Chapter 16

Reaction Energy

Thermochemistry

The study of the transfers of energy as heat that accompany chemical reactions and physical changes.

Heat and Temperature Calorimeter – measures the energy absorbed or

released as heat in a chemical or physical change. Temperature – measures the average kinetic energy

of the particles in a sample of matter. Greater the kinetic energy of the particles in a

sample, the higher the temperature is and the hotter it feels.

Joule – unit of heat as well as all other forms of energy.

Heat – energy transferred between samples of matter. (moves spontaneously from matter at a high temperature to matter of a lower temperature.

Specific Heat – quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius.

The quantity of energy transferred as heat during a temperature change depends on the nature of the material changing temperature, the mass of the material changing temperature, and the size of the temperature change.

The difference depends on the metal’s differing capacities for absorbing this energy.

A quantity called specific heat can be used to compare heat absorption capacities for different materials.

cp = q / m x delta T (Specific heat at a given pressure)

q = cp x m x delta T (Quantity of energy gained or lost with a change in

temperature.)

Enthalpy of Reaction pg534 The energy absorbed as heat during a

chemical reaction at constant pressure is represented by delta H. Enthalpy Change – amount of energy

absorbed by a system as heat during a process at constant pressure. Delta H = Hproducts – Hreactants

Enthaply of Reaction is the quantity of energy transferred as heat during a chemical reaction. (Heat of reaction)

Thermochemical Equation pg535 An equation that includes the quantity of

energy released or absorbed as heat during the reaction. Examples:

2H2(g) + O2(g) 2H2O(g) + 483.6 kJ (exothermic) Enthaply change is negative = energy released

Delta H = - 483.6 kJ 2H2O(g) + 483.6 kJ 2H2(g) + O2(g) (endothermic)

Enthaply change is positive = energy absorbed Delta H = + 483.6 kJ

See rules: pg 537

Enthalpy of Formation pg537

Formation of a compound from its elements in their standard form. The molar enthalpy of formation is

the enthalpy change that occurs when one mole of a compound is formed from its elements in their standard state at 25oC and 1 atm. Delta Ho

f

Appendix Table A-14 pg 862

Enthalpy of Reaction Calculations pg 539 Pg.552 Problem 15a

-1207.6 -634.9 -393.5 CaCO3(s) -------> CaO(s) + CO2(g) -1207.6 0 0 0 CaCO3 ------- Ca + C + 3/2O2 Hf

0 = +1207.6 kJ 0 0 -634.9 Ca + 1/2O2 - CaO Hf

0 = -634.9 kJ 0 0 -393.5 C + O2 --- CO2 Hf

0 = -393.5 kJ __________________________________________ CaCO3(s) ------ CaO(s) + CO2(g) H = +179.2 kJ

(endothermic and unstable) Hint: Hf

0 = heat of formation + = endothermic - = exothermic More negative the enthalpy of formation is, the more stable a

compound is.

Driving Force of Reactions Two factors that determines if a reaction will occur

spontaneously (Define spontaneously?): Change in energy Randomness of the particles

Tendencies (Enthalpy + Reaction): Exothermic: energy released, less energy in

products, products resist to change, more stable. Natural direction that leads to a lower energy state.

Endothermic: energy absorbed, not spontaneous, products are at higher potential energy, and are less stable. Need outside influence (heat etc..) to proceed.

Tendencies (Entropy + Reaction): Increase [(+) delta S] in randomness (entropy)

reaction is spontaneous.

Free Energy Processes in nature are driven in two directions: toward

least enthalpy and toward largest entropy. When these two oppose each other, the dominant factor determines the direction of change. This combined enthalpy-entropy function is called the free energy, G, of the system; it is also called Gibbs free energy.

Natural processes proceed in the direction that lowers the free energy of a system.

Only the change in free energy can be measured. Delta G0 = delta H0 – T delta S0

The reaction will be spontaneous if you have a negative delta G.

The reaction will not be spontaneous if you have a positive delta G.

Another option: See table 2 pg 549