Chapter 5 - ThermochemistryBeing able to measure the amount of energy that

flows either into or out of a system is an important part of chemistry. Chemists can measure the amount of energy released by burning fuel. Food also contains energy that can be measured in calories. How can one figure out how much potential energy a particular type of food contains, or chemical reaction may have?

Thermochemistry

• Energy• Is defined as the ability to do work.• Work = (force Applied) x (distance object move)

W = F . d

Thermochemistry

• Energy• Energy can take on different forms;• Heat• Light• Sound

Thermochemistry

• Energy • Kinetic Energy (Latin: Kinesis = to move)• The energy of motion.• If an object, molecule, or atom moves as a result of a force being applied, we can say that it possesses kinetic energy.

Thermochemistry

• Energy • Kinetic Energy (Latin: Kinesis = to move)• In chemistry, molecules that move possess

kinetic energy.• Kinetic energy creates heat.• Heat is a measure of kinetic energy.

Thermochemistry

• Energy • Kinetic Energy

KE = 1/2m.v2

m = mass (kg)v = velocity (m/s)

Thermochemistry

• Energy • Potential Energy • A stick of dynamite has potential energy.• When it explodes, the potential energy is

converted into kinetic energy (heat).

Thermochemistry

• Energy • Potential Energy • The amount of potential energy contained in

food is a popular topic.• Some foods contain a very large amount of kinetic energy.

Thermochemistry

• Energy • Potential Energy • The amount of potential energy contained in

food is expressed in Calories.• There are 813 Calories in a Cinnabon.• There is enough energy in 1 Cinnabon to heat

813000 grams of water by 1°C. That’s 813 L (215 gallons!)

Thermochemistry

• Energy • Potential Energy

Eel = kQ1Q2

dThe electrostatic potential energy is equal to the product of two electrical charges times a proportionality constant (k = 8.99 x 109 J.m/C2) and inversely proportional to the distance separating them, d (meters).

Thermochemistry

• Energy• Units for Measuring Energy• The most common units for energy is the

Joule (J), calorie (cal), and the Calorie (Cal).• 1 calorie is defined as the amount of energy

required to raise the temperature of 1.o gram of water by 1.0°C.

Thermochemistry

• Energy• Units for Measuring Energy• Therefore 1 Calorie is the amount of energy

required to raise 1.0 kg of water by 1.0°C.

1 calorie = 4.18 Joules1000 calories = 1 Calorie

Thermochemistry

• Energy• Units for Measuring Energy• Convert 2.0 x 103 calories into Joules.

• 4.5 x 104 J = ? Cal

Thermochemistry

• Energy• Chemical Reactions and Energy• A chemical reaction that releases energy is

called an exothermic reaction.• The prefix ‘exo’ means outside, or to leave.

Thermochemistry

• Energy• Chemical Reactions and Energy• An Endothermic reaction absorbs energy

from its surroundings.• The prefix ‘endo’ means to enter or absorb.• An ice-pack absorbs energy from your body in the form of body heat.

Thermochemistry

• Energy• Physical Changes and Energy• Condensation and freezing are examples of

an exothermic process.• Energy has to be released when a substance

changes from a gas to a liquid, or a liquid to a solid.

Thermochemistry

• Energy• Physical Changes and Energy• Vaporization, evaporation, and melting are

examples of an endothermic phase change.• Energy is absorbed when a substance

changes from a liquid to a gas or a solid to a liquid.

Thermochemistry

• Energy• Heat Capacity (Specific Heat)• The amount of energy required to raise the

temperature of a substance by 1°C.• The Specific Heat Capacity (Specific Heat) of

a substance is the amount of energy required to raise the temperature of 1.0 gram of the substance by 1.0 °C.

Thermochemistry

• Energy• Heat Capacity (Specific Heat)• The unit of the calorie is defined using the

specific heat of water.• 1 calorie is the defined as the amount of

energy required to raise the temperature of 1.0 gram by 1.0°C.

Thermochemistry

• Energy• Energy Calculations• We can relate energy, mass, and temperature with the

following equation;

q = m x c x ΔT

q = energy (J. cal, or Cal)m = mass (g or Kg)c = specific heat constant (J/g.°C)

The units of specific heat may vary. (J/g.°C) (cal/g.°C) (cal/kg.°C) (Cal/g.°C) (Cal/kg.K)

Thermochemistry

• Energy• Specific Heat Values

Thermochemistry

• Energy• The sign of ‘q’• If ‘q’ has a positive value, energy is being

absorbed by the substance.

