Heat, Q - Common Form of Energy Measured in Joules, J

Thermochemistry

Heat, Q - common form of energy measured in Joules, J

1 calorie (cal) = 4.184 J

1000 cal = 1 Calorie (Cal) food

Exothermic Reaction (Rxn): energy is given off by rxn (-DH)

Tends to be spontaneous

Activation Energy, Ea: minimum amount of energy needed to start a rxn.

A + B à C + D + energy

Endothermic Rxn: energy is gained by rxn (+DH)

A + B + energy à C + D

Enthalpy, H : measure of E in a system

Heat of formation, DHfo (enthalpy of formation)

DHr = SHf(products) – SHf(reactants)

-DH èexothermic + DH è endothermic

1. How much heat will be released if 1.0g of hydrogen peroxide (H2O2) decomposes in a bombardier beetle to produce a steam spray such as the one shown in the photograph on the opposite page?

2 H2O2(l) à 2H2O(l) + O2(g) DH= -190 kJ

P387 practice problems #1, 2

Hess’s Law: the enthalpy change for a rxn is the sum of the enthalpy changes for a series of rxns that add up to the overall reaction.

In determining overall DH, path is not important. The DH for the original rxn will be equal to the SDH of the multiple rxns that make it up.

Like simultaneous eqns, you can add chemical eqns.

C + O2 à CO2 DH= -393.5 kJ

2CO à CO2 + C DH= -172.5 kJ

2CO + O2 à 2CO2 DH= -566.0 kJ

2. The combustion of sulfur can produce SO2 as well as SO3 depending upon the supply of oxygen.

From the following reactions and their enthalpy changes,

2 SO2(g) + O2(g) à 2 SO3(g) DH= -196 kJ

2 S(s) + 3 O2(g) à 2 SO3(g) DH= -790 kJ

p391 #3, 4

Calorimetry: Measuring energy gained or released during chemical or physical changes

Specific heat: energy needed to raise 1 gram of a substance 1 degree Celsius

Q=mcDt Q, heat in J

m, mass in g

c, specific heat in J/goC

Dt, change in temperature in oC or K

Law of conservation of energy: energy cannot be created nor destroyed,

but may be transformed into different forms

Qgained =Qlost

m1c1Dt1= m2c2Dt2

3. When a 4.25-g sample of solid NH4NO3 dissolves in 60.0g of water in a calorimeter, the temperature drops from 21.0o C to 16.9oC. Calculate DH for the solution process.

NH4NO3(s) → NH4+(aq) + NO3- (aq) DH= ?

P397 #5, 6