Chemical Systems and Equilibrium

Dynamic Equilibrium in a Chemical Systems (Section 7.1)  When a reaction is reversible, it means that it can go both forwards and backwards.  The forward reaction is usually the favourable reaction (where reactants are converted into products).  The backward reaction is where the products become changed back into the original reactants.

Dynamic Equilibrium In a closed system, after awhile an equilibrium mixture is reached, where a certain proportion of the mixture exists as reactants, and the rest exists as products. REVIEW: A closed system means that none of the products or reactants can escape to the outside environment. -> However, energy can be transferred. Heat Energy Exchanged

Equilibrium When equilibrium has been reached, it does not mean that the reactions have stopped.  It means that the forward reaction is making products at the same rate that the backward reaction is making reactants.  This is symbolized by a double arrow between the reactant and product sides of the equation

Dynamic Equilibrium Dynamic means moving or changing, to remind you that the reaction has not stopped. The chemical change in the left-to-right direction is referred to the forward direction The chemical change in the right-to-left direction is referred to the reverse direction The example of the dissolved CO 2 in a soft drink is referred to as a solubility equilibrium

Solubility Equilibrium -> Solid ionic compounds dissolve to form an aqueous solution until the point of saturation -> After saturation any solid added appears to remain as a solid but, an equilibrium system is set up in which the rate of dissolution equals the rate of recrystalization -> If both rates are equal, no observable changes occur

Phase Equilibrium Phase equilibrium is an equilibrium system involving the processes of evaporation and condensation H 2 O (g)  H 2 O (l)

Phase Equilibrium -When liquids are placed in any container – initially evaporation occurs. -Some molecules in the liquid gain enough energy from collisions to leave the surface of the liquid and move into the gas phase. -BUT  as the number of molecules in the gas phase increases, more of them collie with the liquid surface and lose enough energy to condense. -Eventually, the rate at which these two processes occur will become equal, so no further changes will be observed.

Reaction Equilibrium 2 NO 2 (g)  N 2 O 4 (g) + energy In this reversible reaction, the product reacts to form the reactant The formation of products and the re-formation of reactants are two opposing reactions that are occurring at the same time (double arrows) in a closed system When the two reactions occur at the same rate, and no observable changes are taking place, chemical equilibrium is reached

Reaction Equilibrium In order for chemical equilibrium to be reached and maintained, the system must be closed: To recognize when equilibrium is established, a constant property of the system must be observed: -> Colour intensity -> Concentration -> Pressure No materials enter or leave the system!

Percent Reaction at Chemical Equilibrium Percent Reaction = Actual Product Yield x 100% Theoretical Percent Yield The actual product yield is the measured concentrations of the products reached at equilibrium The theoretical yield is the concentration of the products determined using stoichiometry if the reaction were to go to completion

Chemical Reaction Reactions fall loosely into THREE categories: 1)Reactions that favour reactants 2) Reactions that favour products 3) Reactions that achieve noticeable equilibrium

Reactions that Favour Reactants When the reactions favours the reactants very strongly the PERCENT REACTION is much less than 1%. Therefore, mixing reactants has no observable result. (No reaction occuring)

Reactions that Favour Products When the reactions favours the products very strongly the PERCENT REACTION is more than 99% Therefore, they are quantitative and are generally written with a single arrow to indicate that the effect of the reverse reaction is negligible.

Reactions that Achieve Noticeable Equilibrium The PERCENT REACTION is between 1% and 99% Therefore, if the percentage is less than 50%; reactants are favoured but still in equilibrium...if the percentage is greater than 50%; products are favoured but still in equilibrium

Calculating Concentrations In the past (Gr. 11), we have dealt with stoichiometry of reactions that have always been assumed to go to completion. -> Since we now have to deal with reversible reactions – we use an ICE table to organize our information. I -> Initial Concentration C -> Change in Concentration E ->Equilibrium Concentration

Calculating Concentrations at Equilibrium

Example 1 What are we given?

Example 1 What are we given?

Example 1 What are we given? What do we need to find?

Example 1 What are we given? What do we need to find?

Calculating Concentrations at Equilibrium What formulas will we need? C= n/V molar ratios ICE table

Calculating Concentrations at Equilibrium What formulas will we need? C= n/V molar ratios ICE table S:[NH 3 ] i = 4.0 mol [NH 3 ] e =2.0 mol 2.0 L2.0 L = 2.0 mol/L = 1.0 mol/L

Calculated from given information

Based upon the molar ratios from the balanced chemical equation

Determine the value of ‘x’ using the concentrations for ammonia and solve for the equilibrium concentrations:

Since we have calculated the equilibrium concentration for ammonia in the previous steps we can use that information to determine what the value of “x” is. From the ICE table we know that at equilibrium the concentration of ammonia is 2.0 – 2x we can solve for ‘x’

Since we have calculated the equilibrium concentration for ammonia in the previous steps we can use that information to determine what the value of “x” is. From the ICE table we know that at equilibrium the concentration of ammonia is 2.0 – 2x we can solve for ‘x’

Now that we know that x = 1.5 mol/L we can Determine the equilibrium concentrations of all products.