Equilibrium chapter 19 brief notes

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Kc continued

HOMOGENEOUS EQUILIBRIUM = contains equilibrium species that all have the SAME state/phase

HETEROGENEOUS EQUILIBRIUM = contains equilibrium species that have DIFFERENT states/phases

In homogeneous equilibria, the Kc expression contains concentrations of ALL THE species. However, in heterogeneous equilibria, the concentrations of the SOLIDS and the LIQUIDS are essentially CONSTANT. So, any species that are solids or liquids are OMITTED from the Kc expression. They are AUTOMATICALLY incorporated within the overall equilibrium constant. Kc, therefore, ONLY INCLUDES SPECIES THAT ARE (g) OR (aq)!!!!!!!!

When calculating the units for Kc, also only include the aqueous or gaseous species

May be asked to calculate the value for Kc given the MOLES - use the ICE method to work out concentrations, then work out Kc

WHAT CAN AFFECT THE POSITION OF AN EQUILIBRIUM = changes in concentration, changes in temperature, presence of a catalyst

WHY CHANGES IN CONCENTRATION HAS NO EFFECT ON THE EQUILIBRIUM CONSTANT = the rate constants BALANCE OUT - as the products go up, the reactants will balance out. Therefore, it has NO effect on the equilibrium constant

WHY THE PRESENCE OF A CATALYST HAS NO EFFECT ON THE POSITION OF EQUILIBRIUM = the rate of the forward and backwards reactions are increased by the SAME AMOUNT - unchanged equilibrium position

WHY CHANGES IN TEMPERATURE DO EFFECT THE EQUILIBRIUM CONSTANT = changes the rate constants of both the forwards and reverse reactions and is therefore likely to change the vale of the equilibrium constant

If the reaction is EXOTHERMIC, an increase in temperature DECREASES Kc (more reactants form - dividing by a bigger amount = lower value)

If the reaction is ENDOTHERMIC, an increase in temperature INCREASES Kc (more products form - diving by a smaller amount = bigger value)

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Kp

There is a DIRECT RELATIONSHIP between Kp and Kc

Kc = the equilibrium values are expressed as CONCENTRATIONS in mol dm-3

Kp = the equilibrium values are expredded as PARTIAL PRESSURES

For reactions involving GASES, it's difficult to measure the concentration of a particular gas. Instead, the QUANTITY OF EACH GAS IN AN EQUILIBRIUM MIXTURE is described, in terms of the PRESSURE it exerts - its PARTIAL PRESSURE (p) (contribution that that gas makes towards the total pressure)

Partial pressure (Pa) = mole fraction x total pressure

Mole fraction = number of moles of the substance/total number of moles in the gas mixture

The sum of the mole fractions in a gas mixture must equal 1! (like all fractions)

Work out Kp similarly to Kc, but use the partial pressures instead. Stoichiometry values in the equations still used as the powers in the expression, as with Kc

Suitable units for partial pressures include: kPa, Pa or atm. Can use any, as long as THE SAME UNIT IS USED FOR ALL GASES!)

Kp only includes GASES, because only gases have partial pressures. In kP, any other species must be IGNORED eg. ignore any (l), (aq) or (s)

Working out units = same method for Kc and Kp - Kc use moldm-3, Kp use atm, kPa or Pa

Both Kc and Kp are only affected by temperature

SUMMARISE WHAT TO INCLUDE IN THE CONSTANT EXPRESSIONS:

Kc = (aq) and (g)

Kp = (g)