Approaching Equilibrium Lesson 1

Irreversible reactions Most Chemical reactions are considered irreversible in that products are not readily changed back into reactants. Mg + 2HCl  MgCl2 + H2 When magnesium reacts with acid it is not easy to unreact it and get back the magnesium. Wood reacting with oxygen When wood burns it is pretty difficult to un-burn it back into wood again!

Many biochemical reactions are reversible Irreversible reactions Although most chemical reactions are difficult to reverse it is possible to find reactions ranging from irreversible to the fully reversible. Indeed many of the biochemical reactions that take place in living things are reversible. Many biochemical reactions are reversible There are also some very important industrial reactions, like the Haber Process, that are reversible.

Simple reversible reactions Heating copper sulphate The change from blue hydrated copper sulphate to white anhydrous copper sulphate is one of the most commonly known reversible reactions. Heat hydrated copper sulphate anhydrous copper sulphate steam CuSO4.5H20  CuSO4 + 5H2O

Simple reversible reactions Heating ammonium chloride Ammonium salts are made by reacting ammonia with an acid but some of these salts will decompose back into reactants when heated. Heat Heat makes the solid disappear as it changes into gases. Solid reappears as it changes back again in the cool part of the tube. ammonium chloride ammonia hydrogen chloride NH4Cl(s) NH3(g) + HCl(g)

Approaching Equilibrium   Many chemical reactions are reversible if the activation energy is low and the system is closed. Reactants ⇌ Products

Approaching Equilibrium Many chemical reactions are reversible if the activation energy is low.   Reactants ⇌ Products Forward Reaction is left to right   Reverse Reaction is right to left

Dynamic Equilibrium Recall that we proposed that reverse reactions could occur with separate activation energies.

Dynamic equilibrium A reversible reaction is where products can, under appropriate conditions, turn back into reactants. There will be a range of conditions over which both the forward and backward reaction will take place and this can lead to a state of balance with both reactants and products present in unchanging amounts. This is called a dynamic equilibrium. A B A B these combine these decompose

Lets see what happens to some reactants if they are placed in a reaction vessel and allowed to react.  

Approaching Equilibrium Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium Reactants Products Forward Rate Reverse Rate Are Equal

Approaching Equilibrium Reactants Products Forward Rate Reverse Rate Are Equal Are Constant

Graphing the Approach to Equilibrium   product  Concentration reactant Time

Graphing the Approach to Equilibrium   reactant  Concentration product Time

Graphing the Approach to Equilibrium   Forward rate  Concentration Reverse rate Overall rate Time

At dynamic equilibrium: The concentrations of A and B are constant In general terms: At dynamic equilibrium: The concentrations of A and B are constant The forward and reverse reaction rates are equal

[ ] of A decreases while [ ] of B increases till equilibrium is reached. Equilibrium is reached when rate of forward reaction is the same as the reverse reaction.

Approaching Equilibrium Reactant concentrations start high and decrease as the reaction proceeds. The forward rate, which depends on collisions of the reactants, also decreases.   Product concentrations start at zero and increase as the reaction proceeds. The reverse rate, which depends on collisions of the products, also increases. Eventually the forward rate is equal to the reverse rate and the concentrations are constant. This is equilibrium.

Characteristics of a System at Equilibrium   The Forward rate = The Reverse rate The Reactant and Product concentrations are constant The Macroscopic (observable) properties are constant The system is Dynamic as the forward and reverse reactions continue. The equilibrium can be approached from starting with reactants or starting with products.

Approaching Equilibrium from Products Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium from Products Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium from Products Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium from Products Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium from Products Reactants Products Forward Rate Reverse Rate

Approaching Equilibrium from Products Reactants Products Forward Rate Reverse Rate Are Equal Are Constant Same as before

If you start with products all aspects of the approach to equilibrium are reversed Products and Reverse Rate decrease Reactants and Forward Rate increases

Conditions Necessary for Equilibrium Closed system Constant temperature Ea is low so the reaction is reversible

Characteristics of equilibrium Discussion Time Characteristics of equilibrium A tell B ---------------------------, B rephrase A says: That’s exactly what I mean, Or rephrase his/her statement Then continue with next statement

Summary of characteristics of Equilibrium