Every chemical reaction, whether it occurs in a laboratory, within a living organism, or in the vastness of the universe, involves a change in energy. Chemical reactions are essentially processes where existing chemical bonds are broken and new chemical bonds are formed. Breaking bonds requires an input of energy, while forming new bonds releases energy. The overall energy change of a reaction is the net difference between the energy absorbed to break bonds and the energy released when new bonds form. This energy is most commonly observed as heat, but it can also manifest as light, sound, or electrical energy. Understanding these energy changes is fundamental to comprehending why reactions occur and how we can control them.

A chemical reaction that releases energy into its surroundings, typically in the form of heat, is known as an exothermic reaction. The term "exothermic" literally means "exo" (out) and "thermic" (heat). In an exothermic reaction, the energy released when new bonds are formed is greater than the energy absorbed to break the original bonds. This net release of energy causes the temperature of the surroundings to increase. If you were holding the reaction vessel, it would feel warmer, or even hot.

Observable signs of an exothermic reaction often include:
* Increase in temperature of the reaction mixture or its surroundings. This is the most common and direct indicator.
* The production of light, such as in combustion (burning) reactions, where flames are visible.
* The production of sound, as sometimes heard during rapid combustion or explosions.

Real-world examples of exothermic processes are abundant:
* Combustion (Burning): This is the most common example. The burning of fuels like wood, natural gas (methane), propane, or coal releases significant amounts of heat and light energy. This energy is harnessed for heating homes, powering vehicles, and generating electricity. For example, the burning of methane: CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l) + Energy (Heat + Light)
* Neutralization Reactions: As discussed in Chapter 5, the reaction between an acid and a base is typically exothermic. When hydrochloric acid reacts with sodium hydroxide, the test tube feels warm: HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l) + Heat
* Respiration: The metabolic process in living organisms where glucose reacts with oxygen to release energy for cellular activities is a complex series of exothermic reactions.
* Hand Warmers: These products often contain iron powder that reacts exothermically with oxygen in the air, releasing heat.

In contrast to exothermic reactions, an endothermic reaction is a chemical reaction that absorbs energy from its surroundings, typically in the form of heat. The term "endothermic" means "endo" (in) and "thermic" (heat). In an endothermic reaction, the energy absorbed to break the original bonds is greater than the energy released when new bonds are formed. This net absorption of energy causes the temperature of the surroundings to decrease. If you were holding the reaction vessel, it would feel cooler, or even cold.

Observable signs of an endothermic reaction often include:
* Decrease in temperature of the reaction mixture or its surroundings. This is the most common and direct indicator.
* The absorption of light, as seen in photosynthesis where light energy is converted into chemical energy.

Real-world examples of endothermic processes include:
* Photosynthesis: This vital process in plants absorbs light energy from the sun to convert carbon dioxide and water into glucose (chemical energy) and oxygen: 6CO₂ (g) + 6H₂O (l) + Energy (Light) → C₆H₁₂O₆ (aq) + 6O₂ (g)
* Cold Packs: Instant cold packs used for sports injuries typically contain ammonium nitrate and water in separate compartments. When the barrier is broken, the ammonium nitrate dissolves in water in an endothermic process, absorbing heat from the surroundings and making the pack feel cold.
* Melting of Ice: While a physical change, it is an endothermic process as ice absorbs heat from its surroundings to change into liquid water.
* Baking Bread: The baking process involves various endothermic reactions where heat from the oven is absorbed to transform the dough.

To better understand the energy changes during a reaction, chemists use energy profile diagrams. These diagrams plot the energy of the system against the progress of the reaction.
* Reactants: The starting materials of the reaction, shown on the left side of the diagram at a certain energy level.
* Products: The substances formed by the reaction, shown on the right side of the diagram at a certain energy level.
* Activation Energy (Ea): This is the minimum amount of energy that reactant particles must possess when they collide in order for a chemical reaction to occur. It represents an energy barrier that must be overcome for bonds to break and new bonds to form. On an energy profile diagram, it is represented by the height of the "hump" between the reactants and the products.

Energy Profile Diagram for an Exothermic Reaction: In an exothermic reaction, the products are at a lower energy level than the reactants. The difference in energy between reactants and products is released as heat. The enthalpy change (ΔH) for an exothermic reaction is negative, indicating a release of energy.
* Energy Profile Diagram for an Endothermic Reaction: In an endothermic reaction, the products are at a higher energy level than the reactants. Energy is absorbed from the surroundings to reach this higher energy state. The enthalpy change (ΔH) for an endothermic reaction is positive, indicating an absorption of energy.

While energy changes tell us if a reaction releases or absorbs heat, they don't tell us how fast the reaction will occur. Many reactions, even exothermic ones, require a significant amount of activation energy to get started. This is where catalysts come into play. A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently altered in the reaction itself. Catalysts work by providing an alternative reaction pathway that has a lower activation energy (Ea). By lowering this energy barrier, more reactant particles possess the minimum energy required for effective collisions, thus speeding up the reaction.

Catalysts are indispensable in various industries and biological systems:
* Industrial Processes:
* Haber Process: Ammonia (NH₃), a vital component for fertilizers, is produced using an iron catalyst to speed up the reaction between nitrogen and hydrogen.
* Contact Process: Sulfuric acid production relies on a vanadium(V) oxide catalyst.
* Catalytic Converters: Found in cars, these use platinum, palladium, and rhodium catalysts to convert harmful exhaust gases (like carbon monoxide and nitrogen oxides) into less harmful substances (carbon dioxide, nitrogen, and water).
* Biological Systems (Enzymes): Living organisms utilize biological catalysts called enzymes. These highly specific protein molecules facilitate nearly all biochemical reactions in our bodies, allowing them to occur rapidly at body temperature, which would otherwise be too slow to sustain life.