Chemistry 1011 Slot 5 Chemistry 1011 TOPIC Thermochemistry TEXT REFERENCE Masterton and Hurley Chapter 8

Chemistry 1011 Slot 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  t. –Calculate one of the variables in the relationship q = m.c.  t or q = C.  t, 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).

Chemistry 1011 Slot 5 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

Chemistry 1011 Slot 5 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

Chemistry 1011 Slot 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

Chemistry 1011 Slot 5 Heat Flow q is + when heat flows into the system from the surroundings q is - when heat flows from the system to the surroundings

Chemistry 1011 Slot 5 Exothermic and Endothermic Processes In a chemical process, the system would be the reaction mixture – reactants and products In an endothermic process (q > 0) heat flows from the surroundings into the system. In an exothermic process (q < 0) heat flows to the surroundings from the system.

Chemistry 1011 Slot 5 Measuring Heat Flow Heat flow, q, is measured in Joules The relationship between q and temperature change,  t, is given by: q = m.c  t (  t = t final – t initial ) or by q = C.  t c is the specific heat of a substance C is the heat capacity of the system C = m.c

Chemistry 1011 Slot 5 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

Chemistry 1011 Slot 5 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.