Chapter 16: Thermochemistry

• Chemical reactions involve changes in energy – Breaking old bonds of reactants and forming new

bonds in products.

• The study of the changes in energy in chemical reactions is called thermochemistry

• These energy changes are usually in the form of heat.

INTRODUCTION TO THERMOCHEMISTRY

Heat and Its UnitsHeat can be thought of as a flow of energy

due to a difference in temperatures.

Heat always flows from a region of higher

temperature to region of lower temperature.– This is the 2nd law of Thermodynamics

The joule (J) is the SI unit of energy (heat).

A joule is a very small unit of heat; therefore, kilojoules (kJ) are commonly used.

1 kJ = 1000 J

Another common unit is the calorie (cal)

1 cal = 4.184 J

Specific Heat• All substances change temperature when they are

heated, but how much they change varies significantly from one substance to another.

• The specific heat of a substance is the amount of energy required to raise the temperature of one gram by one Celsius degree (1°C).

• Each substance has its own unique specific heat that can be used to identify it.

• Specific heat varies depending on the state of the material.

Substance Specific Heat J/(g°C)

Substance Specific Heat J/(g°C)

Water (liquid) 4.18 Iron 0.449

Ethanol 2.42 Copper 0.385

Ice 2.01 Silver 0.235

Aluminum 0.903 Lead 0.129

• The specific heat of a substance can be used to determine the amount of heat needed to cause a specific temperature change.

• The equation that relates these quantities is:

q = m x Cp x T

• q is the heat lost or gained, m is the mass of the sample, cp is the specific heat (of the element or compound), and ∆T is the difference between temperatures.

Sample ProblemsProblem #1:

Gallium is a solid metal at room temperature but melts at 29.9°C. If you hold gallium in your hand, it melts from your body heat. How much heat must 2.50 g of gallium absorb from your hand to raise its temperature from 25.0°C to 29.9°C? The specific heat of gallium is 0.372 J/g°C.

Problem #2:

A chemistry student finds a shiny rock that she suspects is gold. She determines the mass of the rock to be 14.32 g. She then finds that the temperature of the rock rises from 25°C to 52°C upon absorbing 174 J of heat. Determine the specific heat of the rock. If the specific heat of gold is 0.128 J/g°C, is the rock composed of gold?

One Minute Paper• You have one minute to answer these two

questions concerning today’s lesson.

– What was the most important thing you learned?

– What is still muddy?