ThermochemistryChapter Seventeen

Page 504

Assess prior knowledge…

What does a thermometer measure?

Can a thermometer measure heat?

Energy TransformationsEnergy is the capacity for doing work or

supplying heat.Energy is not matter – it is detected because

of its effectsThermochemistry is the study of energy

changes that occur during chemical changes or changes in state.

All matter has a certain amount of energy stored within it.This is referred to as chemical potential

energy.It is based on the substance's atomic structure.

Energy Transformations (cont'd)Can you think of substances we purchase

that house significant chemical potential energy?

Energy changes occur as work, heat or a combination of both.

Heat (q), is energy that transfers from one object to the next because of temperature differences between the two.Adding heat (energy) causes a rise in the

temperature.

HeatHEAT ALWAYS FLOWS FROM A WARMER

OBJECT TO A COOLER OBJECT.

THE BOLD BLACK LETTERS MEANS THE STATEMENT IS REALLY, REALLY IMPORTANT.

Does the heat transfer continue indefinitely?

Heat TransferWhen it comes to heat transfer, the terms

system and surroundings are often usedThe system is the object or part of the

universe you focus your attention onThe surroundings is everything else (when it

comes to heat transfer, it is typically the 'immediate surroundings').

Thermochemistry is concerned with the heat flow between the system and its surroundings. If we rely on the law of conservation of energy,

when the system loses energy, the surroundings must gain energy and vice versa.

Endothermic ProcessesThe system absorbs heat and the

surroundings cool down.Heat is defined as positive (+q).To help you remember, endo- means "within".

Exothermic ProcessesThe system releases heat and the

surroundings warm up.The heat is defined as negative (-q).The prefix exo- means "outside of"

Page 507, Q. 1 - 2

Units for Measuring Heat Flow

Heat flow is measured in two common units, the calorie (cal) and the joule (J).

The joule is the SI unit for energy.One joule of heat raises the temperature of

1g of water 0.2390°C

1 J = 0.2390 cal 4.184 J = 1 cal

Heat Capacity and Specific HeatThe amount of heat needed to raise the temperature

of an object by 1°C is called its heat capacity.It depends on its mass and chemical composition.The greater the mass, the greater its heat capacity.

Why does the parking lot at Walmart heat up faster than a swimming pool?

Specific heat is merely an extension of heat capacityIt is the amount of heat it required for 1g of substance

to raise its temperature by 1°C.Table 17.1, page 508

To calculate specific heat (C):

heat (joules or calories)mass (g) x change in temperature (°C)

the units of specific heat are then…

J/g·°C or cal/g·°C

Questions 3-4, 9-11, pg. 510