Thermodynamics

Principles of Chemical Reactivity

Basic PrinciplesThermodynamics: The science of heat

and workEnergy: the capacity to do work

-chemical, mechanical, thermal, electrical, radiant, sound, nuclear-affects matter by raising its temperature,

eventually causing a state change-All physical changes and chemical

changes involve energy

Potential Energy: energy that results from an object’s

position -gravitational, chemical, electrostatic

Kinetic Energy: energy of motion

Basic Principles

Law of Energy Conservation:Energy can neither be created nor

destroyed-a.k.a. The first law of thermodynamics

-The total energy of the universe is constant

Temperature vs. Heat:– Temperature is the measure of an object’s

heat energy– Heat ≠ temperature

The Measurement of Heat

Thermal Energydepends on temperature and the amount (mass or volume) of the object

-More thermal energy a substances has the greater the motion its atoms/molecules have-Total thermal energy of an object is the sum of the individual energies of all atoms/molecules/ions that make up that object

SI unit: Joule (J)1 calorie = 4.184 JEnglish unit = BTU

Converting Calories to Joules

Convert 60.1 cal to joules

System: object or collection of objects being studied– In lab, the system is the chemicals inside the

beaker

Surroundings: everything outside of the system that can exchange energy with the system– The surroundings are outside the beaker

Universe: system plus surroundings

Exothermic: heat transferred from the system to the surroundings

Endothermic: heat transferred from the surroundings to the system

Basic Principles

Specific Heat Capacity (C)

amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius

SI Units: Specific heat capacity =J/g. °CSpecific heat of water = 4.184 J/g. °C

Heat Transfer

Heat transfer equationused to calculate amounts of heat (q) in

a substance

T C mq J

g J/g·°C

°C

q1 + q2 + q3 … = 0 or qsystem + qsurroundings = 0

Heat Transfer

Calculate the amount of heat to raise the temperature of 400 g of water from 10.0 oC to 100 oC

Heat Transfer

Calculate the amount of heat energy (in joules) needed to raise the temperature of 12.50 g of water from 45.0°C to 79.0°C

Heat Transfer

Specific heat of gold is 0.13

Therefore the metal cannot be pure gold.

A 1.6 g sample of metal that appears to be gold requires 5.8 J to raise the temperature from 23°C to 41°C. Is the metal pure gold?

Jg.°C

Changes of State

occurs when enough energy is put into a substance to over come molecular interactions

Solid-liquid: molecules in a solid when heated move about vigorously enough to break solid-solid molecular interactions to become a liquid

Liquid-gas: molecules in a liquid when heated move about more vigorously enough to break liquid-liquid molecular interactions to become a gas

Note: This happens in reverse by removing heat energy

Energy and Changes of StateHeat of fusion:

heat needed to convert a substance from a solid to a liquid (at its melting/freezing point)

333 J/g for water

Heat of vaporization: heat needed to convert a substance from a liquid to a gas (at its boiling/condensation point)

2256 J/g for water

Example: Calculate the amount of heat involved to convert 100.0 g of ice at -50.0°C to steam at 200.0°C.

The First Law of Thermodynamics

This law can be stated as, “The combined amount of energy in the universe is constant”

Also called-The Law of Conservation of Energy:– Energy is neither created nor destroyed in

chemical reactions and physical changes.

Changes in Internal Energy (E)

E is negative when energy is released by a system

-Energy can be written as a product of the process

kJ 10 3.516- E

kJ 10 3.516 OH 6 CO 5 O 8 HC3

3)(22(g)2(g))(125

Changes in Internal Energy (E)

E is positive when energy is absorbed by a system undergoing a chemical or physical change

– Energy can be written as a reactant of the process

kJ 10 3.516 E

O 8 HC kJ 10 3.516 OH 6 CO 53

2(g))(1253

)(22(g)

Enthalpy Changes for Chemical Reactions

Exothermic reactions: release energy in the form of heat to the surroundings (H < 0)

-heat is transferred from a system to the surroundings

Endothermic reactions: gain energy in the form of heat from the surroundings (H > 0)

-heat is transferred from the surroundings to the system

For example, the combustion of propane:

Combustion of butane:

Enthalpy Changes for Chemical Reactions

Exothermic reactions generate specific amounts of heat– Because the potential energies of the products are

lower than the potential energies of the reactants

Endothermic reactions consume specific amounts of heat– Potential energies of the reactants are lower than the products

H for the reverse reactionis equal, but has the opposite sign to the forward reaction

Thermochemical Equations

balanced chemical reaction with the H value for the reaction

H < 0 designates an exothermic reaction: heat is a product, the container feels hot

H > 0 designates an endothermic reaction: heat is a reactant, the container feels cold

kJ 3523 - H OH 6 CO 5O 8 HC orxn)(22(g)2(g))12(5

CalorimetryAn experimental technique that measures the heat

transfer during a chemical or physical process

Constant pressure calorimetry:A styrofoam coffee-cup calorimeter is

used to measure the amount of heat produced (or absorbed) in a reaction

– This is one method to measure qP (called H) for reactions in solution

qreaction + qsolution = 0

Note: Assuming no heat transfer to the surroundings

CalorimetryIf an exothermic reaction is performed in a

calorimeter, the heat evolved by the reaction is determined from the temperature rise of the solution– This requires a two part calculation

When we add 25.00 mL of 0.500 M NaOH at 23.000oC to 25.00 mL of 0.600 M CH3COOH already in the calorimeter at the same temperature, the resulting temperature is observed to be 25.947oC. Determine heat of reaction and then calculate the change in enthalpy (as KJ/mol) for the production of NaCH3COO.

CH3COOH(aq) + NaOH(aq) NaCH3COO(aq) + H2O(l)