Equilbrium Flashcards
(25 cards)
IRREVRSIBLE Vs REVERSIBLE reactions
IRREVERSIBLE:
- Goes in one direction
- Goes to completion
- Shown by arrow pointing from reactants
to products
- The products do not convert back into the original reactants
- E.G decomposition of hydrogen peroxide
REVERSIBLE:
- The reactants are converted to the products at the same time as the products are converted to reactants
- The reversible sign shows the reaction is in EQUILBRIUM
DNAMIC EQUILBRIUM ~ Definition
DYNAMIC EQULIBRIUM:
- the rate of the forward reaction is equal to the rate of the backward reaction
- the concentrations of the reactants and products remain constant
DYNAMIC EQUILBRIUM ~ Explained
Occurs when a reversible reaction takes place in a CLOSED SYSTEM:
- neither the products or reactants can enter or leave
- the temperature, pressure or concentration of the reactants and products remains unchanged
- The reactants are forming the products as FAST AS the products are forming the reactants
The POSITION of equilibrium
- Indicates the EXTENT of the reaction
- The PROPORTION of products to reactants in the equilibrium mixture
Le Chatelier’s Principle
According to le Chatelier’s principle:
’ When a system in equilibrium is subjected to an external change , the system readjusts itself to MINIMISE the effect of tat change’
The position of equilibrium shifts to CANCEL OUT any changes in:
- Temperature
- Pressure
- Concentration
Gas Pressure
Pressure = Force/Area
- The greater number of MOLES of a reactant or product in an equilibrium mixture…
- the greater the gas pressure it exerts
TEMPERATURE ~ effect on equilibrium reaction
INCREASE TEMPERATURE:
- Equilibrium moves in the ENDOTHERMIC
direction
- REDUCES the temperature of the system
- MINIMISES the change
DECREASE TEMPERATURE:
- Equilibrium moves in the EXOTHERMIC direction
- INCREASES the temperature of the system
- MINIMISES the change
CONCENTRATION ~ effect on equilibrium reaction
INCREASE CONCENTRATION OF REACTANTS:
- Equilibrium moves to the RIGHT
- REDUCES concentration of reactants
- MINIMISES the change
DECREASE CONCENTRATION OF REACTANTS:
- Equilibrium moves to the LEFT
- INCREASES concentration of reactants
- MINIMISES the change
INCREASE CONCENTRATION OF PRODUCTS:
- Equilibrium moves to the LEFT
- REDUCES concentration of products
- MINIMISES the change
DECREASE CONCENTRATION OF PRODUCTS:
- Equilibrium moves to the RIGHT
- INCREASES concentration of products
- MINIMISES the change
PRESSURE ~ effect on equilibrium reaction
INCREASE PRESSURE:
- Equilibrium moves to the side with FEWER
MOLES
- DECREASES the pressure of the system
- MINIMISES the change
DECREASE PRESSURE:
- Equilibrium moves to the side with MORE
MOLES
- INCREASES the pressure of the system
- MINIMISES the change
Only effects reactions that have BOTH:
- Species in GASEOUS STATE
- Different number of MOLES on either side
of the reaction
CATALYST ~ effect on equilibrium reaction
- Provides an alternative pathway with a lower activation energy
- INCREASE THE RATE of BOTH the forward and backward reaction TO THE SAME EXTENT
- The equilibrium is ATTAINED FASTER
- NO EFFECT ON POSITION OF EQUILBRIUM~ remains the same
The HABER process & conditions we would EXPECT
N2(g) + 3H2(g) (reversible sign) 2NH3(g)
Enthalpy change = -92 KJmol-1
Reactants obtained from:
- Nitrogen ~ the fractional distillation of AIR
- Hydrogen ~ by reacting METHANE
and WATER
LOW TEMPERATURE:
- Reaction is exothermic
- Position of equilibrium to the right
- Favours production of ammonia
HIGH PRESSURE:
- 4 moles of reactants ——- 2 moles of reactants
- Position of equilibrium shifts to right as there are fewer moles of product
- Favours the production of ammonia
What are the COMPROMISE temperature and pressure conditions for the Haber Process?
