ThermochemistryPart II : Enthalpy

Review: Temperature A measure of the average kinetic energy of

the particles that make up a substance or system

Change in temp represents a change in kinetic energy

Systems and surroundings: System: area where reaction takes place

(generally is the reaction) Surroundings: outside of the system

Closed verses open systems

Open-system both matter and energy

can freely cross from the system to the surroundings and back.

Ex: an open test tube

Closed-System energy can cross the

boundary, but matter cannot.

Ex: a sealed test tube

Isolated-System neither matter nor

energy can cross between the system and the surroundings.

Ex: The universe there are no

surroundings to exchange matter or energy with (as far as we know!)

Enthalpy (∆H) a.k.a heat of reaction Chemical rxn (and phase changes) involve

changes in potential energy not kinetic Enthalpy is the potential energy change of a

system during process like chem rxn (or phase change)

Units: kJ/mol

Chemical Rxns: The internal energy of a reactant or product

cannot be measured, but their change in enthalpy can

Enthalpy changes represent the diff between PE of products and PE of reactants

PE changes result from bonds being broken and/or formed

Breaking a bond requires energy!

Creating a bond releases energy !

(Strength of bond determines the energy needed to break or create bond)

Exothermic and Endothermic Rxns:

Exo verses Endo: If energy is released out of the system then it is

considered to be an exothermic enthalpy change (exit).

If energy is absorbed into the system then it is considered to be an endothermic enthalpy change (enter).

There are several different enthalpies that we will be looking at :

Enthalpy of Rxn (∆Hrxn) Standard enthalpy of Rxn (∆H0

rxn) -- at standard temp (25°C) and pressure (100 kPa)

Standard molar enthalpy of formation (∆H0f)

Standard molar enthalpy of combustion (∆H0

comb)

We use tables to find these VALUES!

Standard enthalpy of Rxn (∆H0rxn)

Exothermic and endothermic 3 ways to represent:

Thermochemical equation (includes energy in eq) Chem eq + ∆H0

rxn beside Enthalpy diagrams

Exothermic:

∆H0 is always negative!!!!!

Energy written on product side

1. H2 + ½ O2 H2O + 285.8 kJ

2. H2 + ½ O2 H2O ∆H0rxn= -285.8 kJ/mol

Exothermic cont’d:

Hint: Arrow down for neg

and up for pos arrow goes toward the

products

Endothermic:

∆H0 is always positive!!!!!

Energy written on reactant side

1. 117.3 kJ + MgCO3 MgO + CO2

2. MgCO3 MgO + CO2 ∆H0rxn= 117.3 kJ/mol

Endothermic cont’d:

Hint: Arrow down for neg

and up for pos arrow goes toward the

products

Compare:

Enthalpy dependent on molar amounts: Linearly dependent (multiply by the stoich

coefficients)

Ex:

Standard molar enthalpy of formation (∆H0f)

Is the quantity of energy that is absorbed (+) or released (-) when one mole of a compound is formed directly from its elements (so this is why you see ½ as coefficient)

Ex:

Standard molar enthalpy of combustion (∆H0

comb)

Enthalpy associated with combustion of 1 mol of substance

Ex:

HW Pg 643 # 15, 16, 17, 18