THERMOCHEMISTRYTHERMOCHEMISTRYTHERMOCHEMISTRYTHERMOCHEMISTRY

Chapter 11Chapter 11

HEAT CAPACITY AND SPECIFIC HEAT

• The amount of energy needed to to increase the temperature of an object exactly 10C is the heat capacity of that object. (J/0C)

• The specific heat capacity or simply specific heat of a substance is the amount of heat/ energy needed to raise the temperature of 1g of the substance 10C. (J/g0C)

Calculating Heat Capacity

• The heat capacity of an object is equal to its specific heat times its mass.

• EX. The heat capacity of a 5 gram piece of pure aluminum is equal to:5g X 0.90J/g0C = 4.50 J/0C

Calculating Specific Heat

• C = __q___ = m X ΔT

= heat(joules or calories)mass(g) X change in temperature (0C)

Sample Problem

• How much heat is required to raise the temperature of 250 g of mercury 52 0C?

The specific heat for mercury is:

0.14 J/g0C

SOLVE for q(heat)

• C = __q___ m X ΔT

• Rearrange the specific heat formula to solve for q.

• q = C x m x ΔT

NOW PLUG AND CHUG…

ANSWER

• q = C x m x ΔT• q = 0.14 J x 250g x 520C = g 0C= 1,800 J or 1.8kJ

CALORIMETRY• Calorimetry is the accurate and

precise measurement of heat change for chemical and physical processes.

• In calorimetry, the heat released by the system is equal to to the heat absorbed by its surroundings.

• What law describes this relationship?

Calorimeter

• To measure heat changes accurately and precisely, the process must be carried out in an insulated container.

• The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter.

Heat of Reaction

• A heat of reaction is the heat change for the equation exactly as it is written.

• For systems at constant pressure, the heat content is the same as a property called the Enthalpy (H) of the system. (q = ΔH)

Thermochemical Equations

• You can treat heat change in a chemical reaction like any other reactant product in a chemical equation.

• A chemical equation that includes the heat change is referred to as a thermochemical equation.

ΔH is Positive

ΔH is Negative

Types of Thermochemical Equations

Endothermic (ΔH is positive):Na2CO2 + H2O + CO2 + 129kJ NaHCO3

Exothermic (ΔH is negative):CaO + H2O Ca(OH)2 + 65.2KJ