THERMOCHEMISTRY

BY LESLIE JASPER

THERMOCHEMISTRY

• Thermochemistry is the study of heat change in chemical reactions.

System

• A system is the specific part of the universe that is of interest to us. Studying the system is important in order to study changes associated with chemical reactions.

Types of Systems

• Open system

• Closed system

• Isolated system

Types of Energy Processes

• Exothermic- this process involves the release of energy during a chemical reaction.

• Endothermic- this process involves the intake of energy during a chemical reaction.

Energy

• Energy is the capacity to do work. Work is measured as force times the distance. The term “energy” is a much used term that represents an abstract concept. All different types of energy can be converted from one form to another.

Types of Energy

• Potential Energy

• Kinetic Energy

• Solar Energy

• Geothermal Energy

• Nuclear Energy

• Tidal Energy

• Hydroelectric Energy

Thermodynamics

• Thermodynamics is the scientific study of the interconversion of heat and other kinds of energy. In thermodynamics we study the changes in the state of a system and properties that determine the state of a system known as state functions.

Enthalpy

• Enthalpy is a state function and is the difference between the enthalpies of the products and the enthalpies of the reactants.

• H= H (products) – H (reactants)

Specific Heat

• Specific Heat is the amount of heat required to raise the temperature of one gram of the substance by one degree Celsius. Specific heat is an intensive property while heat capacity is an extensive property.

Calorimetry

• Calorimetry is the measurement of heat changes. A calorimeter is a closed container designed to monitor heat loss or gain between two substances. Types of calorimeters include:

• Constant-Volume Calorimeter

• Constant-Pressure Calorimeter

Standard Enthalpy of Formation

• This formation is the heat change that results when 1 mole of a compound is formed from its elements at a pressure of 1 atm. Elements are said to be in standard state at 1 atm. This gives rise to the term “standard enthalpy”.

Standard Enthalpy of Reaction

• This is the enthalpy of a reaction carried out at 1 atm.

• aA + bB = cC + dD

Methods to measure standard enthalpy formation

• Direct method- this method measures the standard enthalpy of formation when compounds are readily synthesized from their elements.

• Indirect method- this method measures the standard enthalpy of formation when the reaction is going too slow or side reactions are producing undesirable side reactions and not readily synthesized.

Hess’s Law

• Hess’s Law states that when reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

First Law of Thermodynamics

• This law states that energy can be converted from one form to another, but cannot be created or destroyed.

Enthalpy of Solution

• Also known as the heat of solution. This is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent.

• H(soln) = H(soln) – H(components)

Lattice Energy

• Lattice Energy is the energy required to completely separate one mole of a solid ionic compound into gaseous ions.

Heat of Dilution

• Heat of Dilution is the heat change associated with the dilution process. If a certain solution process is endothermic, more heat will be absorbed by the solution from its surroundings. If the process is exothermic, more heat will be liberated if more solvent is added to the solution.

Temperature

• Temperature is a relative measure of how hot or how cold an object is. Temperature can also be known as a measure of the concentration of heat. It is a measure of the average random motion of the unit of particles (kinetic energy) of an object.

Types of Thermometers

• Thermometers are used to measure temperature and include:

• Glass mercury-filled thermometers• Bi-Metallic Stemmed• Thermisters• Digital or Thermocouple• Deep Fry or Candy• Refrigerator or Oven• Types of disposable thermometers• Laser thermometers (new)

Three Major Types of State Functions

• Entropy (S)

• Gibb’s free energy (G)

• Enthalpy (H).

Entrophy

• Entrophy is the disorder of the universe. Entropy is constantly increasing in the universe or at least reaches a maximum at equilibrium if the system is closed and unable to exchange heat or mass with its surroundings.

Gibbs Free Energy

• Gibbs Free Energy is a measure of non-pV work that must go into a reaction to make it occur (when G is positive) or work that a reaction can do (when negative). It can be shown that the change of Gibbs Free Energy over temperature in a reaction is the change of the total entropy of the reacting system plus its surroundings during a reaction at constant temperature and pressure.

Thermal Runaway

• Thermal runaway begins when the heat produced by the reaction exceeds the heat removed. The surplus of heat raises the temperature of the reaction mass, which causes the rate of reaction to increase. This in turn accelerates the rate of heat production. This reaction can be hazardous and/ or cause an explosion. An example would be sulfuric acid and water.

Calorie

• A calorie is the quantity of heat required to raise the temperature of one gram of water one Celsius degree. A calorie is actually called a kilocalorie. Food producers feel it is pleasing to the consumer to not hear the prefix “kilo” in front of calorie.

Clearing up the Myth

• They say that a pregnant woman has to “eat for two”. This is not true since a pregnant woman only has to consume 300 more kilocalories than a non-pregnant woman. This can be accomplished by an extra serving from each of the five food groups daily.