This document provides information on energy changes that occur during chemical reactions. It discusses exothermic and endothermic reactions, and defines important terms like heat of reaction (ΔH). Several factors that affect the rate of chemical reactions are also explained, such as temperature, concentration, and the presence of catalysts. Common applications of chemical reactions like combustion and fire control are described. The document is submitted as part of a lesson on changes, energy, and time for a chemistry class.

1. UNIT XII
CHANGE, ENERGY
AND TIME
Submitted by:
AILEEN GOMEZ
KRISTINE ANN PIMENTEL
ALBERT BERNAL
Submitted to:
MRS. LOURDES MEJIA

2. LESSON A. ENERGY CHANGES IN CHEMICAL REACTIONS
Every day you encounter changes in matter, which involve energy changes. You
cook food with the heat released from the burning of kitchen fuel- wood, charcoal, or
liquefied petroleum gas. The food you eat serves as the fuel for your body, enabling
you to do all your daily activities and for your body organs to carry out their vital
functions.
The process of digestion and body metabolism involves a series of chemical
changes, whereby the complex food molecules- fats, proteins, and carbohydrates are
changed into simpler molecules. The simpler food molecules enter the cells and
participate in cellular respiration. This oxidation process, which releases energy which
is used for basal metabolism, growth and repair of body tissues, and physical activities.
During a chemical or a physical change, energy is absorbed or released as heat. Heat
is a common term we often use, but in chemistry, it has a different meaning. Heat is
energy transferred as a result of a difference in temperature. As we study the flow and
transformation of heat, we keep track of the energy of a system and its surroundings.
You have learned that a system is anything we isolate for the purpose of study. This
may be a solution in beaker, a gas in a covered flask or reaction mixture. Everything
else around the system constitutes its surroundings.
EXOTHERMIC and ENDOTHERMIC PROCESSES
When atoms in the reactants are rearranged during a chemical reaction, products
with different energies are produced.

3. Chemical reactions can occur spontaneously if the reactants possess more potential
energy (stored energy) than the products. This type of reaction occurs spontaneously
because of the downhill energy path (from more potential energy to less). These
reactions are called exothermic (heat-producing) reactions, because potential energy
is converted to heat as the reactions proceed.
Exothermic reaction:
Energy lost by system= energy gained by surroundings
Conversely, endothermic (heat-absorbing) reactions do not occur spontaneously
because of the uphill energy path that exists. The products of endothermic reactions
contain more potential energy than the reactants. As a result, energy must be added to
trigger an endothermic reaction.
Endothermic reaction:
Energy gained by the system= energy lost by the surroundings

4. HEAT OF REACTION (∆H)
Think of a chemical reaction as a process in which a system changes from initial state
the reactants to a final state (the products). Many chemical reactions take place in
containers open to the atmosphere and therefore occur at a constant pressure of
about 1 atm. To keep track of energy changes at constant pressure, chemist uses the
term enthalpy, H, and the enthalpy change or heat of reaction, ∆H. The enthalpy H is
energy or heat content of the substance. It is dependent on the nature of the
substance, its physical state and temperature. The heat of reaction, ∆H is equal to the
heat released or absorbed at constant pressure during a chemical reaction.
A reaction may be written in general form:
A + B C + D
Each reactant and product has an enthalpy H. the sum of the enthalpies of reactants
(∑Hr) and the sum of the enthalpies of products (∑Hp).
∑Hr= HA + HS
∑Hp= HC + HD
∆H = ∑Hp - ∑Hr
When the reaction is endothermic, energy is transferred to a constant pressure system
as heat, the enthalpy increases by that amount.
∑Hp> ∑Hr
Thus for endothermic process, ∆H> 0 or positive.

