Chapter 17ThermochemistryThermochemistry

Law of conservation of energy

• The law of conservation of energy states that energy can be neither created or destroyed but can be converted from one form to another.

• Heat, which is represented by the symbol q, is energy that is in the process of flowing from a warmer object to a cooler object.

Heat

• When the warmer object loses heat, its temperature decreases.

• When the cooler object absorbs heat, its temperature rises.

Measuring heat

• In the metric system, the amount of heat required to raise the temperature of one gram of pure water by one degree Celsius (1°C) is defined as a calorie (cal).

• The SI unit of heat and energy is the Joule (J). One Joule is the equivalent of 0.2390 calories, or one calorie equals 4.184 Joules.

• Temperature is a measure of the average kinetic energy of the particles in a sample of matter.

• Temperature is measured in °C or K

Temperature

Important Important ConceptConceptHeat and temperature

are not the

same thing

• Specific heat (c) is the amount of energy that is required to raise the temperature of one gram of pure water by one degree Celsius (1°C).

Specific Heat

• The specific heat of water is 4.18 J/g∙°C

Specific Heat• Because different substances have

different compositions, each substance has its own specific heat.

Substance Specific Heat J/g ∙ °CWater 4.18Ethyl Alcohol 2.44Aluminum 0.897Copper 0.385Gold 0.129

• The heat absorbed or released by a substance during a change in temperature depends not only upon the specific heat of the substance, but also upon the mass of the substance and the amount by which the temperature changes.

Calculating Specific Heat

Calculating Specific Heat

• The formula for calculating specific heat is:

• q = the heat absorbed or released

• c = the specific heat of the substance

• m = the mass of the sample in grams

• ∆T is the change in temperature in °C

Calculating Specific Heat

• ∆T is the difference between the final temperature and the initial temperature or, Tfinal – Tinitial.

Try this Problem• How much heat is

given off when an 869 g iron bar cools from 940.0°C to 50.0°C?

c Fe = 0.444 J/g • °C

m = 869 g

ΔT = Tfinal – Tinitial = 50.0°C – 940.0°C = -890°Cq = c m ΔT

q = 0.444 J/g • °C x 869 g x –890.0°C

q = -343,000 J

Enthalpy• Enthalpy is the amount of energy

absorbed or lost by a system during a process at constant pressure.

• Types of enthalpy– Heat of Reaction– Heat of Formation– Heat of Combustion– Heat of Vaporization– Heat of Fusion

Thermochemical Equation

• A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products and the energy change, usually expressed as the change in enthalpy, ∆H.

• 2H2(g) + O2(g) 2H2O(g) + 967.2 kJ

Enthalpy and Change in Enthalpy• Although you cannot measure the

actual energy or enthalpy of a substance, you can measure the change in enthalpy, which is the heat absorbed or released during a chemical reaction.

• The change in enthalpy for a reaction is called the Heat of Reaction (∆Hrxn).

• You have already learned that ∆ means a change in the property.

Heat of Reaction

• ∆Hrxn is the difference between the

enthalpy of the substances that exist at the end of the reaction and the enthalpy of the substances present at the start.

The formula for Heat of Reaction is

ΔHrxn = H°f products - H°f reactants

Heat of Combustion

• The Heat of Combustion is the amount of energy released as heat by the complete combustion of one mole of a substance.

• The formula for Heat of Combustion is ΔHc = H°f products - H°f reactants

• As you can see, it is the same as Heat of reaction except for the beginning: ΔHc

Heat of Formation

• Heat of formation (H°f) is the energy needed to form one mole of a compound from its elements.

• Elements have 0 H°f

• A Heat of Formation table is found in your textbook on page 902

Heat of Vaporization

Heat of Fusion• Heat can also be absorbed or

released during a change of state.

• The heat required to vaporize one mole of a liquid is called its heat of vaporization (∆Hvap).

• The heat required to melt one mole of a solid is its heat of fusion (∆Hfus).

Solve This Problem• ΔH°f CO2(gas) = -393.0 kJ/mol

• ΔH°f H2O(liquid) = -285.8 kJ/mol

• ΔH°f C5H12(gas) = -145.7 kJ/mol

• Calculate Heat of Combustion for this reaction C5 H12 (g) + 8O2(g) 5CO2(g) + 6 H2O(l)

ΔHc = H°f products - H°f reactants

ΔHc = [(5)(-393.0 kJ/mol) + (6)(-285.8 kJ/mol)] - [-145.7 kJ/mol + 0 kJ/mol]

= -3534.1 kJ or -3530 kJ

Endothermic and Exothermic

• Endothermic reactions absorb energy from the system. The container gets cold.

• Exothermic reactions release energy from the system. The container gets hot.

Endothermic Reactions

• The products have more energy than the reactants. The products have absorbed energy.

Exothermic Reactions

• The products have less energy than the reactants. The products have given off energy.

Changes in Enthalpy

Entropy (S)• Entropy is a measure of the degree of

randomness of the particles in a system.• Gases have high entropy and solids have low

entropy.

Does your bedroomhave high Entropy?

Reaction RateReaction rate is the change in concentration of

reactants per unit time as a reaction proceeds.

Factors that influence reaction rate:1. Nature of reactants – some substances tend to

reactfaster than other.

2. Surface area – The more surface area, the faster a reaction will occur.

3. Temperature – Higher temperatures make a reactionrate increase.

4. Concentration – Higher concentrations make a reaction rate increase.

5. Presence of a catalyst – Can speed up or slow down a reaction depending on the catalyst.

Phase Changes• Phase changes occur when a

substance changes from one state to another.

• For a phase change to occur, energy must be added to or taken away from the substance.

• Phase changes are physical changes.

Phase Phase ChangesChanges Heating CurvesHeating Curvesandand

Energy Changes Terminology

• At the melting/freezing point– Heat of Fusion– Heat of solidification

• At the boiling/condensation point– Heat of Vaporization– Heat of Condensation