Chapter 3: Energy and Its Conservation 3.1 Types of Energy 3.2 Thermodynamics 3.3 Energy Changes in...

Chapter 3: Energy and Its Conservation

3.1 Types of Energy

3.2 Thermodynamics

3.3 Energy Changes in Chemical Reactions

3.4 Measuring Energy Changes: Calorimetry

3.5 Enthalpy

3.6 Energy Sources

Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

3.1 Types of Energy

Learning objective:

Recognize the types of energy of interest to chemists.


3.1 Types of Energy

Energy – the ability to do work

1. Kinetic Energy2. Potential Energy:

a. Electricalb. Chemicalc. Mass

3. Thermal Energy4. Radiant Energy


Kinetic Energy

Every moving object has kinetic energy which is dependent on the velocity (v) and mass (m) of an object:

Joule (J): the SI unit of energy

2

2

s

m kg 1 J 1 joule 1

2kinetic

1E mv

2


Potential Energy

Electrical Energy - energy from positive and negative ions held a small distance apart.

1 2electrical

q qE = k

r

q1 and q2 are the charges of two ionsr is the distance between the ions in pmk = 2.31 x 10-16 J pm


Potential Energy

Chemical Energy: energy resulting from attraction of the electrons and nuclei in molecules (bond energy)


Potential Energy

Mass: transformation of mass into energy (E = mc2)Thermal Energy: the total energy of random

movements of moleculesRadiant Energy: energy as a result of electromagnetic

radiation.


Energy Transfer and Transformations

Energy can be transferred from one type to another.

Energy transformations accompany chemical reactions.


3.2 Thermodynamics

Learning objective:

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


3.2 Thermodynamics

The study of energy transfers and transformations

“how much energy goes where”

Terms: System: whatever we want to

describe and study by itself. Surroundings: everything else but

the system Boundary: - what separates the

system from its surroundings


Conservation of Energy

Energy is neither created nor destroyed in any process, although it may be transferred from one body to

another or transformed from one form into another

or, restated

Energy may be transferred as work or heat, but no energy can be lost, nor can heat or work be obtained from

nothing


Heat

Thermal energy that is exchanged with its surroundings is referred to as heat (q) and is measured in joules (J).


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 by 1° C.


Temperature Change

qΔT =

nC

Where q is the amount of heat transferred, n is the number of moles of material and C is the molar heat capacity of the substance in J mol-1 °C-1.


So you can also say that:

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

qsurroundings = – qsystem

Heat transferred q m C T

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

If Tf < Ti, then the T should be negative.


Example 3 - 1

Calculate the temperature change that results from adding 250 J of thermal energy to each of the following (a) 0.75 mol of Hg; (b) 0.35 mol of Hg; (c) 0.35 mol of H2O.


Example 3 - 2

An aluminum frying pan that weighs 745 g is heated on a stove from 25°C to 205°C. What is q for the frying pan?


Work

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

The amount of work depends on the magnitude of the force

wsurroundings = – wsystem


First Law of Thermodynamics

Simply a restatement of the law of conservation of energy

E = q + w


State and Path Functions

State Functions: properties that depend only on the conditions that describe the system. Energy is a state function.

Path Functions: properties that depend on how the change occurs. Distance travelled is a path function.


Distance Travelled is a Path Function…

…but distance betweentwo cities is

a state function


Thermodynamic Path Functions

Energy is a state function, but heat and work are path functions



Learning objective:

Understand the origins of energy changes in chemical reactions



Bond breakage requires energy Bond formation releases energy.


Example 3 - 3

The Haber reaction for the formation of ammonia releases energy:

N2 + 3 H2 2 NH3 E = 40.9 kJ

How much energy is released in the production of 1.00 kg of ammonia?


Path Independence

A change in any state function is independent of path.

Thus, the energy change in a chemical reaction is independent of the manner in which the reaction takes place.


Bond Energies

Bond energy (BE): energy required to break a bond, always positive

Usually expressed in kJ/mol

H2 (g) H (g) + H (g) Ebond breaking = BE = + 435 kJ/mol

And the reverse process:

H (g) + H (g) H2 (g) Ebond making = BE = 435 kJ/mol


Table 3 – 2: Average Bond Energies


Reaction Energy

Bond energies can be used to estimate the energy change that occurs in a chemical reaction

rxn (bonds broken) (bonds formed)E BE - BE


Example 3 - 4

The world produces tens of billions of vinyl chloride annually. Most is converted to the polymer poly(vinyl) chloride (PVC), which is used to make piping, siding, gutters, floor tiles, clothing and toys. Vinyl chloride is made in a two-step process. The balanced overall equation is as follows:

Based on average bond energies, what energy change accompanies the formation of one mole of vinyl chloride? Does the synthesis require an input of energy?