5. When the reaction is exothermic, energy is released as heat from a constant pressure
system, the enthalpy of the system decreases by that amount.
∑Hp <∑Hr
For an exothermic process, ∆H<0 or negative.
HEAT FROM CHEMICAL REACTIONS- SOME APPLICATIONS
Industries also need a continuous supply of energy. Most industries use chemical
energy from petroleum and its products. This energy is released when these material
are burned.
In all these changes, energy flow either from the system to the surroundings or from
the surroundings to the system but the total energy of the system and of all its
surroundings remains constant. This reflects the law of conservation of energy.
LESSON B. NATURE AND RATE OF CHEMICAL REACTIONS
The rate of reaction describes how fast reactants are used up and products are
formed. Reactions proceed at different rates. Some are very fast, like explosion or
firecrackers. Others, such as the reaction involved in cooking or rusting take a
moderate amount of time, from several minutes to month. Still others take much
longer. The human aging process continues for decades and the formation of soil from
dead plants take hundreds and thousands of years.

6. Chemical reactions are processes by which atoms or molecules are redistributed,
resulting in different substances with unique properties. Many industries rely on large-
scale chemical reactions to make products, such as alloys, fertilizers, and building
materials (including glass and concrete), that are vital to modern life. Chemical
reactions are classified into different categories according to the mechanics of the
reactions. The original elements or compounds involved in a chemical reaction are
called reactants, and the chemicals that result are called products.
HOW CHEMICAL REACTIONS TAKE PLACE
All natural system in the universe continuously seeks a state of maximum stability.
Spontaneous changes follow this tendency. Spontaneous processes occur on their
own, that is, without any external driving force. They take place in a definite direction.
WHAT MAKES REACTION OCCUR?
Combustion or burning meets the two conditions for spontaneity. First the reaction
results in greater disorder in the system. Second, the energy of the system decreases.
Collisions may occur between any molecules at any given time. For a chemical
reaction to take place, the molecules must collide. Not all collisions, however, result in
a chemical reaction. Reactions occur only when two conditions are met. First, the
colliding molecules must have the sufficient energy to reach an activated state. Second,
the molecules must be properly oriented with respect to each other on collisions.
At any given temperature, the particles in a system possess a definite amount of
energy corresponding to the average energy of all particles present. The energy of most

7. particles is near the average value. Some particles, however, have higher than the
average energy; others have lower than the average energy.
The reactant molecules must have a minimum energy. The minimum energy required
to start a reaction is called the energy of activation.
FACTORS AFFECTING REACTION RATE
Reaction rate is measured in terms of the amount of reactants being used up the
amount of products being formed per unit of time. Some reactions occur
instantaneously, like an explosion; others occur very slowly, like the fading of paints;
still others occur at a moderate rate. The rate of a reaction may also vary depending on
the prevailing conditions.
Chemical reactions occur when certain physical and chemical factors make conditions
energetically favorable for the reactants to combine into products. Some factors, such
as the potential energy (stored energy) associated with the reactants, can trigger a
spontaneous chemical reaction. If the products have a higher level of entropy (disorder
among the particles) than the reactants, this difference can also initiate a chemical
reaction. External factors, such as heat or the presence of a catalyst (a substance that
increases reaction rate without being chemically changed), can trigger or increase the
rate of a reaction.
Several factors affect the rate of a chemical reaction. Most factors can be controlled.
This is advantageous for manufacturing chemist involved in a chemical processes.
Manufacturing processes would be more economical if more products are formed in
the shortest time possible and in the most efficient way. Undesirable reactions, such as

