Thermochemistry

Thermochemistry is the study of heat changes that occur during chemical reactions.

Energy - the capacity for doing work or supplying heat.When energy is stored within the structure of substances, it is called chemical potential energy.

Ex. When gasoline burns, the potential energy stored in its chemical bonds is released to do work, such as propelling a car.

Heat (q) - energy that is transferred from one object to another because of a temperature difference between them. Heat always flows from a warmer object to a cooler object. Heat cannot be measured directly.

In chemistry, we take the system’s point of view. The system is our reaction. The surroundings are everything else in the universe. The direction of heat flow is given from the point of view of the system. Engineers use the opposite point of view.

There are 2 types of reactions when it comes to heat:

1. Endothermic process - system absorbs heat from the surroundings. (heat flows into the system)

Heat is positive; q>02. Exothermic process - system releases heat to the surroundings.(heat flows out of the system)

Heat is negative; q<0

Ex. Endothermic or exothermic process?

-evaporating alcohol-leaves burning-boiling water-water cooling-melting ice-freezing water

Law of Conservation of Energy- in any chemical or physical process, energy is neither created nor destroyed.

Ex. Heat may be lost by the system, but it is not destroyed. It is transferred to the surroundings.

Energy units:calorie (cal) and joule (J)

calorie - quantity of heat needed to raise the temperature of one gram of water 1oC.

4.18 joule = 1 calorie

Heat capacity- amount of heat needed to raise the temperature of an object 1oC. It depends on mass and composition.

Ex. It takes more heat to increase the temperature of a large pot of water than a small cup of water. It takes more heat to raise the temperature of water than metal.

Specific Heat Capacity(C)-The amount of heat it takes to raise the temperature of 1 g of a substance 1oC.

specific heat units = cal/goC or J/g0C

Water has a uniquely high specific heat compared to other substances.

To calculate the heat energy required for a temperature change, use the following formula: q = (m)(C)(T)q = heat energy either absorbed or released

(J or cal)m = mass (g)C = specific heat of the object

(cal/goC or J/goC)T= change in temperature of object (oC)

(use p. 159 specific heat table)

Example 1

How much energy is required to heat an iron nail with a mass of 7.0 g from 25oC

until it becomes red hot at 752oC?

Example 1

q = mC TC Fe= 0.45J/goCT = 752-25 = 727oCq = 7.0g(0.45J/goC) 727oCq = 2300 J

Example 2

If 5750 J of energy are added to a 455 g piece of glass at 24.0oC, what is the final temperature of the glass?

(C= 0.50J/goC)

Example 2

q = mC TC glass= 0.50J/goCT = ?5750J = 455g(0.50J/goC) TT = 25oCTfinal = 24.0 + 25 = 49oC

Example 3

A 30.0 g sample of an unknown metal is heated from 22.0oC to 59.2oC. During the

process, 1.00 kJ of energy is absorbed by the metal. What is the specific heat of the metal?

Example 3

C = q/(m T)T = 59.2-22.0 = 37.2oCC = 1000J/ (30.0g)(37.2oC)C = 0.896 J/goC

Example 4

If it takes 3590 calories to heat up a sample of water by 12.2oC, what is the

mass of the water?

q = mC T3590 cal = m (1.00cal/goC)12.2oCm = 294 g

Calorimetry is the measurement of heat change for chemical and physical processes.

heat released by system = heat absorbed by surroundings

Calorimeter- insulated device used to measure heat change in a chemical or physical process.

Ex. A 25 g sample of a metal at 75.0oC is placed in a calorimeter containing 45 g of H2O at 23.0oC. The temperature stopped changing at 29.4oC. What is the specific heat of the metal?q = mC Tqwater = qmetal

Twater = 29.4-23.0 = 6.4oCTmetal = 75.0-29.4 = 45.6oCqwater = 45g(4.18J/goC)6.4oC = 1204J1204J = 25g(Cmetal)45.6oCCmetal = 1.1J/goC

Enthalpy (ΔH)-heat content of the system

Enthalpy change ( H ) is used to represent the amount of energy absorbed or lost by a system as heat during a process at constant pressure H = q = m C T

Exothermic reactions have - H

Endothermic reactions have + H

Thermochemical equations –includes the quantity of energy released or absorbed as heat during the reaction

Physical states must be included.

CH4(g) + 2O2(g) CO2(g) + 2H2O(g) + 890 kJ H = -890 kJ (energy is released)exothermic

2H2O + 241.8 kJ 2H2(g) + O2(g) H = +241.8 kJ (energy is required)endothermic

Exothermic - energy is a productEndothermic - energy is a reactant

H is also called the heat of reaction which is the quantity of energy released or absorbed as heat during a chemical reaction.