Ch.6 - Thermochemistry See all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds
Sections
Internal Energy
Calorimetry
Hess's Law
Enthalpy of Formation
End of Chapter 6 Problems
Additional Practice
Units of Energy
Endothermic & Exothermic Reactions
Additional Guides
Enthalpy
Jules Bruno

Enthalpy, ΔH, is an extensive property and state function that represents the total amount of heat within a chemical reaction or system. 

Thermochemistry

Internal Energy, work and heat 

The First Law of Thermodynamics states that the total energy in an isolated system is constant. The meaning of this definition is that energy can change from one form to another, but never be created nor destroyed. This relationship is illustrated by the equation: 

Equation-internal-energy-heat-work-formula-thermodynamics-unitInternal Energy of a System

U, sometimes shown as E, represents the internal energy of the system or chemical reaction. The variables of q and w represent heat and work respectively. 

Heat and Enthalpy of Reaction 

At constant temperature heat, q, is equal to the enthalpy of reaction, ΔHRxn

Heat-enthalpy-Constant-PressureHeat and Enthalpy

Work 

Work is the amount of energy required to move an object against an external force. The most common form is pressure-volume work and is represented as: 

Work-Equation-Expansion-CompressionWork Equation

The symbols of P and V represent pressure and volume respectively and by using the Ideal Gas Law we can incorporate moles, the gas constant R, and temperature. 

Ideal-Gas-LawDerived work equation

Under constant volume, ΔV = 0. When placing it into the first equation for work we obtain w = 0. 

Exothermic vs. Endothermic Processes

In an exothermic process a system or chemical reaction releases heat from the system to the surroundings. This causes the molecules to slow down as they lose thermal energy and begin to form bonds. 

Exothermic-Reaction-Bond-FormingExothermic Reaction (Bond Forming)The releasing of thermal energy by the reactant molecules also produces an energy diagram where the products are lower in energy than the reactants. 

Energy-Diagram-ExothermicEnergy Diagram (Exothermic Process)

In an endothermic process the system absorbs heat from the surroundings. By absorbing their thermal energy, the molecules speed up enough to break their bonds. 

Endothermic-Reaction-Bond-BreakingEndothermic Reaction (Bond Breaking)

The absorbing of thermal energy by the reactant molecules also produces an energy diagram where the products are higher in energy than the reactants. 

Energy-Diagram-EndothermicEnergy Diagram (Endothermic Process)

Thermochemical Equations

thermochemical equation is a stoichiometric question that now incorporates the heat or enthalpy of a reaction, ΔHRxn. Whereas a normal stoichiometric reaction deals with a mole to mole ratio, a thermochemical equation deals with a ΔH to mole ratio. 

Stoichiometric-ThermochemicalStoichiometric vs. Thermochemical

Let’s take a look at a typical thermochemical equation practice question. 

PRACTICE: How much heat (in kJ/mol) must be absorbed to produce 35.4 g NH3?

Thermochemical-EquationThermochemical Question 

STEP 1: Calculate the molecular mass of NH3

Calculating-Molar-Mass-NH3Calculating Molar Mass (NH3)STEP 2: Convert the grams of NH3 into moles. 

Grams-Moles-ConversionGrams to Moles Conversion 

STEP 3: Perform a mole to ΔH comparison to find the amount of heat released or absorbed. 

Thermochemical-EquationThermochemical Equation

Using the balanced chemical equation allows you to determine the amount of heat, which in this case will be in units of kilojoules. 

Mole-to-EnthalpyMole to Enthalpy ConversionEnthalpy of Formation 

In a formation equation 1 mole of a product is formed from the standard states of its elements. ΔH­is used to represent the enthalpy of formation for a compound or element. 

For example, the formation of Ca(OH)2 comes from the standard states of carbon, oxygen and hydrogen combining together. 

Formation-EquationFormation Equation

By using the enthalpies of formation for each compound we can calculate the enthalpy of reaction, ΔHRxn, by doing products minus reactants

Additional Topics

Enthalpy is also discussed in Chemical Thermodynamics, which looks at the spontaneity of chemical reactions and their connection to entropy and Gibbs Free Energy. In addition we deal with enthalpy when discussing Hess’s Lawcalorimetry, specific heat capacity, and the standard states of elements.  


Jules Bruno

Jules felt a void in his life after his English degree from Duke, so he started tutoring in 2007 and got a B.S. in Chemistry from FIU. He’s exceptionally skilled at making concepts dead simple and helping students in covalent bonds of knowledge.