350-500C:
- The forward reaction is EXOTHERMIC
- At a LOW temperature the equilibrium shifts towards the PRODUCTS
- Favours the production of ammonia
- HOWEVER temperature must be INCREASED to increase the RATE of reaction
100-200atm:
- A HIGH pressure shifts the position of equilibrium towards the PRODUCTS
- Favours the production of ammonia
- INCREASE the rate of reaction
- HOWEVER pressure must be LOWERED to:
- reduce cost
- Prevent danger for work force and
surrounding areas
The CATALYST used in the Haber Process and why?
IRON:
- Used in the form of a SOLID GAUZE to maximise SURFACE AREA
- INCREASES the rate of reaction by providing an alternative route with a lower activation energy
- As a result, LOWER temperature can be used and costs are reduced
- The LIFE and ACTIVITY of the catalyst is INCREASED when the operating temperature is LOWER.
What is the COMPRISED YIELD of ammonia from the Haber Process and WHY?
15%:
- The removal of ammonia caused the position of equilibrium to shift to the RIGHT
- This INCREASES the yield of ammonia produced
- It takes MORE TIME for a higher conversion of ammonia to take place at the industrial conditions when the product is removed in a LARGER amount
The DRAWBACKS of using HIGH PRESSURES in industrial processes
VERY COSTLY TO:
-Compress gases to high pressures
- Produces thick pipes and reaction vessel to withstand high pressures
UNSAFE:
- Risk of leakage , e.g toxic ammonia
- Endangers the workforce & surrounding
area
Any industrial process aims to achieve the:
- Highest possible yield
- In the shortest time (Fastest Rate)
- At the lowest cost
This requires a COMPROMISE between temperature and pressure conditions and the use of a catalyst.
How do we get VIRTUALLY 100% yield of ammonia from the Haber Process?
- The unreacted gases are RECYCLED
- Until eventually they are all converted to ammonia
The Equilibrium Law
- Gives the CINCENTRATION of reactants and products when a reversible reaction reaches EQUILIBRIUM
The Equilibrium Constant (Kc)
- Allows the CONCENTRATIONS of the reactants and products at equilibrium to be CALCULATED
aA + bB (reversible sign) cC + dD
Kc = [products] / [reactants]
= [C]^c[D]^d / [A]^a[B]^b
- [] are the equilibrium concentrations in mol dm^-3
UNITS for Equilibrium Constant
- Write out units in equation
Examples:
mol dm^-3/ mol dm^-3 x mol dm^-3
- This would cancel out to give dm^-3 mol^-1
- Units with POSITIVE powers placed first
- If there are no units , ‘NO UNITS’ must be written
Homogenous Equilibria
- ALL equilibrium species are in the SAME STATE or PHASE
- The Kc expression will include the CONCENTRATIONS of ALL species in the equilibrium
Heterogeneous Equilibria
- The equilibrium species are in DIFFERENT STATES or PHASES
- The Kc expression DOES NOT include the CONCENTRATIONS of SOLIDS or LIQUIDS.
- This is because their concentration remain EFFECTIVLY CONSTANT
- Only the concentrations of GASEOUS and AQUEOUS species are included
Significance of a LARGE Kc value
- > 100
Kc fraction:
- Top is LARGE
- Bottom is SMALL
Concentration of the products > concentration of reactants
POSITION OF EQUILIBRIUM:
RHS for the concentration of PRODUCTS to be HIGH
Significance of a SMALL Kc value
- <0.01
Kc Fraction:
- Top is SMALL
- Bottom is LARGE
Concentration of reactants > concentration of products
POSITION OF EQILIBRIUM:
LHS for the concentration of the reactants to be HIGH
