Unit 6: Thermochemistry

Coffee Cup Calorimeter

An insulated system where a reaction occurs in a solution with constant volume and pressure.

• Work = 0

• ΔH = ΔE = Q

Piston Calorimeter

Used for gas forming reactions with variable volume but constant pressure.

• ΔH = ΔE + PΔV= Q-PΔV + PΔV = Q

Bomb Calorimeter

Designed for gas forming reactions with constant volume but varying pressure.

• ΔH = ΔE + VΔP

Heat of Formation

The change in enthalpy when a compound is formed from its elements under standard conditions.

ΔH

Change in enthalpy, representing heat absorbed or released in a reaction.

ΔE = Q + (-PΔV)

Equation relating internal energy change to heat and pressure-volume work.

-Qrxn = Qcal + QH2O

Total heat of reaction equals the heat absorbed by the calorimeter plus the heat absorbed by water.

Endothermic Process

A process that absorbs heat from its surroundings, resulting in a positive ΔH.

Exothermic Process

A process that releases heat to its surroundings, resulting in a negative ΔH.

Specific heat of water

4.184 J/gºC, the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.

Heat of Vaporization

The amount of energy required to convert a substance from a liquid to a gas (2260 J/mol)

Heat of Fusion

The amount of energy required to melt a solid into a liquid (334 J/g)

Hess’s Law

The principle that states the total enthalpy change during a reaction is the sum of the enthalpy changes for each step of the reaction.

Bond dissociation energy (BDE)

The energy required to break a bond in a molecule (Bonds broken - bonds formed)

Activation Energy

The minimum energy required for a chemical reaction to occur. Affected by catalyst.

Calorimetry

The measurement of heat changes in chemical reactions.

High Specific Heat

Substances that require more heat to change temperature, indicating lower thermal conductivity.

Variable Volume but Constant Pressure

Condtition in which a system can change volume while keeping pressure unchanged.

Specific Heat of Ice

2.09 J/g C

Specific Heat of liquid water

4.184 J/g C

Specific Heat of water vapor

2.02 J/g C

Relationship between Hfus and Hvap

Hvap>Hfus

Relationship between Hf(l), H(g), and Hvap

Hf(l) + Hvap = H(g)