Chapter 7 - Reversible Reactions and Chemical Equilibrium
Chapter 7.1: Recognizing Equilibrium
- Equilibrium: products and reactants are occurring simultaneously at the same rate. water in a closed jar has the number of evaporating molecules and number of condensing molecules equal
- Chemical processes that reach equilibrium:
- Homogeneous equilibrium: the equilibrium reactions in which the reactants and products are in the same phase
- Heterogeneous equilibrium: the equilibrium of reactions in which the reactants and products are in different phases
Chapter 7.2: Thermodynamics and Equilibrium
- Favorable change: change that has a natural tendency to happen under certain conditions
- Enthalpy: Exothermic: when products have less enthalpy than reactants, it releases energy
- Endothermic: when reaction absorbs energy
- Temperature: changes with the direction
- Entropy S: a tendency towards randomness or disorder in a system
- The second law of thermodynamics: total entropy of the universe is constantly increasing must add together changes in the entropy of the system, ∆Ssys, and changes in the entropy of the surroundings, ∆Ssur
- Free energy: available energy; useful work obtained from reaction
- Also called gibbs free energy G
- ∆G = ∆H − T∆S
- Change in free energy
- Change in enthalpy
- Change in entropy kelvin temperature
- ∆G
- Negative: forward reaction is favorable
- Zero: reaction at equilibrium
- Positive: reaction favorable in reverse direction only
Chapter 7.3 The equilibrium constant
- Law of chemical equilibrium: At equilibrium, there is a constant ratio between the concentrations of the products and reactants in any change
- equilibrium constant keq or kc: forward rate constant divided by the reverse rate constant
- kf / kr = keq
- kc: uses concentrated values when concentration is at equilibrium
- ICE table: table is used to record the initial, change, and equilibrium values of the reacting species,
- K > 1, products are favoured. The equilibrium lies far to the right. Reactions where K is greater than 1010 are usually regarded as going to completion
- K ≈ 1, there are approximately equal concentrations of reactants and products at equilibrium
- K < 1, reactants are favoured. The equilibrium lies far to the left. Reactions in which K is smaller than 10−10 are usually regarded as not taking place at all
7.4 Predicting the Direction of a Reaction
- Reaction quotient, Qc: expression that is identical to equilibrium constant expression, but concentration not necessarily at equilibrium
- Qc = ( [R]c[S]d ) / ( [P]a[Q]b )
- If Qc equal to kc then it is Qc close to equilibrium
- Le Châtelier’s principle: a dynamic equilibrium tends to respond so as to relieve the effect of any change in the conditions that affect the equilibrium
- Common ion effect: common ion involves adding ion to a solution in which theion is already present, the equilibrium shifts away from the added ion
- applies Le Châtelier’s principle to ions in an aqueous solution
- Endothermic change (∆H > 0):
- An increase in temperature shifts the equilibrium to the right, forming more products.
- A decrease in temperature shifts the equilibrium to the left, forming more reactants.
- Exothermic change (∆H < 0):
- An increase in temperature shifts the equilibrium to the left, forming more reactants.
- A decrease in temperature shifts the equilibrium to the right, forming more products.
- Reducing the volume of an equilibrium mixture of gas (constant temperature), causes sift a shift in equilibrium in direction with few gas molecules
- Catalyst does not affect the position of equilibrium, only affects the time
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