Unit 9 Thermochemistry Chapter 17

This tutorial is designed to help students understand scientific measurements.

Objectives for this unit appear on the next slide.

◦ Each objective is linked to its description.

◦ Select the number at the front of the slide to go directly to its description.

Throughout the tutorial, key words will be defined.

◦ Select the word to see its definition.

Objectives

13 State the laws of thermodynamics

14 State the difference between temperature and heat

15 Identify variables and their labels for heat quantities and solve thermochemistry problems

16 Calculate heat-lost heat-gained quantities

(calorimetry)

17 Interpret heating/cooling curves and identify the

phase changes.

13 The Laws of Thermochemistry

There are four laws of thermochemistry.

The first three were written and then

another was added.

◦ The one that was added was thought to be

important enough that it should be listed first.

Zeroth Law of Thermodynamics

States that if two systems are at a thermal

equilibrium with a third, they are at a

thermal equilibrium with each other.

◦ Equilibrium occurs with there is no transfer of

heat from one system to another.

First Law of Thermodynamics

States that energy can be transformed

but it cannot be created or destroyed.

◦ This is sometimes known as the Law of

Conservation of Energy.

◦ Since heat is a type of energy, this law applies

to thermodynamics.

Second Law of Thermodynamics

States that heat always travels from hot to

cold.

◦ Heat cannot be transferred from a colder

object to a warmer object.

Third Law of Thermodynamics

States that absolute zero is the lowest

temperature and all molecular motion

stops at this point.

◦ Essentially, this means you cannot have a

temperature lower than 0 K.

Laws of Thermodynamics Recap

0th Law

◦ Thermal systems will form equilibrium

relationships when mixed.

1st Law

◦ Energy cannot be created or destroyed.

2nd Law

◦ Heat flows from hot to cold.

3rd Law

◦ All molecular motion stops at absolute zero.

14 Temperature vs. Heat

Temperature is often confused with heat but

the two are quite different.

Temperature is a measure of the average

kinetic energy of molecules.

Heat is the measure of the total kinetic energy

of molecules.

Temperature vs. Heat

All molecules are in a state of motion.

The motion is measured by kinetic energy.

However, not all molecules are moving at

the same speed and thus do not have the

same kinetic energy.

The average is taken to determine the

speed of the majority of the molecules.

The total is determined for a purpose

that will be discussed in Unit 9.

15 Heat Quantities

As discussed in Unit 10, temperature is

the average kinetic energy of molecules.

Heat is the total kinetic energy of

molecules.

Heat is measured in Joules or kiloJoules.

To calculate heat, the following equation is

used:

q=mc∆T

Heat Quantities

q=mc∆T For the equation above:

◦ Q = heat measured in joules

◦ m = mass of the object in grams

◦ c = specific heat of an object in 𝐽

𝑔°𝐶

◦ ∆T = change in temperature

Specific Heat

Every substance has a certain specific heat.

This is the amount of heat required to raise the temperature of one gram of an object by one degree Celsius.

Essentially, this number indicates how well on object holds heat.

◦ The higher the amount, the longer it will take to heat. Consider water.

It has a high specific heat. Therefore, a lake can be cool even on a warm summer day. It takes the sun a long time to increase the temperature of the water.

◦ The lower the value, the shorter it will take to heat. Consider a sandy beach.

It has a low specific heat. Therefore, it heats up fast on a warm summer day thus making it painful to walk on.

Heat Quantities

Since we have our equation, q=mc∆T, it is possible to calculate missing variables.

Consider:

A copper wire gained 570 J of heat when its temperature was increased from 25°C to 35°C. What is the mass?

q = 570 J q=mc∆T

m = ? Grams

c = 0.384 𝐽

𝑔°𝐶 570 J = X x 0.384

𝐽

𝑔°𝐶 x 10°C

∆T = 35°C - 25°C = 10°C

x = 150 grams

These

amounts

are found

on the back

of the

Periodic

Table.

16 Heat Lost/Heat Gained

According to the laws of thermodynamics, when two systems are placed together they will form an equilibrium.

Therefore, heat will travel from the warmer system to the colder system.

Because energy cannot be created or destroyed, the heat lost by one system equals that gained by the other system.

Calorimetry

A common use for the heat lost/heat

gained concept is calorimetry.

Calorimetry is a technique used to

determine information about an unknown

object.

It works by heating up an object and then

placing it into water. The temperature of

the water and the object will equilibrate

to the same temperature.

Calorimetry

1. Heat object

2. Place object in water

3. Determine temperature

of system

Calorimetry

Consider: An sample metal with mass of 931 grams was heated to

45°C and placed in 200 grams of water. The water

increased from 15°C to 25°C. What was the metal?

Sample Water

Mass (grams) 931 200

Specific Heat (J/g°C) 0.449 4.18

Intial Temp. (°C) 45 15

Final Temp. (°C) 25 25

Heat (J) 8360 8360

These numbers will be the same for each column

Q=mc∆T

Iron

17 Heating Curves

As a substance gains heat, its molecules

will speed up.

Since molecules are speeding up, it is

possible for a phase change to occur.

During a phase change, the temperature

does not increase even though the heat

will.

Heat (J)

Tem

pera

ture

(°C

)

20

90

Liquid

Gas

Solid

Melting/Freezing

Vaporization/Condensation

Melting Point = 20°C

Boiling Point = 90°C

Heating Curves and Heat Quantities

A problem occurs when calculating a heat

quantity when a phase change occurs.

For instance, if ice is at 0°C and melts to a

liquid, it gained heat.

◦ However, using q=mc∆T would not work

because there was no ∆T.

Heat of Fusion/Heat of Vaporization

When a heat quantity needs to be calculated during a

phase change, the equation is modified.

Since each substance has a different specific heat for

each phase and the temperature does not change, these

variables are replaced.

◦ c∆T becomes ∆Hfusion for melting/freezing

◦ c∆T becomes ∆Hvaporization for

condensation/vaporization

q=mc∆T q=m∆H

Heating Curves

q=mc∆T

q=mc∆T

q=mc∆T

q=m∆Hvaporization

q=m∆Hfusion

On the slants use

q=mc∆T. On the flats

use q=m∆H

Heating Curves

Consider:

◦ A 15 gram sample of ice was heated from

-10°C to 25°C. How much heat was gained?

This reaction undergoes a phase change at 0°C.

A 15 gram sample of ice was heated from -10°C to

25°C. How much heat was gained?

This this reaction touches three parts of

the curve, it will take three equations to

determine the heat gained.

◦ First, -10°C to 0°C q=mc∆T

◦ Second, melting q=m∆Hfusion

◦ Third, 0°C to 25°C q=mc∆T

If these values are added together, we will

have the total heat gained.

-10°C to 0°C q=mc∆T

◦ q=15 grams x 2.10 𝐽

𝑔°𝐶 x 10°C=315 J

Melting q=m∆Hfusion

◦ q=15 grams x 334 J/g = 5010 J

0°C to 25°C q=mc∆T

◦ q=15 grams x 4.18 𝐽

𝑔°𝐶 x 25°C = 1568 J

1568 J + 5010 J + 315 J = 6893 J gained

A 15 gram sample of ice was heated from -10°C to

25°C. How much heat was gained?

This concludes the tutorial on

measurements.

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