Reactivity 2.3
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What is physical equilibrium?
equilibrium that exists during a physical change, rate of forward process = rate of reverse
What does dynamic mean?
both forward and reverse processes still occur, their rates are just equal
What is a condition necessary to establish equilibrium?
closed system - no matter can escape
Chemical vs physical equilibria:
same characteristics, but one is with a chemical rather than physical change
Characteristics of equilibria:
• closed system • rate of forward = rate of reverse • concentrations remain constant (not equal)
How to graphically identify when equilibrium position has been reached?
concentration of reactants and products constant
What does the Kc (equilibrium constant) tell us?
ratio that provides information about composition of a reaction mixture at equilibrium extent of reaction
Kc=
concentrations of products/concentrations of reactants all raised to power of coefficients
What does it mean if an equilibrium position lies to the right/left?
lies to right - higher conc. of products lies to left - higher conc. of reactants
What does a small/great Kc mean?
small - higher conc of reactants - equilibrium lies to left large - higher conc of reactants - equilibrium lies to right
scale of Kc
Kc«1 - almost only reactants Kc<1 - high conc. reactants Kc=1 - appreciable amounts of both Kc>1 - high conc. products Kc»1 - almost all
What is Le Chateliers principle? What do we use it for?
when an equilibrium system is subjected to change, the system will respond to minimise the effect of change helps us to predict effect of equilibrium position when things change
What are the types of changes that can be made to a system at equilibrium?
• changing concentration by removing products/reactants • changing pressure by changing volume • temperature
What happens to an equilibrium when we change concentrations?
system counteracts by using up additional reactant/product removing product - equilibrium shifts to right to increase yield of products removing reactant - shifts to left, increase reactant yield
Effect of changing concentration on Kc
None - will eventually even out
Effect of changing pressure on equilibria
increase - favours side with less moles, since gas particles collide more frequently decrease - favours side with more moles, particles with lower frequency of collision
What affects the Kc?
only temperature
Does pressure affect Kc?
no
What happens when we change temperature in equilibria?
heat added - favours endothermic reaction heat removed - favours exothermic reaction
Effect of temperature on Kc
position shifted to left - Kc decreases position shifted to right - Kc increases
What happens when we add a catalyst to an equilibrium?
equilibrium reached sooner, but no effect on position of equilibrium or Kc rate of forward and reverse increased by the same
What is the reaction quotient and how can we use it?
Q = ratio between concentration of products and reactants NOT AT EQUILIBRIUM comparing Q to Kc, we can tell in which direction the reaction will proceed to reach equilibrium
Q =
conc. reactants/conc. products raised to power of coefficients
Q > Kc
conc of products is greater than at equilibrium reverse reaction favoured reaction proceeds to left