Week 8

• One of the major role of a chemist is to be

  able to determine whether or not a reaction will

  occur given the conditions like temperature and

  concentration.
• On the other hand, the reaction is said to

  be nonspontaneous if a reaction does not

  occur under specified conditions.
• Factors that determine whether a reaction will proceed

  • Enthalpy
  • Entropy
  • Temperature
• In order to determine the spontaneity of a certain

  process, we should know two things about the system.

  One is change in enthalpy and the other one is change

  in entropy.
• Enthalpy is also known as heat energy
• Entropy (represented by S) is described as the measure of the

  randomness or disorder of a system.
• The greater the disorder of a

  system, the greater its entropy.
• A negative entropy is the one with greater order as in condensation

  of water vapor (gas to liquid) or in the case of formation of a solid

  precipitate in a chemical reaction where the reactants are in liquid

  form.
• Gibbs free energy, denoted G, combines enthalpy and entropy into a single value.
• If ΔG is positive, then the reaction is nonspontaneous
• If ΔG is zero, then the reaction is at equilibrium
• A positive entropy means increasing disorder just like during evaporation (liquid to gas) or melting (solid to liquid).
• A reaction is at equilibrium state if the rate of the forward reaction equals the rate of the backward reaction.
• Dynamic Equilibrium - There is no net change in concentrations of reactants and products.
• If the value of Qc is less than that of the EQUILIBRIUM CONSTANT (Kc), then the reaction proceeds on a forward direction.
• All reactions tend towards a state of chemical equilibrium, the point at which both the forward process and the reverse process are taking place at the same rate.
• It is important to remember that even though the concentrations are constant at equilibrium, the reaction is still happening.
• Homogeneous Equilibria - applies to reactions in which all reacting species are in the same phase.
• Heterogeneous Equilibria - Results from a reversible reaction involving reactants and products that are in different phases.
• Variables that can be controlled experimentally

  • Concentration
  • Pressure
  • Volume
  • Temperature
• Le Chatelier’s Principle - A rule which helps to predict the direction in which the equilibrium reaction will go through when a change in concentration, pressure, volume, or temperature occurs.
• Le Chatelier’s Principle states that if an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position.
• The word “stress” means a change in concentration, pressure, volume, or temperature that removes the system from the equilibrium state.
• FACTORS THAT AFFECT CHEMICAL EQUILIBRIUM

  • Changes in Concentration
  • Changes in Volume and Pressure
  • Changes in Temperature
  • The Effect of a Catalyst
• Adding additional reactant to a system will shift the equilibrium to the right, towards the side of the products.
• If we add additional product to a system, the equilibrium will shift to the left, in order to produce more reactants.
• Changes in pressure do not affect the concentrations of reacting species in condensed phases (in aqueous solution) because solids and liquids are virtually incompressible.
• The greater the pressure the smaller the volume
• To decrease the pressure, you must shift to the side of the equation that has fewer moles of gas.
• The direction of the net reaction will go to the lesser moles of gas.
• The change in temperature changes the value of the equilibrium constant.
• To understand the effect of temperature, we must know whether the reaction is endothermic (absorption of heat) or exothermic( release of heat).
• The value of the equilibrium constant increases when the heat is added (increase in temperature) and decreases when the heat is removed (cooling the system)
• Increase in temperature favors the endothermic reaction (forward reaction, i.e. left to right)
• The function of a catalyst is to speed up the reaction by lowering the activation energy.
• The catalyst has no effect on the equilibrium system.
• The driving force for a spontaneous process is an increase in the entropy of the universe.
• The degree of disorderliness can be determined by (a) number of arrangement of molecules can have in a system (b) the number of types of molecules as well as the number of molecules.
• If you change the temperature of a reaction, then Kc also changes.
• The reaction must be balanced with the coefficients written as the lowest possible integer values in order to get the correct value for Kc.