactor headspace, Use of reflux will also help water removal

Step Growth polymerisation

Does not require initiation - All molecules can react. Reaction not confined to Monomer:Chain as with Chain Growth. Proceeds by a series of individual reactions between Monomers:Monomer, Monomer:Chain and Chain:Chain. Often involves elimination of a small molecule eg water. In which case called a Condensation Polymerisation. Reversible reaction so need to remove / exclude condensate during reaction and processing.

what size molecules prefer to react in step growth polymerisation?

smaller molecules because with large molecules mass blocks the reactive centre/site

chain growth polymerisation Mw vs conversion

Rapid growth which then remains quite constant until monomer depleted. Instantaneous jump to high Mw

step growth polymerisation Mw vs conversion

Initial slow growth but Increases rapidly at high conversion. Mw doesn't increase much till high conversion reached

Step-growth process

Polymer growth occurs by reaction between monomers, dimers, trimers and higher oligomers/polymers.

DP low to moderate

Monomer consumed rapidly but molecular weight increases slowly

No initiator needed - same reaction mechanism throughout

No termination step - end-groups remain active Polymerisation rate decreases steadily as functional groups are consumed

DP

The degree of polymerization: the number of repeat units in an average polymer chain at time t in a polymerization reaction

No/N or 1/(1-p)

Chain-growth process

Chain length increases by successive additions of a monomer molecules to the end of the growing chain.

Achieve very high DP

Monomer consumed slowly but molecular weight increases rapidly

Initiation and propagation mechanisms are different Usually chain growth is terminated

Polymerisation rate increases initially as initiator is generated and remains relatively constant until monomer depleted

Carothers Equation

Predicts molecular weight

DP=1/(1-p)

Extent of reaction

p

p = (No-N)/No

Number of functional groups remaining at anytime

N

DP max

DP max = (1 + r) / (1 - r)

r = molar ratio of reactants