8. corrosion of metals or the decomposition of food products, can be delayed or slowed
down, or even prevented.
TEMPERATURE EFFECTS ON REACTION RATE
An increase temperature supplies energy to the reactant particles. Thus at higher
temperatures, more reactant particles have enough energy to reach the activate state.
All reactions increase in rate when temperature increases. Generally, reaction
rates double for every 10 Co
.
The effect of temperature on the rate of reaction also applies to body processes. On
the other hand, persons exposed to cold weather or other environmental agents have
to guard against hypothermia. Hypothermia is a fall in the temperature of the inner
part of the body.
PRESCENCE OF A CATALYST
Some reactions take place very slowly even with a high concentration of reactants.
Substance causing reactions occur faster but are themselves unchanged at the end of
the reaction are called catalyst. On the other hand, inhibitors are substance that slows
down chemical reactions.
Catalysts are substances that trigger or speed up chemical reactions (without
chemically altering the catalysts in the process). A catalyst combines with a reactant to
form an intermediate compound that can more readily react with other reactants. An
example of this is the formation of sulfur trioxide (SO3), which is an important
ingredient for producing sulfuric acid (H2SO4). Without a catalyst, sulfur trioxide is made

9. by combining sulfur dioxide (SO2) with molecular oxygen: 2SO2 + O2 → 2SO3. Because
this reaction proceeds very slowly, manufacturers use nitrogen dioxide (NO2) as a
catalyst to speed production of SO3:
Step One: NO2 (catalyst) + SO2 → NO + SO3 (SO3 is extracted and combined with steam
to produce sulfuric acid)
Step Two: NO (from Step One) + O2 → NO2 (catalyst that is reused in step one)
In the above reactions, nitrogen dioxide (NO2) acts as a catalyst by combining with
sulfur dioxide (SO2) to form both sulfur trioxide (SO3) and nitrogen monoxide (NO). The
sulfur trioxide is removed from the process (to be used in the production of sulfuric
acid). Nitrogen monoxide (NO) is subsequently combined with molecular oxygen (O2) to
produce the original catalyst, nitrogen dioxide (NO2), which can be continually reused
to catalyze sulfur trioxide (SO3).
A catalyst has specific action. A catalyst for one reaction may not affect other
reactions. Catalase is also found in blood of animals, chicken liver, yeast, fresh beef and
fresh pork.
WHAT ARE ANTIOXIDANTS?
Antioxidants are used widely in various industries, especially in food and paint
manufacture. They prevent the easy spoilage of food and the fading of paints, both of
which are oxidation processes, by being oxidized first. They use up oxygen, thus
protecting foods and paints from oxidation.

10. THE CHEMISTRY OF FIRE CONTROL
In industry, energy from fire is used to refining metals and making glass ceramics
and metal ware. Fire from burning fuels also produces stem for electric power for
generation. While fire is useful in many ways, uncontrolled fire can cause damage or
loss of life and property.
Putting out fires requires the application of a number of chemistry principles that you
have studied. In most cases, water is used to extinguish fires. Every gram of water
vaporized to steam absorbs 2259.4 joules (540 calories) of heat from the burning
substance. This has a considerable cooling effect. Furthermore, when water changes to
steam, its volume increases 1700 times. This large volume of steam displaces an equal
volume of air surrounding the fire, thus preventing contact of the burning material with
air. However, water cannot be used to put all types of fires. It cannot be used to
extinguish fires caused by oil, kerosene, gasoline and other liquid fuels that float on
water.
Water should be used with care when putting out electrical fires. Water itself is a
poor electrical conductor, but it contains impurities which make it a conductor. As a
general rule, water should not be used to extinguish chemical fires caused by carbide
(kalburo), peroxides, metallic sodium, and magnesium dust. These substances react
with water producing much heat. The products are flammable gases that easily ignite in
the heat.
Carbon dioxide is used in certain fire extinguishers. Carbon dioxide extinguishers are
used for fires involving flammable liquid and electrical equipment. Carbon dioxide is
noncombustible and unreactive toward most substances. It does not conduct
electricity. As a gas, it can penetrate and spread over all parts of fire.

11. In case you want to slow down a chemical reaction, the opposite conditions are
required (low concentration of reactants, lower temperature, absence of catalyst or
addition of an inhibitor, and less area of contact between reactants). The rate of
reaction as affected by the nature of reactants is beyond your control. It depends on
the chemical nature of the substances.