3 - Thermochemistry (3.1 - 3.2)

8/10/2019 3 - Thermochemistry (3.1 - 3.2)

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3.1 Types of Energy

3.2 Thermodynamics

3.3Energy Changes in

Chemical Reactions

3.4 Measuring EnergyChanges: Calorimetry

3.5 Enthalpy (H)

Thermochemistry


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3.1 Types of EnergyLearning Objective

Recognize the types of energy of interest to chemists.


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The Types of Energy

Energy the ability to do work

Energy

KineticEnergy

PotentialEnergy

Electrical

Chemical

Mass

ThermalEnergy

RadiantEnergy


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Kinetic Energy

Kinetic Energy

Energy due to motion

Energy associated with randommolecularmotion, thermal energy

All moving objects have kinetic energy,dependent on velocity (v) and mass (m)

Joule (J): the SI energy unit


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Potential Energy

Potential Energy Energy due to condition, position or

composition

Energy associated with forces ofattraction orrepulsion betweenobjects

Three Types:

Electrical Energy Chemical Energy Mass


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Potential Energy

1) Electrical Energy Energy from positive andnegative ions held a small distance apart.

q1 and q2 are the charges of two ions

r is the distance between the ions in pm.

(k = 2.31 x 10-16 J pm)


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Potential Energy

2) Chemical Energy Energyresulting from attraction of theelectrons and nuclei in molecules

(bond energy)

3) Mass: Transformation of mass

into energy


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Thermal Energy the total energy of random

movements of molecules energy as a result of

electromagnetic radiation.

Thermal & Radiant Energy

Radiant Energy


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Energy Transfer &

Transformations Energy can be transferred

from one type to another.

Energy transformationsaccompany chemicalreactions.


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3.2 ThermodynamicsLearning Objective

Understand the first law of thermodynamics and the conceptsof heat and work.


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Thermodynamics

Key terms:System

Surroundings

Boundary

The study of energy transfers and transformations

how much energy goes where?


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Three Systems

Open

System

Closed

System

Isolated

System

Energy

&

Matter

Exchanged

Energy

Exchanged

No Matter

Exchanged

No

Energy

&

Matter

Exchanged


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1st Law of Thermodynamics

The Law of Conservation of Energy Energy can neither be created nor destroyed

Energy can be converted fromone form to another.

Energy of the universe is constant

The 1st

Law of Thermodynamics Application of the conversation of energy Energy may be transferred as work or heat, but no

energy can be lost, nor can heat or work beobtained from nothing


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Transfer of Energy

Two Ways! Heat Energy transferred due to a temperature

difference. Energy, as heat, flows from warm to

cool.

Work Transfer of energy by the action of a force

through a distance.

Temperature reflects the random motion ofparticles and related to kinetic energy


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Heat

Thermal energy that is exchanged with its surroundingsis referred to as heat (q) and is measured injoules (J).

Some heat flows can be determined from

temperature changes:


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Heat Flows and Temperature

1. T depends on q, the amount of heat

transferred.

2. T depends on the direction of heat flow:

If a substance absorbs heat, T > 0

If a substance releases heat, T < 0

3. T depends inversely on the amount of

material.

4. T depends on the identity of the material.

Molar heat capacity (C) amount of heat needed

to raise the temperature of 1 mol of substance by1 C.


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Temperature Change

Where

q is the amount of heat transferred

n is the number of moles of material C is the molar heat capacity of the substance

(C has units of J mol1 C1)

S


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Se can also say that

Energy transfer is directional, so we must keep track ofthe signs associated with heat flows

Reminder:

T = final temperature initial temperatureRemember to use the correct algebraic sign!

(If Tf< Ti, then the T should be negative)


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An iron kettle weighing 1.35 kg contains 2.75 kg of water at 23 C. Thekettle and water are heated to 95.0 C. How many joules of energy areabsorbed by the water and by the kettle?

( ;

)

Example 1

W k


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Work

Work (w): energy used to move an object against anopposing force

Work is a product of force (F) and displacement (d):

The amount of work depends on the magnitude of theforce

Work done by a gas: Pressure-Volume Work

Work involved in the expansion or compression of gases

W k i Ch i l P


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Work in a Chemical Process

Consider a gas confined to a cylinder with a moveablepiston of area A.

W k i Ch i l P


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Work in a Chemical Process

The external pressure is force per unit area

When the gas expands it pushes against the piston

and the piston moves a distance

h


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Work in Gases

Expansion

Work done by the gas Vf> Vi, therefore V > 0

w < 0

Esystem decreases

Compression Work done on the gas

Vf< Vi, therefore V < 0

w > 0 Esystem increases

E l 3


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A gas is allowed to expand from a volume of 2.3 L to a volume of 5.8 L.During the process, 460 J of heat is transferred from the surroundings tothe gas. (1.00 L atm = 101.325 J)

a) How much work has been done if the gas expands against a

vacuum?b) How much work has been done if the gas expands against a pressure

of 1.3 atm?

Example 3

E l 2


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Breathing requires work, even if you are unaware of it. The lung volumeof a 70 kg man at rest changed from 2.20 L to 2.70 L when he inhaled,while his lungs maintained a pressure of approximately 1.0 atm.

How much work, in liter-atmospheres and joules, was required to take a

single breath? (1.00 L atm = 101.325 J)During exercise, his lung volume changed from 2.20 L to 5.20 L on eachin-breath. How much additional work, in joules, did he require to take a

breath while exercising?

Example 2

First Law of Thermodynamics


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First Law of Thermodynamics

Simply a restatement of the law of conservation ofenergy

Internal energy (E) The sum of the potential andkinetic energies of all the particles in the system!

A change in the internal energy of a system is due tothe energy transfer, the result of a flow of work (w)

and/or heat (q).

State & Path Functions


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State & Path Functions

State Function A property of the system thatdepends only on its present state, and not how it gotthere. It is independent of pathway.

Path Function properties that depend on how thechange occurs. Distance travelled is a path function.

In thermodynamics

Work (w) and heat (q) are path dependent functions.

How much work is done and how much heat is transferreddoes depend on the pathway taken.

Distance Travelled is a Path Function


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Distance Travelled is a Path Function

Thermodynamic Path Functions


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Thermodynamic Path Functions

Energy is a state function, but heat and workare path functions


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3 - Thermochemistry (3.1 - 3.2)

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