Unit 9 ThermochemistryChapter 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.

Objectives13 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 ThermodynamicsStates 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 ThermodynamicsStates 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 ThermodynamicsStates that heat always travels

from hot to cold.◦Heat cannot be transferred from a

colder object to a warmer object.

Third Law of ThermodynamicsStates 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 Recap0th 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. HeatAll 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∆TFor the equation above:

◦Q = heat measured in joules◦m = mass of the object in grams◦c = specific heat of an object in ◦∆T = change in temperature

Specific HeatEvery 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 QuantitiesSince 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∆Tm = ? Gramsc = 0.384 570 J = X x 0.384 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 GainedAccording 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.

CalorimetryA 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 CurvesAs 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°CBoiling Point = 90°C

Heating Curves and Heat QuantitiesA 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 VaporizationWhen 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 CurvesConsider:

◦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∆TIf 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 x 10°C=315 J

Melting q=m∆Hfusion

◦q=15 grams x 334 J/g = 5010 J0°C to 25°C q=mc∆T

◦q=15 grams x 4.18 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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