Introduction to Thermochemistry

Chemistry JokeA Bear-y Good Pun!

The vet at the zoo had a problem: beryllium.He had only two options, to curium or to barium. He lead the dentist to boron the bear’s tooth.I’d give a nickel to have seen what happened when the bear woke up. Now the vet and the dentist argon. It’s ironic what can happen when you have a beryllium and try to curium.

Law of Conservation of Energy

• In any chemical or physical process, energy is neither created nor destroyed.

• All the energy in a process (or reaction) can be accounted for as work, stored energy, or heat.

Heat

• The energy that is transferred from one body to another because of temperature differences

• Flows from warmer → cooler object–Only changes caused by heat can be

detected – like changes in temperature

Units of Heat

• Joule (J) is the SI unit of heat and energy.

• calorie (cal) is another commonly used unit of energy defined as the quantity of heat needed to raise the temperature of 1g of water 1oC.

• 4.184 J = 1 cal

Enthalpy• Heat can be represented by either q or H.• For systems at constant pressure, the heat

content, q, is the same as a property of the system called enthalpy, H.

• Since most reactions are carried out at constant pressure, q (heat) and H (enthalpy) are often used interchangeably.

• Changes in enthalpy = H• q = H at constant pressure

Calorimetry

- The measurement of the heat into or out of a system for either chemical or physical processes is called calorimetry.

• Calorimeter—the device used to measure this absorption or release of heat

Foam cups are excellent heat insulators and are commonly used as simple calorimeters.

• In any chemical reaction or change of physical state, heat is either released or absorbed.

• In studying which direction heat flows, we use these two definitions:– the system - the part of the universe on

which you focus your attention– the surroundings - includes everything

else in the universe

System vs. SurroundingsWhich way does the heat flow?

• Endothermic Process: Heat Absorbed– Heat going into the system from the surroundings.– Surroundings cool down.– Example: Chemical Ice Pack – q or ∆H is positive

• Exothermic Process: Heat Released– Heat leaving the system and going into the surroundings.– Surroundings heat up.– Example: Combustion– q or ∆H is negative

Exothermic or Endothermic

Thermochemical Equations

• A thermochemical equation shows the heat change.

• The heat is usually given in kJ and can be included in the reaction itself or directly after the equation.

• The heat change for a reaction is called Hrxn.

Exothermic Reactions

• Energy is written as a product, or• H is negative

CaO (s) + H2O(l) Ca(OH)2 + 65.2 kJ

or

CaO (s) + H2O(l) Ca(OH)2 H = -65.2 kJ

Endothermic Reactions

• Energy is written as a reactant• H is positive

2NaHCO3 + 129 kJ Na2CO3 + H2O +CO2

or

2NaHCO3 Na2CO3 + H2O(g) +CO2 H = 129 kJ

Exothermic or Endothermic?

C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l) + 1235 kJ

PCl5(s) PCl3(g) + Cl2 H = 87.9 kJ

H2(g) + F2(g) 2HF(g) H = -536 J

CO2(g) + 393.5 kJ C(s, graphite) + O2 (g)

Exo

Exo

Endo

Endo

H is positiveEnergy is a Reactant

H is negativeEnergy is a Product

Potential Energy Diagrams

Phase Changes• Physical processes can also have energy

changes.• Which phase changes are exothermic and

which are endothermic?

• Gas → Liquid? ____________• Liquid → Solid? ____________• Solid → Liquid? ____________• Liquid → Gas? _____________

Exo

Exo

Endo

Endo

Phase Changes

• There are special names for the heat required to melt or vaporize a substance.

• The molar heat of fusion is the amount of heat required to melt one mole of a substance: Hfus.

• The molar heat of vaporization is the amount of heat required to vaporize one mole of a substance: Hvap.

Heating Curve• A heating curve

is a graph that shows phase changes in a plot of temperature vs. time.

• The slanted lines show heating and an increase in kinetic energy.

• The flat lines show phase changes and an increase in potential energy. (Hfus, Hvap)

Heating CurveHvap

Hfus

a → b solid warmingb → c solid becoming a liquidc → d liquid warmingd → e liquid becoming a gase → f gas warming

Chemistry Joke

Q: Is silicon spelled the same way in Spanish?

A: Si!