5.1.2 How Far Flashcards
(33 cards)
how do you find the units for Kc
1) substitute the units into the Kc expression
2) cancel out common units and show the final units in a single line
- remember, all concentrations are given as mol dm-3
- and you can have no units
what is homogeneous equilibrium
contains equilibrium species that all have the same state/phase
-Kc would contain ALL species
what is heterogeneous equilibrium
- contains equilibrium species that have different states
- for calculating Kc, you ONLY USE gases and aqueous species, as concentrations of solids and liquids are essentially constant, so are automatically incorporated into the total expression
what is the first step in calculating Kc, where the concentrations are not given, so you can’t just input them into the expression
MUST FIND THE AMOUNT OF MOLES AT EQUILIBRIUM:
using a RICE table: where you write the balanced equation given at the top
1) Reacting amount: the molar numbers in the balanced equation
2) Initial moles: given for reactants, and always 0 for products
3) Change in mole: gathered from the moles of product made, as will be subtracted from reactants in CORRECT MOLAR RATIO
4) Equilibrium moles: calculated at the end once you have applied the change in moles
what is the second step in calculating Kc, once the moles at equilibrium have been established
- need to find the concentration in moldm-3
- use the same volume as given in the question for all values
- and divide each equilibrium moles by it
- REMEMBER IT IS IN dm-3, NOT cm-3, SO CONVERT IF NECESSARY
what is the last step of Kc, once you have found the concentration of each species
- write the Kc expression
- and input in your concentrations
- remembering to square values if in molar value 2
what is Kp
the equilibrium constant in terms of partial pressure
what is Kp used for
used for gases, where it is easier to measure partial pressure rather than concentration
what can be said about the relationship between Kc and Kp
- concentration and pressure are directly proportional
- so Kp and Kc have a direct relationship to eachother too
what can be said about gases in the same conditions of temperature and pressure
under the same conditions of temperature and pressure, the same volume of different gases will contain the same number of moles of gas molecules (via Avogadro’s Law)
what is mole fraction
the proportion of a gas by its volume to the total volume of gases in a gas mixture
how is mole fraction calculated
moles of A / total moles in gas mixture
- if given as percentages, just do percent over 100
what is the symbol of mole fraction
x(A) = mole fraction of A
what is partial pressure
p = the contribution a gas makes towards the total pressure P
- sum of all p=P (sum of all partial pressures is the total pressure)
how do you find the partial pressure of a gas
p(A) = x(A) x P
- the mole fraction of A times by the total pressure
how do you write out a Kp constant
- same process of Kc (products over reactants, and use powers to show the moles), but [] are replaced with p()
what is important to remember when writing out the Kp constant
- ONLY includes gases, as only they have partial pressures
- all other species MUST BE IGNORED
- units individually of p() can be kPa, Pa, atm, as long as you stay constant throughout
- the power is determined by balancing numbers
how do you calculate Kp
1) need to find the mole fractions of each species (if moles not given at equilibrium, us ICE table) - via adding all the moles, and dividing each mole by this amount
2) find the partial pressures of each species - via multiplying the mole fraction by the total pressure
3) input the p() values into the Kp constant and determine units
- REMEMBER ONLY GASES
what are the basic 3 principles of Le Chatelier’s principle
1) as concentration increases, equilibrium position shifts to decrease this concentration
2) as pressure increases, equilibrium shifts towards the side with fewer molecules to decrease this pressure
3) as temperature increases, the equilibrium shifts in the endothermic direction, to decrease this temperature
what do the equilibrium constants of K (c/p) indicate
the extent of a chemical equilibrium
- value of 1 showcases it is halfway
- high value 100 shows in favour of products
- low value 0.001 shows in favour of reactants
when will the value of an equilibrium constant K change
ONLY WITH TEMPERATURE
- so if the temperature stays constant, no other modification will cause it to change
in an exothermic reaction, what happens to the equilibrium constant K when you increase temperature
- K decreases with increasing temperature
- and increasing the temperature decreases the equilibrium yield of products (equilibrium shifts to the left)
what are the effects of Kp decreasing when temperature increases in an exothermic reaction
- at equilibrium, there is a set Kp value
- if temperature increases, Kp decreases
- so the system is no longer at equilibrium, as the ratio of p(x)/p(y) is now greater than Kp
- so the equilibrium partial pressures must now change to give new values of Kp
- p(products) must decrease
- and p(reactants) must decrease
- so position of equilibrium shifts to the left
- new equilibrium is reached, where p(x)/p(y) is now equal to Kp again
what happens to K when you increase the temperature of an endothermic reaction
- K increases
- as temperature increases of endothermic reaction
- so position of equilibrium shifts to the right
- increasing the equilibrium yield of products
