3.7 - Equilibria Flashcards
Features of a dynamic equilibrium
- forwards and backwards reaction happen at same rate
- concs. of reactants + products do not change
- closed system
Enthalpy change of forward and backward reaction
same but one is exo and one ends (so one is + and one -)
Effect of increasing temperature on equilibrium
shifts equilibrium position in endothermic direction
(always say whether forward reaction is exo/endo)
Effect of increasing pressure on equilibrium
shifts equilibrium position to side with fewest molecules of gas
Effect of catalyst on equilibrium
increases rate of both forward and reverse reactions by same amount so equilibrium position unchanged
Importance of compromise between chemical equilibrium and reaction rate
- high pressures expensive to generate
- high temps increase energy costs
- high pressures have safety risk
- rate may be too slow with too low temps
Define homogeneous equilibria
contains equilibrium species all in the same state/phase
Define heterogeneous equilibria
contains equilibrium species that have different states/phases
Which physical states of chemicals appear in Kc and which don’t?
yes: gas or aq
no: liquids and solids
why?: conc. of solids/liquids essentially constant
Define mole fraction
the proportion by volume of a gas to the total volume of the gases in a mixture
(sum of mol fractions in mixture is always 1)
Define partial pressure
contribution a gas makes towards total pressure
- sum of PPs = total pressure
- units: kPa, Pa, atm
Only factor that affects Kc
temperature
What is Kp for?
gases only
Equation for partial pressure
mol fraction x total pressure
Effect of Kc on equilibrium position
Kc>1
- shifts right
- higher product yield
Kc<1
- shifts left
How Kc is affected by temperature
forward reaction endo: Kc increases as temperature increases
forward reaction exo: Kc decreases as temperature increases
How does e.g increasing pressure affect Kp and equilibrium position
(assume more molecules of gas on left)
- denominator increases more than numerator
- to restore Kp, denominator must decrease and numerator must increase
- increasing pressure causes equilibrium to shift right towards side with fewest gas molecules
Which constant has curved brackets?
Kp
State Le Chatelier’s principle
position of equilibrium shifts to minimise effect of any change
How to find delta H from graph
(-) gradient x R
How to find delta S from graph of ln Kp against 1/T
- extrapolate line to y-intercept
- delta S = intercept x R
When can mol (not concs) be used to calculate Kc?
same no. of mol on both sides of eq/volume cancels
