Enthalpy and Entropy

An exothermic reaction gives out energy from the system to the surroundings.
1. The temp of surroundings increases
2. ^H is negative
An endothermic reaction takes energy into the system from the surroundings.
1. The temp of surroundings decreases
2. ^H is positive
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
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
Any other reaction is can be given the symbol ^h(theta)r, if the are done under standard conditions
Measuring enthalpy in the lab
1. Measure the temperature rise when a known volume of water is heated by the complete combustion of a measured mass of fuel
2. energy transferred= mass of water x heat capacity of water x temp. change
  1. This works out the enthalpy change for 1 mole of fuel used
Entropy is a measure of the number of ways in which particles can be arranged- it is a measure of disorder
1. Gases> liquids > solids - in order of greatest entropy
  1. Mixtures have a greater entropy than the unmixed constituents
  2. If the number of particles INCREASES during a the course of a reaction then entropy usually increases
Energy Density
1. energy density = enthalpy change of combustion x amount of fuel

Resource summary

	Created by poppyekoronka about 10 years ago