Learning objective:

Apply the principles of calorimetry to determine energy changes in a chemical reaction



Calorimeter: a device used to measure heat flows that accompany chemical reactions. A Styrofoam cup is a simple calorimeter.

Exothermic: if the reaction releases heat.Endothermic: if the reaction absorbs heat.

qreaction = - qcalorimeter qcalorimeter = CcalT


Example 3 - 5

A calorimeter is calibrated with an electrical heater. Before the heater is turned on, the calorimeter temperature is 23.6 °C. The addition of 2.02 x 103 J of electrical energy from the heater raises the temperature to 27.6 °C. Determine the total heat capacity of this calorimeter.


Example 3 - 6

Ammonium nitrate (NH4NO3, M = 80.05 g/mol) is used in cold packs to “ice” injuries. When 20.0 g of this compound dissolves in 125 g of water in a coffee-cup calorimeter, the temperature falls from 23.5 °C to 13.4 °C. Determine the q for the dissolving of the compound. Is the process exothermic or endothermic?


Molar Energy Changes

Energy change is an extensive quantity – it is dependent on the amount of substance.

molar

ΔEΔE =

n


Example 3 - 7

A 0.125 g sample of octane (C8H18, M = 114.2 g/mol) is burned in excess O2 in the constant-volume calorimeter described in Example 3 – 5. The temperature of the calorimeter rises from 21.1 to 32.9 °C. What is Emolar for the combustion of octane?


3.5 Enthalpy

Learning objective:

Understand and calculate enthalpy and internal energy


3.5 Enthalpy

Enthalpy (H) is a thermodynamic state function that describes heat flow at constant pressure.

Enthalpy change, H: heat transferred into or out of a system at constant pressure. For a reaction, it can be calculated according to:

Hreaction ≈ Ereaction + RTngas

H = E + pV


Example 3 - 8

Find the difference between molar H and E for the combustion of octane at 298 K.


Energy and Enthalpy of Vapourization

Changes of state always take place at a constant temperature

Heat of vapourization Hvap: heat required to convert liquid to gas

Hvap ≈ Evap + RTvap


Enthalpies of Formation

A formation reaction produces 1 mol of a chemical substance from the elements in their most stable formsThere is a single product with a stoichiometric

coefficient of 1.All the starting materials are elements, and each is in

its most stable form.Enthalpies of reactions involving gases vary with

pressure, so pressures must be specified.Enthalpies of reactions occurring in solution vary with

concentration, so concentrations must be specified.


Remember State Functions

State function - a quantity whose value is determined only by the state of the system. It does not depend on the path taken to get there.

Standard state - is the most stable form of a substance at T = 25 °C and p = 1 bar, and 1 M if it is in solution

The superscript ° indicates standard conditions.


Therefore...

The standard enthalpy change of a reaction for the formation of 1 mole of a compound directly from its elements is called the standard molar enthalpy of formation, Hf°

Mn (s) + O2 (g) MnO2 (g) Hfo = -520.0 kJ/mol

Br2 (l) Br2 (g) Hfo = 30.9 kJ/mol


Enthalpy Changes for Chemical Reactions

Hess’s Law: the enthalpy change for any overall process is equal to the sum of enthalpy changes for any set of steps that leads from the starting materials to the products.

To calculate the total enthalpy change for a reaction, H°rxn:

H°reaction = p H°f,p- r H°f,r


Enthalpy Changes Under Nonstandard Conditions

Energies and enthalpy change as temperature, concentration, and pressure change.

Therefore, H depends on these variables, too.


3.6 Energy Sources

Learning objective:

Be familiar with our sources of energy


3.6 Energy Sources

Energy and Civilization: advances of civilization can be viewed as the results of people figuring how to increase the availability of energy


Ultimate Energy Sources

The vast majority of our energy sources originate in solar energy.

Photosynthesis converts some solar energy into more concentrated forms:6 CO2 (g) + 6 H2O (l) + 2880 kJ C6H12O6 (s) + 6 O2 (g)

One nonsolar source is nuclear energyAnother is the Earth’s hot interior (geothermal) energy


Future Resources

Economically, we want energy sources to be high intensity, and readily extracted and transported.

Environmentally, they would be renewable and environmentally benign.


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