reacting feasibility, enthalpy & entropy

• when a chemical reaction takes place, there's often an exchange of heat energy between the reaction and its surroundings
• the study of the changes of energy between reaction and its surroundings is called thermochemistry
• the change in energy can also be called enthalpy change
• an enthalpy change can be either exothermic or endothermic
• enthalpy changes can be measured directly or indirectly:

  • direct methods include calorimetry, in which temperature changes are measured as a reaction happens
  • indirect methods include Hess's Law and bond enthalpy calculations
• the standard enthalpy of formation is defined as the enthalpy change when one mole of a substance is formed by its elements in their standard states
• the standard state of an element is the most stable state of the substance under standard conditions
• standard conditions are a pressure of one atmosphere and a specific temperature, usually room temperature 298K (kelvin) (25 degrees)
• the standard enthalpy of formation of a substance can be calculated from standard enthalpy changes which are determined experimentally

  delta H = delta Hf (products) - delta Hf (reactants)
• the enthalpy formation of an element in its most stable form is 0
• a reaction that occurs at a particular temperature is called a feasible reaction
• the enthalpy change by itself isn't enough to determine whether a reaction will be feasible, the entropy of a system has to be taken into account
• entropy is the degree of disorder within a system, its given the symbol (S) and is measures in J K-1 mol-1
• the standard entropy of a substance or absolute entropy values are calculated for one mol of a substance based on the scale where the substance has an entropy of 0 at 0K
• the third law of thermodynamics states the entropy of a perfect crystal at 0K is zero
• the higher the absolute entropy values the greater the degree of disorder in a substance
• solids are more ordered than liquids which are more ordered than gases
• theres an increase in entropy from solid to liquid, yet the increase in entropy from liquid to gas is much larger as gas is very disordered
• entropy will increase as temperature increases
• as entropy increases in a solid, the particles gain energy and vibrate more, so increasing their disorder
• as a solid melts and forms a liquid, theres a large increase in disorder at that particular temperature which is observed by a rapid vertical line

  this temperature is the melting point

  as the liquid continues to increase in temperature, so does the disorder until another rapid rise is observed

  this temperature is the boiling point
• entropy is a state function (solid, liquid or gas) and values can vary with temperature and pressure
• in general the higher the molecular weight of substances in the same state, the higher the entropy value
• the entropy change in a reaction can be calculated from the absolute entropy values

  delta S = S (products) - S (reactants)
• from a reaction equation, its possible to predict whether the entropy change is positive or negative

  if a random reaction has decreased or increased in entropy its either positive or negative respectively
• the second law of thermodynamics states that for a reaction to be feasible, the total entropy of a reaction system and its surroundings must be positive
• heat energy released by the reaction system into the surroundings increases the entropy of the surroundings
• heat energy absorbed by the reacting system from the surroundings decreases the entropy of the surroundings
• standard states of a substance is when it's at its most stable form