Warm Up Can you explain why the colder block melts the ice faster? Colder block is actually the same temperature as warmer block … both are at room temperature.

Warm Up

Can you explain why the colder block melts the ice faster?

Colder block is actually the same temperature as warmer block … both are at room temperature.

Block made of metal simply conducts heat faster. Heat flows from your hand faster into it and out of block into ice faster as well.

The Flow of Energy – HeatChapter 8.1

Introduction to Thermochemistry

• Thermochemistry (or Thermodynamics) is the study of heat, energy, and temperature changes that occur during chemical reactions (One of the main courses of study in Physical Chemistry)

• Law of Conservation of Energy (1st Law of Thermodynamics) – The law of conservation of energy states that energy may neither be created nor destroyed. Therefore the sum of all the energies in the system is a constant. (Whether in a reaction flask or the universe)

James Joule (1818-1889)

Joule studied the nature of heat, and discovered its relationship to mechanical work. This led to the Law of conservation of energy, and this led to the development of the First law of thermodynamics. The SI derived unit of energy, the joule, is named for James Joule. He worked with Lord Kelvin to develop the absolute scale of temperature. Joule also made observations of magnetostriction, and he found the relationship between the current through a resistor and the heat dissipated, which is now called Joule's first law.

Temperature vs. Heat

• Temperature is the measure of the average kinetic energy of the particles in a substance– Measure using a thermometer– Rises when heat is added– Falls when heat is removed

• Heat (q) is the energy that is transferred between two objects– Cannot be measured directly– Can only detect the changes caused by heat

Thermochemical Reactions

• System – reactants and products• Surroundings – the reaction container

Exothermic Rxn Endothermic Rxn

Heat/Energy flows out of the system

Heat/Energy flows into the system

Heat/Energy is a product

Heat/Energy is a reactant

q is negative q is positive

Exothermic vs. Endothermic

Exothermic Endothermic

Heat Heat

State Properties

The state of a system is described by giving its …

-composition -temperature-pressure-energy-entropy-volume

These above are some examples of state properties

State properties depend only on the state of a system, not the way the system reached that state.

Measuring Heat

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

• The calorie (cal) is also used to measure heat capacity and is defined by the amount of heat needed to raise the temperature of 1g of pure water by 1C

• 1 J = 0.2390 cal 1 cal = 4.184 J1 kcal = 4.184 kJ

• NOTE: The energy contained in food is measured in Calories (Cal) = 1000 calories = 1 kcal

Hmmmm…..Which of these pictures contains more heat?

Heat Capacity vs Specific Heat

• Heat capacity (C) is the amount of heat needed to raise the temperature of a substance by 1Cq = C x Dt (Dt = tfinal – tinitial) (units J/C or J/K)

• Specific heat (c) is the amount of heat needed to raise the temperature of one gram of a

substance by 1Cq = c x m x Dt (Dt = tfinal – tinitial) (units J/gC or

J/gK)

Examples of specific heat capacities

Susbtance

Specific heat (cal/gc)

Water 1.00

Ice 0.50

Steam 0.40

Paper 0.33

Aluminum

0.21

Sand 0.19

Iron 0.11

Copper 0.09

Gold 0.03

Homework

Read 8.1 and 8.2Problems 1, 3, and 5

Due tomorrow