• If ‘q’ has a negative value, energy is being released from the substance.

Thermochemistry

• Energy Problems• How many Joules of energy is required to

heat 1.0 L of water from 23.0°C to 100.0°C? (cH2O = 4.18 J/g.°C)

Thermochemistry

• Energy Problems• How many calories of energy is lost when

a 400.0 gram piece of iron cools from 200.0°C to 50.0°C? (cFe = 0.11 cal/g.°C)

Thermochemistry

• Calorimetry• A calorimeter is an insulated container

that can measure the amount of energy that either flows into

or out of an object or chemical reaction.• Water is usually used to capture, or release the energy.

Thermochemistry

• Calorimetry• When using a calorimeter, it is assumed that

the energy lost or gained by the object or reaction, is equal to the energy lost or gained by the water in the calorimeter.

q (lost by object) = q (gained by calorimeter)

orq (gained by object) = q (lost by calorimeter)

Thermochemistry

• Calorimetry• For an exothermic reaction, we can assume

that; -q (reaction or object) = +q (calorimeter)

-mcΔT (reaction or object) = +mcΔT (calorimeter)

Thermochemistry

• Calorimetry• A 200.0°C piece of lead, with a mass of 12.0 g,

is placed into a calorimeter that holds 300.0 grams of water. The temperature of the water rose by 18.0°C. Calculate the experimental value for the specific heat constant for lead. (specific heat of water = 4.18 J/g . °C)

Thermochemistry

• Calorimetry• 10.0 mL of a 1.0 M aqueous solution of HNO3 is

added to 10.0 mL of a 1.0 M aqeous solution of NaOH. Each solution is recorded as having a temperature of 23.0°C before they are mixed. After the two solutions are mixed, the temperature increases to 30.0°C . Assuming that each solution is mostly water, calculate the enthalpy change (ΔH) of this chemical reaction.

Thermochemistry• Energy and Phase Changes• The graph shows the continuous heating of

water versus time.• Describe what is happening.

Thermochemistry• Energy and Phase Changes• The temperature of a substance remains

constant when it is going through a phase change.

• We can calculate the amount of energy absorbed in the sloped regions by using q=mcΔT.

Thermochemistry• Energy and Phase Changes• We can calculate the amount of energy

absorbed during the phase changes by using the heat of fusion,

ΔHfus, or the heat

of vaporization, ΔHvap.

Thermochemistry• Energy and Phase Changes• Calculate the amount of energy absorbed when

5.00 grams of ice melts and changes into water. ΔHfus H2O = 334 J/g

Thermochemistry• Energy and Phase Changes• Calculate the amount of energy released when

18.00 grams of steam condensed and changes into water.

ΔHvap H2O = 2260 J/g

Thermochemistry• Energy and Phase Changes• Calculate the amount of energy that is required

to heat water from -20.0°C to 80.0°C. ΔHfus H2O = 334 J/g

Thermochemistry• Energy and Phase Changes• Cooling Curves

Thermochemistry

• Thermochemical Equations• The change in the amount of energy of a

chemical equation is expressed as an Enthalpy Change (ΔH).

• Endothermic Reaction = +ΔH• Exothermic Reaction = -ΔH

Thermochemistry

• Thermochemical Equations• We can calculate the theoretical ΔH of a

chemical reaction if we know the Heats of Formation (ΔHf) of the reactants and products.

N2(g) + 2H2(g) 2NH3(g)

ΔHf N2 = 0 kJ/mol

ΔHf H2 = 0 kJ/mol

ΔHf NH3 = -46 kJ/mol

Thermochemistry

• Thermochemical Equations• Hess’s Law – The Heat of a chemical

reaction (ΔHreaction) can be calculated by taking the difference of the heat of formation of the product(s) (ΔHproducts) and heat of formation of the reactant(s) (ΔHreactants).

• ΣΔHreaction = ΣΔHproducts - ΣΔHreactants

Thermochemistry

• Thermochemical Equations

Thermochemistry

• Thermochemical Equations• Hess’ Law• Calculate ΔHreaction for the combustion of

methane, CH4(g) using Hess’ Law.

Thermochemistry

• Thermochemical Equations• Hess’ Law• Calculate ΔHreaction for the neutralization

reaction between HCl and NaOH.