Chemistry 1011

1
Chemistry 1011

• TOPIC
• Thermochemistry
• TEXT REFERENCE
• Masterton and Hurley Chapter 8

2
8.1 Principles of Heat Flow

• YOU ARE EXPECTED TO BE ABLE TO
• Define and give examples of state properties.
• Identify endothermic and exothermic processes and
  define each in terms of heat flow.
• Relate heat flow to specific heat, m and Dt.
• Calculate one of the variables in the
  relationship q m.c.Dt or q C.Dt,
  given the remaining variables.
• Identify phase changes, define latent heat of
  fusion and latent heat of vaporization and
  calculate one of the variables in the
  relationship q m.latent heat, given the
  remaining variables (Laboratory 1).

3
Some Definitions

• Energy
• The capacity to do work
• Heat
• A form of energy
• Thermochemistry
• The study of the heat flow that accompanies
  chemical reactions
• A System and its Surroundings

4
State Properties

• The State of a system
• Its composition, temperature, volume, pressure
• State properties
• Depend only on the state of the system, not on
  how it got there
• Eg pressure and volume

5
Heat Flow

• Heat flows from a hotter to a colder
• Put a pot of water on the stove
  The water in the pot is the system
• Heat the pot heat flows from the surroundings
  to the system
• The heat flow, q, for the system is positive

6
Heat Flow

• q is when heat flows into the system from the
  surroundings
• q is - when heat flows from the system to the
  surroundings

7
Exothermic and Endothermic Processes

• In a chemical process, the system would be the
  reaction mixture reactants and products
• In an endothermic process (q gt 0) heat flows from
  the surroundings into the system.
• In an exothermic process (q lt 0) heat flows to
  the surroundings from the system.

8
Measuring Heat Flow

• Heat flow, q, is measured in Joules
• The relationship between q and temperature
  change, Dt, is given by
• q m.c Dt (Dt tfinal tinitial) or by
• q C. Dt
• c is the specific heat of a substance
• C is the heat capacity of the system
• C m.c

9
Phase Changes

• Most substances can exist in the solid, liquid or
  gas phase
• The phase depends on the temperature and pressure
  conditions
• Change of phase takes place at the
• freezing/melting point - fusion (Melting)
• or boiling point - vaporization (Condensation)
• Sublimation is the change of phase from solid to
  gas without passing through the liquid phase

10
Latent Heat

• During a phase change, heat is either absorbed or
  released, but the temperature does not change
• The latent heat of fusion (vaporization) of a
  substance is the heat absorbed or released when 1
  gram of the substance changes phase.