Ch.6 - Thermochemistry WorksheetSee 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

In a formation equation one mole of product is formed from the standard or elemental forms of each element. 

Formation Equation

Concept #1: Understanding a Formation Equation


Welcome back, guys. In this new video, we're going to take a look at the standard heats of formation.
We're going to say in a formation equation, one mole of a compound forms from its elemental forms. We're going say that delta F represents our heat of formation. And it's just the enthalpy change for the chemical reaction when all the substances are in their standard states.
Basically, a formation equation will be the one I provide here. The natural state of carbon in our environment is C graphite. The natural form of hydrogen in our environment is H2. Remember, hydrogen is a diatomic molecule. We're going to say that they combine together to give us one mole of this compound. CH4 is known as methane. This would be the formation equation for one mole of methane compound. This represents a formation equation. We have the elements in their natural states as reactants combining together to give us one mole of some type of product. 

Concept #2: Understanding Heats of Formation


Now what we should remember, when it comes to the heats of formation, there's certain things that we need to remember. We're going to say one, an element in its elemental form or standard state has a delta H of formation of zero. When I mean natural state or standard state, that means that the element is either by itself, like sodium solid, or it's connected to copies of itself like P4 or Cl2 or S8. These guys are connected to copies of themselves. If an element is in its standard state, it's delta H value is zero.
We're going to say most compounds have a negative delta H of formation. Because, remember, we said when you're forming bonds, that's an exothermic process. We said earlier, exothermic reactions have a delta H value that is negative.
Finally, we can use these heats of formations in order to find the enthalpy of our reaction. To find the enthalpy of our reaction, we just simply do products minus reactants. 

Enthalpy of a Reaction

The enthalpy value associated with an element or compound can be used to find the enthalpy of a reaction. 

Example #1: The oxidation of ammonia is given by the following reaction:

4 NH3 (g) +  5 O2 (g) →  4 NO (g)  + 6 H2O (g)

Calculate the Horxn if the Hof value for NH3 , NO and H2O are -45.9 kJ/mol, 90.3 kJ/mol and -241.8 kJ/mol respectively.

The enthalpy of a reaction can be determined if we are given the enthalpy of formation value for each compound. 

Practice: Ibuprofen is used as an anti-inflammatory agent used to deal with pain and bring down fevers. If it has a molecular formula of C13H18O2 determine the balanced chemical equation that would give you directly the enthalpy of formation for ibuprofen.

Example #2: Use the following bond strength values (kJ/mol):

C–H  412          C–O  360          C=O  743           

C–C  348          H–H  436          C=C  611

C≡C  837         C≡O  1072        O–H  464

O=O 498

Calculate the enthalpy of the reaction shown in the formula below: 

When given bond energies then calculating the enthalpy of the reaction requires a different equation.