chemistry thermodynamics

7/29/2019 Chemistry Thermodynamics

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1

Energy

The capacity to do work

or to produce heat.




2

Law of Conservation

of Energy

Energy can be converted from one form to

another but can neither be created nor

destroyed.

( E universe is constant)




3

The Two Types of Energy

Potential: due to position or composition -

can be converted to work

Kinetic: due to motion of the object

KE = 1 / 2mv2

(m = mass, v = velocity)




4

Temperature v. Heat

Temperature reflects random motions of

particles, therefore related to kinetic energy

of the system.

Heat involves a transfer of energy between

2 objects due to a temperature difference




5

State Function

Depends only on the present state of the

system - not how it arrived there.

It is independent of pathway.




6

System and Surroundings

System: That on which we focus attention

Surroundings: Everything else in the universe

Universe = System + Surroundings




7

Exo and Endothermic

Heat exchange accompanies chemicalreactions.

Exothermic: Heat flows out of the system(to the surroundings).

Endothermic: Heat flows into the system

(from the surroundings).




8

First Law

First Law of Thermodynamics:

The energy of the universe is

constant.




9

Figure 6.2

The Combustion of Methane

Fi 6 3




10

Figure 6.3

The Energy Diagram for the Reaction of Nitrogen and Oxygen to

Form Nitric Oxide




11

First Law

E = q + w

E = change in system’s internal energy

q = heat

w = work




12

Work

work = force distance

since pressure = force / area,

work = pressure volume

wsystem = PV




13

Figure 6.4

The Volume of a

Cylinder




14

Enthalpy

Enthalpy = H = E + PV E = H PV

H = E + PV

At constant pressure,

qP = E + PV ,

where qP = H at constant pressure

H = energy flow as heat (at constant pressure)




15

Figure 6.5

A Coffee-Cup Calorimeter Made of

Two Styrofoam Cups

Fi 6 6




16

Figure 6.6

A Bomb Calorimeter




17

Heat Capacity

C =heat absorbed

increase in temperature =J

C or J

K




18

Some Heat Exchange Terms

specific heat capacity

heat capacity per gram = J/°C g or J/K g

molar heat capacity

heat capacity per mole = J/°C mol or J/K mol




19

Hess’s Law

Reactants Products

The change in enthalpy is the same whether

the reaction takes place in one step or a

series of steps.




20

Figure 6.7

The Principle of Hess’s Law




21

Calculations via Hess’s Law

1. If a reaction is reversed, H is also reversed.N2(g) + O2(g) 2NO(g) H = 180 kJ

2NO(g) N2(g) + O2(g) H = 180 kJ

2. If the coefficients of a reaction are multiplied

by an integer, H is multiplied by that same

integer.6NO(g) 3N2(g) + 3O2(g) H = 540 kJ




22

Standard States

Compound

• For a gas, pressure is exactly 1 atmosphere.

• For a solution, concentration is exactly 1 molar.

• Pure substance (liquid or solid), it is the pure liquid or

solid.

Element

• The form [N2(g), K(s)] in which it exists at 1 atm and

25°C.




23

Standard Heat of Formation

H f

H of a reaction in which one mole of a product

is made from elements

)()(2

1)(

2

122

g NOgOg N

H f (NO) = H rxn




24

Change in Enthalpy

Can be calculated from enthalpies of

formation of reactants and products.

H rxn° = np H f (products) nr H f (reactants)




25

Figure 6.8

Pathway for the Combustion of Methane

Figure 6 9




26

Figure 6.9

A Schematic Diagram of the Energy Changes for the Reaction

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)




27

Bond Energy

• Bond energy is the energy required to break

a bond.

• Breaking a bond is always endothermic.

• Bond formation is always exothermic.

• May use bond energies to approximate

H rxn

chemistry thermodynamics

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