Thermo Lect 1 2008

Thermochemistry Syllabus • Heat and Temperature • Internal energy and p - V work • The First Law of thermodynamics • Heat Capacity and Enthalpy changes • Applications of Hess’ Law in calculating enthalpy changes

by: Duanne Biggs

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Thermochemistry Thermochemistry is the study of the heat evolved or absorbed in chemical reactions Thermodynamics- study of energy and its transfers Energy- the capacity to do work or produce heat Joule as the SI unit of energy First Law of Thermodynamics- energy of universe is constant 2

Energy • Thermal energy is the energy associated with the random motion of atoms and molecules • Chemical energy is the energy stored within the bonds of chemical substances • uclear energy is the energy stored within the collection of neutrons and protons in the atom • Electrical energy is the energy associated with the flow of electrons 3

Energy • Potential energy- (PE) due to position or composition Ep = mgh – ex. attractive or repulsive forces

• Kinetic energy- (KE) due to motion of the object – Ek = ½mv2 :depends on mass and velocity

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Energy Atoms and molecules • Thermal Energy: This is due to the kinetic energy of atoms and molecules that are in motion. (Can be measured by finding the temperature) • Chemical Energy: This is due to potential energy stored in bonds linking atoms together. (Some of this energy is released when molecules react to form more stable substances.) 5

Energy • Heat is the transfer of thermal energy between two bodies that are at different temperatures. • Temperature is a measure of the thermal energy. • Temperature ≠ Thermal Energy

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Energy Transfer • Two Ways to Transfer Energy: – Heat- (q) transfer of energy between two objects because of a temperature difference – Work- (w) force acting over a distance

• Energy change is independent of pathway because it is a state function • work and heat depend on pathway so are not state functions • State function- depends only on current conditions, not past or future 7

State functions State functions are properties that are determined by the state of the system, regardless of how that condition was achieved. Energy, pressure, volume, temperature

Potential energy of hiker 1 and hiker 2 is the same even though they took different paths. 8

Important Concepts and Terms • A thermodynamic system is the region of the universe that we select for investigation • E.g. Gas in a cylinder may constitute a system. • Everything outside the system is called the surroundings. • The system is separated from the surroundings by its boundary. • Exothermic: energy released by system to surrounding. • flows out of system • container feels hot to the touch • Endothermic: energy absorbed by system from surr. • flows into the system • container feels cold to the touch 9

The system is the specific part of the universe that is of interest in the study.

SYSTEM

Exchange:

open

closed

isolated

mass & energy

energy

nothing

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Internal Energy • (E) sum of potential and kinetic energy in system • can be changed by work, heat, or both • E = PE + KE • ∆E = q + w

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Signs • signs are very important • signs will always reflect the system’s point of view unless otherwise stated ∆E change in internal energy

q heat

w work

exothermic

-

-

-

endothermic

+

+

+

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Work

F P= A

• common types of work w = F × ∆h = P × A × ∆h – expansion- work done by gas – compression- work done on a gas expansion compression

+∆V -∆V

-w +w

w = − P∆V P is external pressure – not internal like we normally refer to 13

Work

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Enthalpy Definition: H ≡ E + PV since E, P and V are all state functions, then H is too for the following, the process is at constant P and the only type of work allowed is PV work • Since we know that • We can write

w = − P∆V ∆H = ∆E + P∆V = qP + w + P∆V = q

P

= q

P

+ (− P ∆ V ) + P ∆ V

• When ∆ H is positive, the system gains heat from the surroundings. • When ∆ H is negative, the surroundings gain heat from the system. • so, qP = ∆H at constant P 15

Enthalpy = Heat of Reaction

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Example 1 • Find the ∆E for endothermic process where 15.6 kJ of heat flows and 1.4 kJ of work is done on system – Since it is endothermic, q is + and w is +

∆E = q + w = 15.6kJ + 1.4kJ = 17.0kJ

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Example 2 • Calculate the work of expansion of a gas from 46 L to 64 L at a constant pressure of 15 atm. – Since it is an expansion, ∆V is + and w is -

w = − P∆V = −(15atm)(64 L − 46 L) = −270 L ⋅ atm

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Example 3 • A balloon was inflated from 4.00 x 106 L to 4.50 x 106 L by the addition of 1.3 x 108 J of heat. Assuming the pressure is 1.0 atm, find the ∆E in Joules. (1 L·atm=101.3 J) – Since it is an expansion, ∆V is + and w is -

∆E = q + w = q − P∆V 8

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∆E = 1.3 ×10 J − (1.0atm)(4.5 ×10 L − 4.00 ×10 L) 8

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∆E = 1.3 ×10 J + (−5.1×10 J ) = 8.0 ×10 J

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