Thermochemistry and Thermodynamics

Energy

• Ability to do work or produce heat

Energy

Kinetic Energy • Energy in motion

Potential Energy • Due to the composition or

position of an object

Law of Conservation of Energy

• Energy cannot be created or destroyed, only converted from one form to another

Let’s talk terminology.

• System: part of a system that a chemist chooses to study

• Surroundings: the rest of the universe

Ice and water = System

Beaker, air, and everything else= surroundings

A system can interact with its surroundings.

Let’s talk terminology.

• Energy transfers involve transfers of heat and/or work.

• Internal Energy (E): total energy within a system

Thermal energy: energy associated with random molecular motions

Chemical Energy: energy associated with the bonds and intermoleuclar attractions

Let’s talk terminology.

• Since energy and work are so closely related, we use the same unit to measure both.

• Joule (J)

• Calorie (cal): 1 cal = 4.186 J

Heat

• The energy transferred between a system and its surroundings as a result of temperature difference

• Symbol: q

Heat

Endothermic • Heat is absorbed

Exothermic • Heat is released

Heat Capacity

• The amount of heat needed to increase the temperature of n object exactly 1°C

• Depends on mass and chemical composition

Specific Heat

• Amount of heat required to raise the temperature of 1 g of that substance by 1°C

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Specific Heat

Calorimetry Problems

q = cp m ΔTcq = amount of heat/energy transferred (Joules)

m = mass of the substance (grams)ΔTc = temperature change of the substance

Tfinal – Tinitial

cp= specific heat of the substance

• How much heat is needed to raise the temperature of 20.0 grams of liquid water from 5 °C to 20 °C?

• How much heat is released by 200.0 grams of a solid aluminum as it cools from 200.0 °C to 150.0 °C?

• How much heat is absorbed by 5.00 grams of copper when it is heated from 0.0 °C to 200.0 °C?

EXIT QUIZ

• How many joules of heat are released when 50 grams of a solid magnesium is cooled from 70 °C to 60 °C? cp = 1.020 J/(g °C).