warmup – endothermic or exothermic?? Write out the complete sentence and fill in the

blank.1. A reaction that releases heat is called

__________2. Heat is a reactant in a ____________ reaction.3. A reaction that feels warm is ____________4. A heat flows into a(n) __________ reaction.

Thermochemical Equations part II

calculate enthalpy of reaction

Objectivesto use thermochemical equations to calculate energy changes that occur in chemical reactions

to classify reactions as exothermic or endothermic

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

All Chemical reactions change the number and type of chemical bonds between the atoms.

2H2(g) + O2(g) 2H2O(l)

Products have:4 H-O bondsthat get formed

Reactants have:

2 H-H bonds1 O=O bondthat get

broken

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/molH – O464 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

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. (Ho

f)

Enthalpies of formation (Hof) for many

chemical substances can be found in tables. (textbook p316)

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

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 H is negative, the reaction is exothermic and releases heat.

If H is positive, the reaction is endothermic and absorbs heat.

If the enthalpy of products is greater, then H is positive,

the reaction is endothermic.It feels cool.

If the enthalpy of reactants is greater, then H is negative,

the reaction is exothermic.

It feels warm.

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 → O2 + 2 H20

Enthalpy Hof of reactants

H2O2 -187.8 kJ/molEnthalpy Ho

f of productsO2 0.0 kJ/molH20 -285.8 kJ/mol

Using the balanced equation:

H =

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 Ho

f of productsH3C6H5O7 -1544 kJ/mol CO2 -393.5 kJ/molNaHCO3 -951 kJ/mol H2O -241.8 kJ/molNa3C6H5O7 -2412.3 kJ/molUsing the balanced equation:

H =

Concepts to know Enthalpy – measure of heat content of a system or

substance Bonds forming release heat. H is negative Bonds breaking absorb heat. H 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. H = (Ho

f products – Hof reactants)

If H is negative, the reaction is exothermic and releases heat.

If H is positive, the reaction is endothermic and absorbs heat.