Enthalpy and Entropy

Enthalpy and Entropy

1. An exothermic reaction gives out energy from the system to the surroundings.

   • The temp of surroundings increases
   • ^H is negative

2. An endothermic reaction takes energy into the system from the surroundings.

   • The temp of surroundings decreases
   • ^H is positive

3. The standard enthalpy of combustion is H(theta) C, is the enthalpy change when 1 mol of substance burns completely in oxygen under standard conditions

4. The standard enthalpy change of formation ^H(theta)f, is the enthalpy change when 1 mol of a substance is formed from its constituent elements. Both the reactants and the products are in their standard state

5. Any other reaction is can be given the symbol ^h(theta)r, if the are done under standard conditions

6. Measuring enthalpy in the lab

   • Measure the temperature rise when a known volume of water is heated by the complete combustion of a measured mass of fuel
   • energy transferred= mass of water x heat capacity of water x temp. change
     • This works out the enthalpy change for 1 mole of fuel used

7. Entropy is a measure of the number of ways in which particles can be arranged- it is a measure of disorder

   • Gases> liquids > solids - in order of greatest entropy
     • Mixtures have a greater entropy than the unmixed constituents
     • If the number of particles INCREASES during a the course of a reaction then entropy usually increases

8. Energy Density

   • energy density = enthalpy change of combustion x amount of fuel