CHEM 1311 Lecture Notes - Lecture 9: Thermal Energy, Food Energy, Endothermic Process
Chapter 10: Energy Changes in Chemical
Reactions
Focus of this section: Energy changes;
Thermochemistry; Calorimetry
Two reasons to run a chemical reaction:
1. we want the products of the reaction
2. we want the energy from the reaction
Energy and Energy Changes:
= Kinetic - energy of motion Ek = 1/2 mu2
“m” is mass “u” is velocity
-- Thermal energy: associated with random motion
• measure with thermometer
= Potential - energy of position (stored energy)
-- Electrostatic energy: results from interaction of
charged particles
-- Chemical energy: stored within structural units of
chemical substances (e.g. bonds)
Energy Units:
• joule (J) - the SI unit
= amount of energy possessed by a 2 kg mass
moving at a speed of 1 m s–1
1 joule = 1 kg · m2 s–2 =
1 J = 1· N m (when discussing force)
• Calorie (cal) - commonly used
with food labels
= 1 calorie = 4.184 J
= 1000 cal = 1 Calorie
(the food calorie)
= energy required to raise
temperature of 1 gram
of H2O by 1 °C
Energy changes in chemical reactions.
• energy flows from a reaction to the environment, or from
environment into the reaction (in form of heat)
-- System = part of the universe being studied
= often the reacting vessel in the chemistry lab
-- Surroundings = the rest of the universe
-- Boundary = separates system and surroundings
Thermodynamics: study of the flow of energy
• Thermochemistry: The study of heat (transfer of
thermal/kinetic energy) in chemical reactions
-- Exothermic process - transfer of heat from system to
the surroundings (PE decreases for system)
2 H2(g) + O2(g) → 2 H2O(l) + energy
-- Endothermic process - transfer of heat from
surroundings to the system (PE increases for system)
energy + 2 HgO(s) → 2 Hg(l) + O2(g)
Endothermic vs. Exothermic
States and State Functions.
• with thermodynamics we follow changes in the state of a
system
-- state defined by macroscopic measures
(composition, energy, temperature, pressure, volume)
• State function = depends only on initial and final states of
the system and not on how the change was
accomplished (i.e does not depend on path)
= Energy, pressure (P), volume (V) and temperature (T)
• Path function = DOES depend on the path
= work
Document Summary
Two reasons to run a chemical reaction: we want the products of the reaction, we want the energy from the reaction. - thermal energy: associated with random motion: measure with thermometer. = potential - energy of position (stored energy) - electrostatic energy: results from interaction of charged particles. - chemical energy: stored within structural units of chemical substances (e. g. bonds) Energy units: joule (j) - the si unit. = amount of energy possessed by a 2 kg mass moving at a speed of 1 m s 1. 1 joule = 1 kg m2 s 2 = 1 j = 1 n m (when discussing force: calorie (cal) - commonly used with food labels. = 1000 cal = 1 calorie (the food calorie) = energy required to raise temperature of 1 gram of h2o by 1 c. Energy changes in chemical reactions: energy flows from a reaction to the environment, or from environment into the reaction (in form of heat)
