Thermochemistry

Thermochemistry

• The study of heat changes in chemical reactions

• Exothermic: reactions that release heat• Endothermic: reactions that absorb heat• Enthalpy: the heat content of a system at

constant pressure

Units of Heat

• 1 Calorie = 1 kilocalorie = 1000 calories

• 1 J = 0.239 cal

• 4.186 J = 1 cal

Food Calorie

Heat Capacity

•The heat capacity of an object depends on:–mass –chemical composition.

•The amount of heat needed to increase the temperature of an object exactly 1°C is the heat capacity of that object.

Specific Heat

q = mcΔTWhere:

q = heat (energy)m = massc = specific heatΔT= temperature change

Specific Heat of Substances

Why is the sand hot and the water cool at the beach?

Jet Stream

Enthalpy

• The heat content of a system at constant pressure is the same as a property called the enthalpy (H) of the system.

Calorimetry• The measurement of

heat changes for physical and chemical processes

Calorimetry

CalorimetryTwo Key Equations

1. qreaction = -qsurroundings

qr = -qs

2. q = mcΔT = ΔHWhere: m = mass

c = specific heatΔT= temperature

change

Calorimetry Example

1. 50 mL of water is placed in a beaker. A piece of calcium is weighed and placed in the cup. The temperature change of the water is measured. Calculate the heat released to the water by the calcium. Note: c = 4.184 J/g°C

2. Calculate the heat released per mole of calcium based on the previous results

Thermochemistry

• Exothermic reaction: Heat is a productC3H8 + 5O2 → 3CO2 + 4H2O + 2043 kJ

ΔH = -2043kJ

H

Reaction Progress (t)

Thermochemistry

• Endothermic reaction: Heat is a reactant

C + H2O + 113 kJ → CO + H2

ΔH = +113kJ

H

Reaction Progress (t)

Enthalpy (ΔH)• Heat content of a system at constant pressure

• ΔH = Hproducts - Hreactants

• ΔH◦ is the standard enthalpy reported at 25 degrees C and 1 atm.

Sign of Sign of ΔΔH H ProcessProcess HeatHeat

++ endothermicendothermic absorbedabsorbed

-- exothermicexothermic releasedreleased

Standard Heats of Formation

•For a reaction that occurs at standard conditions, you can calculate the heat of reaction by using standard heats of formation.

Standard Heats of Formation

Entropy and Reaction Tendency

•There is a tendency in nature to proceed in a direction that increases the randomness of a system.

•A random system is one that lacks a regular arrangement of its parts.

•This tendency toward randomness is called entropy.

•Entropy, S, can be defined in a simple qualitative way as a measure of the degree of randomness of the particles, such as molecules, in a system.

Standard Entropy Changes for Some Reactions

Entropy

•In a solid, the particles are in fixed positions, and we can easily determine the locations of the particles.

•In a liquid, the particles are very close together, but they can move around. Locating an individual particle is more difficult. The system is more random, and the entropy is higher.

•In a gas, the particles are moving rapidly and are far apart. Locating an individual particle is much more difficult, and the system is much more random. The entropy is even higher.

Entropy

• Absolute entropy, or standard molar entropy, of substances are recorded in tables and reported in units of kJ/(mol•K).

• Entropy change, which can also be measured, is defined as the difference between the entropy of the products and the reactants.

• An increase in entropy is represented by a positive value for ∆S, and a decrease in entropy is represented by a negative value for ∆S.

Free Energy•Processes in nature are driven in two

directions: toward least enthalpy and toward largest entropy.

•As a way to predict which factor will dominate for a given system, use this equation:

∆G0 = ∆H0 – T∆S0

•This combined enthalpy-entropy function is called the free energy, G, of the system; it is also called Gibbs free energy.

Relating Enthalpy and Entropy to Spontaneity

∆G0 = ∆H0 – T∆S0

A spontaneous reaction is one that goes to completion unaided

Example: rusting, ice melting