Chapter 11-2: How does temperature affect enthalpy and entropy?

1. Chapter 11-2: How does temperature affect enthalpy and entropy?

2. Thermodynamics Properties • Thermodynamics deals with energy transformation of several different kinds • The three properties that affect the relationship between energy and matter are: • Entropy • Enthalpy • Gibbs Energy

3. Enthalpy (H): • Enthalpy can be thought of as heat or thermal energy = as something is heated the increase in enthalpy is equal to the thermal energy it receives • A change in enthalpy can be measured when a substance is heated and cooled, however enthalpy itself cannot be measured. So scientists have decided to assign all elements under standard conditions a molar enthalpy of zero

4. Properties • Extensive Properties – a property that depends on the amount of material present • Mass • Volume • Heat capacity • Enthalpy • Entropy • Intensive Properties – a property that does not depend on the amount of material present • Density • Pressure • Temperature • Concentration

5. Temperature Affects Molar Enthalpy • Enthalpy increase is equal to the heat input • So molar enthalpy can be found by dividing the enthalpy increase by the number of moles • Temperature also affects entropy because entropy can be directly measured

6. How much does the molar enthalpy change when a 92.3g block of ice is cooled from -0.2⁰C to -5.4⁰C?

7. Things to Notice about that Problem • Mass was given, and not needed • Scientists often find themselves with more information than needed • ΔT = Tf – Ti --- Gives the same result whether or not you change Celsius to Kelvin • In thermodynamics problems it doesn’t always work out this way so getting into the habit of changing the temperature to kelvin

8. Changes of State

9. Homework/Rest of Class Work on problems 9-13 on page 395