THERMOCHEMISTRY

THERMOCHEMISTRY
ENGR. AMIEL ROY B. PILLAZAR, ME

THERMOCHEMISTRY
Thermochemistry is the branch of chemistry concerned
with heat effects accompanying chemical reactions. It deals
with the study of the heat released or absorbed by the
chemical and physical processes. A reaction can be
exothermic (energy is released/liberated) or endothermic
(energy is absorbed).
SYSTEM AND SURROUNDINGS
A primary goal of the study of thermochemistry is to
determine the quantity of heat exchanged between a system
and its surroundings. The system is the part of the universe
being studied, while the surroundings are the rest of the
universe that interacts with the system.
TYPES OF SYSTEMS
There are three types of systems that must be understood in the
study of thermochemistry – open system, closed system, and isolated
system. An open system is a system that freely
exchanges energy and matter with its surroundings. A closed
system is a system that exchanges only energy with its surroundings,
not matter. An isolated system does not exchange energy or
matter with its surroundings.
ENERGY AND WORK
In defining a system and its surroundings, words like energy
and work are used very often. As a result, one's understanding of
a system and its surroundings can increase by understanding
energy and work. Energy is the ability to do work. Work is when
an object moves against a force.
HEAT AND TEMPERATURE
Temperature is that property of matter that determines
the direction in which heat flows spontaneously. Heat is the
energy transferred between the system and its surroundings
as a result of a temperature difference.
HEAT
It is reasonable to expect the quantity of heat (q)
required to change the temperature of a substance depend
on:
◦ how much the temperature is to be changed
◦ the quantity of the substance
◦ the nature of the substance
HEAT
Historically, the quantity of heat required to change the temperature of one gram of
water by one degree Celsius has been called the calorie (cal). The SI unit of heat is
simply the basic SI energy unit, the Joule (J).
1 𝑐𝑎𝑙 = 4.184 𝐽
The quantity of heat required to change the temperature of a system by one degree
is called the heat capacity of the system. If the system is a mole of a substance, we can
use the term molar heat capacity. If it is one gram of a substance, we call it specific heat
capacity, or more commonly, specific heat (c).
𝑐𝑎𝑙 𝐽
𝑐𝑤𝑎𝑡𝑒𝑟 = 1.00 = 4.184
𝑔℃ 𝑔℃
QUANTITY OF HEAT (q)
The quantity of heat (q) can be calculated using the formula:
𝑞 = 𝑚𝑐∆𝑡
where:
q = the quantity of heat
m = mass of the substance
c = specific heat of the substance
∆t = temperature change
EXAMPLES:
1. How much heat is required to raise the
temperature of 7.35 g of water from 21.0 ℃ to
98.0 ℃?
2. How much heat in kilojoules (kJ), is required to
raise the temperature of 8 ounces of ice water
(237 g) from 4.0 ℃ to 37.0 ℃?
LAW OF CONSERVATION OF ENERGY
Another idea that enters into calculations of quantities
of heat is the law of conservation of energy. In
interactions between a system and its surroundings, the
total energy remains constant – energy is neither created
nor destroyed.
𝑞𝑟𝑒𝑙𝑒𝑎𝑠𝑒𝑑 = 𝑞𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑
EXAMPLES:
1. A 150.0 g sample of lead is heated to the temperature of boiling water
(100 ℃). A 50.0 g sample of water is added to a thermally insulated
beaker, and its temperature is found to be 22 ℃. The hot lead is
dumped into the water, and the final temperature of the lead-water
mixture is 28.8 ℃. Calculate the specific heat of lead.
2. When 1.00 kg of lead (C = 0.13 J/g ℃) at 100 ℃ is added to a quantity
of water at 28.5 ℃, the final temperature of the lead-water mixture is
35.2 ℃. What is the mass of the water present?
HEATS OF REACTION
A heat of reaction (qrxn) is a quantity of heat exchanged

between a system and its surroundings, when a chemical
reaction occurs within the system at constant pressure.
CALORIMETRY
Heats of reaction are
experimentally determined
in a calorimeter, a device for
measuring quantities of
heat.
HEATS OF REACTION AND CALORIMETRY
Experimentally, the formula used to find the heat of reaction

is:
𝑞𝑟𝑥𝑛 = −𝑞𝑐𝑎𝑙𝑜𝑟𝑖𝑚
where: 𝑞𝑐𝑎𝑙𝑜𝑟𝑖𝑚 = 𝑞𝑏𝑜𝑚𝑏 + 𝑞𝑤𝑎𝑡𝑒𝑟
also, 𝑞𝑐𝑎𝑙𝑜𝑟𝑖𝑚 = 𝐶∆𝑡
EXAMPLE:
1. The combustion of 1.010 g sucrose in a bomb calorimeter
causes the temperature to rise from 24.92 to 28.33 degrees
Celsius. The heat capacity of the calorimeter assembly is
4.90 kJ/℃. (a) What is the heat of combustion of sucrose,
expressed in kilojoules per mole of sucrose? (b) Verify the
claim of sugar producers that one teaspoon of sugar (about
4.8g) contains only 19 Calories.
THERMOCHEMICAL EQUATION
The heat of reaction (also known as enthalpy of reaction)
is the change in the enthalpy of a chemical reaction that
occurs at a constant pressure. It is a thermodynamic unit of
measurement useful for calculating the amount of energy
per mole either released or produced in a reaction.
∆𝐻 = 𝐻𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 − 𝐻𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠
THERMOCHEMICAL EQUATION
∆𝐻 = 𝐻𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 − 𝐻𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠
Reactions that liberate heat are called exothermic reactions. For

reactions of this type, the products have a lower enthalpy than the
reactants; ∆𝐻 has a negative value.
Reactions that absorb heat are called endothermic reactions. For
reactions of this type, the enthalpy of the products is higher than the
enthalpy of the reactants; ∆𝐻 is positive.
EXAMPLES:
1. The thermite reaction is highly exothermic:
2𝐴𝑙(𝑠) + 𝐹𝑒2 𝑂3(𝑠) → 2𝐹𝑒(𝑠) + 𝐴𝑙2 𝑂3(𝑠) ∆𝐻 = −848 𝑘𝐽
How much heat is liberated when 36.0 g of Al reacts

with excess Fe2O3?
EXAMPLES:
2. Calculate the ΔH of the following reaction
𝐶𝑂2(𝑔) + 𝐻2 𝑂(𝑙) → 𝐻2 𝐶𝑂3(𝑔)
if the standard values of ΔH are as follows: CO2 (g): -

393.509 kJ /mol
H2O (l) : -241.83 kJ/mol
H2CO3(g) : -275.2 kJ/mol.

THERMOCHEMISTRY

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THERMOCHEMISTRY

ENGR. AMIEL ROY B. PILLAZAR, ME

A heat of reaction (qrxn) is a quantity of heat exchanged

Experimentally, the formula used to find the heat of reaction

Reactions that liberate heat are called exothermic reactions. For

2𝐴𝑙(𝑠) + 𝐹𝑒2 𝑂3(𝑠) → 2𝐹𝑒(𝑠) + 𝐴𝑙2 𝑂3(𝑠) ∆𝐻 = −848 𝑘𝐽

How much heat is liberated when 36.0 g of Al reacts

𝐶𝑂2(𝑔) + 𝐻2 𝑂(𝑙) → 𝐻2 𝐶𝑂3(𝑔)

if the standard values of ΔH are as follows: CO2 (g): -

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