Heat & Hess’ LAW

Thermodynamics and Hess’ law

Thermodynamics Study of energy relationships in a

chemical system

We will look at energy in terms of three concepts

1. Temperature2. Heat 3. Enthalpy (H)

Key Terms that you should know/recognize

1. Temperature - measurement of avg. KE of particles in an object

2. Heat - heat is the transfer of energy btw 2 objects due to a ∆ temperature

3. Enthalpy ( H) - total energy in a chemical system- we will be concerned with the TRANSFER of enthalpy (∆ H)

Measuring All these “things”

Temperature ( Celsius and Kelvin) Energy (joules or calories)

• Ability of a system to do work or supply ( or produce) heat

Heat (CANNOT MEASURE) • We can only measure through changes in

temperature

Tro's "Introductory Chemistry", Chapter 3 6

Units of Energy Calorie (cal) is the amount of energy

needed to raise one gram of water by 1 °C.• kcal = energy needed to raise 1000 g of

water 1 °C.• food calories = kcals.Energy Conversion Factors

1 calorie (cal) = 4.184 joules (J)

1 Calorie (Cal) = 1000 calories (cal)

1 kilowatt-hour (kWh) = 3.60 x 106 joules (J)

Change in Heat Example

• Window in the winter time

Energy always flows in the same direction

→ When does the energy flow stop?

Enthalpy (H) Energy is stored in chemicals, found

in bonds that hold atoms together

As chemical rxn takes place, bonds break, new bonds are created, energy is exchanged

Change in energy is change in enthalpy

Exothermic vs. Endothermic

Energy can be released or absorbed

Heat releasing (Exothermic) Reactants --> Products + E H = (-)

Heat Absorbed (Endothermic) Reactants + E --> Products H =

(+)

S + O2 SO2 ∆H= -296 kJ/mol

a. How much heat is released when 275 grams of sulfur is burned?

S + O2 SO2 ∆H= -296 kJ/mol

b. How much heat is released when 25 mol of sulfur is burned in excess oxygen?

S + O2 SO2 ∆H= -296 kJ/mol

c. How much heat is released when 150.0 grams of sulfur dioxide is produced?

Hess’ Law Some chemical reactions we cannot

carry out in a calorimeter they release or absorb tons of energy

We must indirectly calculate the change in energy for these reactions

We use Hess’ Law

Hess’ Law Before we discuss Hess’ Law we must

first talk about Energy

Energy is a state function• This means that the pathway one uses to

get from one energy level to another is not important

Hess’ Law Change in enthalpy for any equation

can be calculated without actually carrying out the reaction

Just simply add up the H of other related experiments

2NO2 (g) --> N2O4 (g)

N2(g) + 2O2(g) --> 2NO2 (g) H = 67.8 kJ

N2(g) + 2O2 (g) --> N2O4 (g) H = 9.67 kJ