1. warmup – endothermic or exothermic??
Write out the complete sentence and fill in the blank.
A reaction that releases heat is called __________
Heat is a reactant in a ____________ reaction.
A reaction that feels warm is ____________
A heat flows into a(n) __________ reaction.

2. Thermochemical Equations part II
calculate enthalpy of reaction

3. Objectives
to use thermochemical equations to calculate energy changes that occur in chemical reactions
to classify reactions as exothermic or endothermic

4. Thermochemical Equations – concept overview
Definition: Enthalpy (H) is a measure of the heat content of a system or substance.
When chemical bonds are formed, heat leaves the system and the chemical potential energy or internal enthalpy of the system goes down, the surroundings get warm. H is negative.
To break or destroy a chemical bond heat is required and chemical potential energy or internal enthalpy of the system goes up, the surroundings get cool. H is positive.
When a chemical reaction takes place the enthalpies (chemical potential heat content) of the products will differ from the enthalpies of the reactants because they have different bonds

5. All Chemical reactions change the number and type of chemical bonds between the atoms.
2H2(g) O2(g)  H2O(l)
Reactants have:
2 H-H bonds
O=O bond
that get broken
Products have:
H-O bonds
that get formed

6. review: Bond Dissociation Energy (BDE):
Each type of bond releases a different amount energy when it is formed and requires that same amount of energy to break it.
BDEs can be found in chemistry tables (page 448)
BDE Examples:
C = O 736 kJ/mol
H – O 464 kJ/mol
Example 1: Water draw it:
H2O has two H-O bonds
Example 2: Carbon Dioxide draw it:
CO2 has two C=O bonds

7. Enthalpy of Formation (also called Heat of Formation)
Every substance releases heat (energy) when each of its bonds are formed and requires that same amount of heat to break them.
The total change in heat to form 1 mole of a substance from its constituent elements is called the enthalpy of formation. (Hof)
Enthalpies of formation (Hof) for many chemical substances can be found in tables. (textbook p316)

8. Enthalpy of Reaction (also called Heat of Reaction)
When a chemical reaction takes place the enthalpies (heat content) of the products will differ from the enthalpies of the reactants because they are bonded differently after the reaction.
The total change in enthalpy (H) that occurs in all the reactants when forming all the products in a reaction is the enthalpy of reaction
= H

9. Enthalpy of Reaction (also called Heat of Reaction or H)
If you know the enthalpy for formation (Hof) of all the products and reactants in a chemical reaction, the change in enthalpy for the reaction, H, can be calculated easily.
If DH is negative, the reaction is exothermic and releases heat.
If DH is positive, the reaction is endothermic and absorbs heat.

10. If the enthalpy of products is greater, then DH is positive, the reaction is endothermic.
It feels cool.
If the enthalpy of reactants is greater, then DH is negative, the reaction is exothermic.
It feels warm.

11. Example 1: decomposition of hydrogen peroxide
Calculate the enthalpy of reaction for decomposition of hydrogen peroxide into oxygen and water and determine if the reaction is exothermic or endothermic.
2 H2O2 → O H20
Enthalpy Hof of reactants
H2O kJ/mol
Enthalpy Hof of products
O kJ/mol
H kJ/mol
Using the balanced equation:
H =

12. Example 2: citric acid and sodium bicarbonate reaction
Calculate the enthalpy of reaction for the reaction between citric acid solution, H3C6H5O7 (lemon juice) and sodium bicarbonate NaHCO3
H3C6H5O7 + 3 NaHCO3 → 3 CO2 + 3 H2O + Na3C6H5O7
Enthalpy Hof of reactants Enthalpy Hof of products
H3C6H5O kJ/mol CO kJ/mol
NaHCO kJ/mol H2O kJ/mol
Na3C6H5O kJ/mol
Using the balanced equation:
H =

13. Concepts to know
Enthalpy – measure of heat content of a system or substance
Bonds forming release heat. DH is negative
Bonds breaking absorb heat. DH is positive
Enthalpy of formation for substances differ because different bonds are involved. Hof
Enthalpy of a reaction is the total Hof for all reactants and products.
DH = (Hof products – Hof reactants)
If DH is negative, the reaction is exothermic and releases heat.
If DH is positive, the reaction is endothermic and absorbs heat.