• Created by: larissa67
  • Created on: 24-08-18 01:44

Reversible reactions

  • Definition; A reaction where the products can react to reform the reactants is called a reversible reaction
  • Reversible reactions often do not have the highest yield formed because the products are reacting together to reform the reactancts, and vice versa. This is called an equilibrium reaction.
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  • Equilibrium has 4 characteristics.
    • 1. It is dynamic
    • 2. Forward & reverse reactions occur at the same rate
    • 3. Concentration of reactants and products stay the same in equi
    • 4. It requires a closed system
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Dynamic Equi

  • Definition; dyanmic equi means that molecules of reactants and products are constantly reacting.
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Same rate of fwd & bckwd reactions

  • Rate of fwd reaction = rate of bckwd reaction
  • Equi can be reached by approached from both directions of the equation
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Closed systems

  • Definition; A closed system is where none of the reactants or products can escape from the reaction mixture.
  • If the reaction takes place in a liquid where no gas is lost, equilbrium of this reaction can be studied in an open system.
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Position of equi

  • "Position of equi" = relative to the amounts of products & reactants that are present in an equi mixture.
  • Both temperature and concentration of products affects the position of the equi
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Le Chatelier's Principle

  • Definition; If one or more factors that affect equi are changed, then equi shifts in the direction to oppose/reduce this change.
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Conc. affects on equi

  • When conc of the REACTANTS is increased;
    • Posi of equi moves to the RIGHT away from the reactants
    • More products are produced as a result of this
  • When conc of the PRODUCTS is increased;
    • Posi of equi moves to the LEFT 
    • More reactants are formed as a result of this 
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Pressure affects on equi

  • Only affects gases
  • When pressure in a system is INCREASED
    • Molecules move closer together
    • Equi shifts to minimise this increase
    • Shifts to the direction of fewer gas molecules
  • Special note; if there are EQUAL moles of gas on each side, this change in pressure does not affect the equi in this system
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Temp. affects on equi

  • When temperature of equi is INCREASED;
    • The reaction will go in the direction that opposes the change, so it will go in the direction where energy is ABSORBED - this is the endothermic reaction
  • When temperature of equi is DECREASED;
    • The reaction will go in the direction that opposes the change, so it will go in the direction where energy is RELEASED - this is the exothermic reaction
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Equi constant, Kc

  • Definition; The Kc is the equilibrium constant. The equi constant is a number that expresses the relationship between the amount of products and reactants that are present in a mixture.
  • The way to work out Kc is to divide the products by the reactants
  • Each species is raised to a power (that is the number of moles)
  • Units of Kc are dependant on the form of the equi expression
  • (c)C + d(D) = A(a) + (b)B
  • Eg. Kc = A(a)  B(b)
  •                C(c) D(d)
  • Special note; in equations that have a solid in, you ignore the solid's concentration because it pretty much stays constant.
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Kc & conc. changes

  • If all other conditions in the equi stay constant, then the value of Kc does not change when the conc. of reactants or products is altered
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Kc & pressure changes

  • If all other conditions in an equi stay constant, chaging the pressure does not have an affect on the Kc.
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Kc & temperature changes

  • ENDOTHERMIC REACTION; Increasing the temperature in an endothermic reaction means that the Kc will increase.
  • EXOTHERMIC REACTION; Increasing the temperature in an exothermic reactiona means that the Kc will decrease. 
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Equi constant, Kp

  • Total pressure of gases in a container is due to the molecules of the gas hitting the sides of the container.
  • Definition; pressure exerted by one gas is called a partial pressure. Each partial pressure adds up to the total pressure of gases.
  • Ptotal = Pa + Pb + Pc
  • Pa, Pb and Pc are each partial pressures of gases in the mixture.
  • Partial pressure equilibrium expressions are written in a similar way to concentration expressions are written.
  • Eg. N2 + 3H2 = 2NH3
  • Kp = p2 NH3
  • .       p N2 x p3 H2
  • The products are written on the top and the reactants are written on the bottom.
  • The moles of each species in the equation are also written as powers in the equations, next to each value
  • Units for Kp are dependant on the form of equilibrium expression (they are worked out by deducing the units of each species)
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Equi & ammonia production

  • Ammonia is made by the Haber process
  • N2 + 3H2 = 2NH3
  • Yield of ammonia INCREASES if the PRESSURE also INCREASES because the position of equilibrium shifts to the right hand side because there are less moles (there are 4 molecules of has on the left hand side and 2 molecules of gas on the right hand side)
  • Kp value INCREASES when the TEMPERATURE DECREASES as the reaction goes in the direction in which energy is released, the fwd reaction is exothermic so the reaction favours ammonia production.
  • Removing ammonia as it is formed means that the postition of equilibrium goes to the right to replace the ammonia that has been removed, so MORE ammonia is produced to keep the Kp value constant.
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Equi & sulfuric acid production

  • SA is made by the conact process
  • 2SO2 + O2 = 2SO3
  • When PRESSURE is INCREASED, more sulfuric acid is produced as there are less moles on the right hand side compared to the left hand side, (there are 2 molecules of gas on the right hand side whereas there are 3 molecules of gas on the left hand side)
  • Despite this, the reaction is carried out at just above atp as the value of Kp is v. high, and position of equi is already far over to the right. Very high pressure is uneccessary because it is expensive and not needed
  • If the TEMPERATURE is DECREASED, it shifts the position of equilibrium over to the right because the reaction goes to where energy is released, and this is an exothermic reaction so equi favours the production of SO3, increasing the value of Kp
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Bronsted-Lowry theory

  • Bronsted-Lowry acid; a proton donor
  • Bronsted-Lowry base; a proton acceptor
  • Amphoteric substances; a substance eg. water that can act both as an acid AND a base
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Conjugate acids & bases

  • If a reactant and a product is linked by the transfer of a proton, this is called a conjuagte pair.
  • In a conjugate pair, the acid has one more proton than the base
  • Eg. NH3 + H20 = NH4+ + OH-
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