Making an Ester, Energy Transfer in Reactions, Catalyst and Bond Energies and Reversible Reactions

C7 Revision

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Making an Ester

Reflux, Distil, Purify, Dry

Refluxing - The Reaction
To make ethyl ethanoate - react ethanol with ethanoic acid using a catalyst to speed it up.
Heating speeds the reaction up - heated in a flask fitted with a condenser which catches vapours and recycles them back into the flask giving time to react.
Separates the ester from all the other things (unreacted alcohol, carboxylic acid, sulfuric acid and water)
Mixtures heated below a fractioning column, as it boils that's where the vapor goes
When it reaches boiling point of ethyl ethanoate, the liquid that flows from the condensor is collected - it's impure.
Liquid is collected (the distillate) and poured into a tap funnel, then the impurities are removed.
Mixture is shaken with sodium carbonate solution to remove acidic impurities.
Ethyl ethanoate doesn't mix with water so they're separated - lower layer can be removed (tapped off)
Any remaining water in the ethyl ethanoate can be removed by shaking it with lumps of anyhydrous calcium chloride which aborbs water. Finally the pure ethyl ethanoate can be separated from the solid calcium chloride by filtration

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Energy Transfer in Reactions

Reactions are Exothermic or Endothermic
Exothermic = gives out energy to the surroundings usually as heat and shown by a rise in temperature
Endothermic = takes in energy from the surroundings usually as heat and shown by a fall in temperature

Energy level diagrams show if it's 'Exo-' or Endothermic
Exothermic shows the products are at a lower energy than the reactants
difference in height represents the endergy given out.
TriangleH = the energy change - Negative as heat is given out. -ve
Endothermic shows the products are at a higher energy than the reactant
- triangleH is +ve. The difference in height shows the energy taken in.

Activation Energy is the energy needed to start a reaction
- the minimum amount of energy needed
If the energy input is less than the activation energy there won't be enought energy to start a reaction.


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Catalysts and Bond Energies

Catalysts lower the Activation Energy by providing alternative routes
The over all energy change for the reaction (triangle H) remains the same.

Energy must always be supplied to break bonds
During reactions - old bonds are broken, new formed
Energy must be supplied to break existing bonds - endothermic
Energy is released when new bonds are formed - exothermic

Every bond has a particular bond energy for it, it varies depending on the compound the bond occurs in
These can calculate the overall energy change for a reaction

Example - the formation of HCL
H-H: +436kJ; Cl-Cl: +242kJ; H-Cl: +431kJ
The energy required to break original bonds is 436+242 = +678kJ
Energy released by forming new bonds is 2 x 431 = +862kJ
More energy is released than it is used to form product: 862 - 678 = 184kJ released
It is a exothermic reaction - TriangleH = -184kJ

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Reversible Reactions

This is one where the products of a reaction can react with each other to convert back to the original reactants
Reversible Reactions will reach dynamic equilibrium
If a reversible reaction takes place in a closed system then the state of equilibrium will always be reached
Closed system - no reactants or products can escape
Equilibrium means the  relative % quantities of reactants and products will reach a certain balance and stay there
Dynamic equilibrium - reactions are still taking place in both directions, the overall effect is nil as the forward and reverse reactions cancel each other out
They take place at exactly the same rate in both directions.

Ionisation of Weak acids is a reversible reaction
When acids are dissolved in water they ionise - hydrogen ions, H+ - makes it acidic
Stong acid (HCL) ionise almost completely in water - lot's of H+
Weak acid (carboxylic acids) ionise slightly, only some Hydrogens are relaeased - small number of H+ formed
Ionisation of a weak acid = reversible since few H+ ions are released, the equilibrium is well to the left.

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First place I've actually found this information, the textbook is terrible, thanks


Thank you so much 

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