Endothermic and Exothermic Reactions

Front

Exothermic Reaction Noun

---

[ek-so-thur-mik ree-ak-shun]

Back

Exothermic Reaction

---

A chemical reaction that releases energy from the system to the surroundings, usually in the form of heat or light.

---

Example: Burning wood is a combustion reaction where fuel (hydrocarbon) and oxygen react to produce carbon dioxide, water, and release energy as heat and light.

Front

Endothermic Reaction Noun

---

[en-do-thur-mik ree-ak-shun]

Back

Endothermic Reaction

---

A chemical reaction that absorbs energy from its surroundings, causing a drop in the temperature of the immediate environment.

---

Example: This diagram shows an instant cold pack's contents. When the inner water bag breaks, it mixes with ammonium nitrate, absorbing heat and making the pack cold.

Front

Enthalpy Noun

---

[en-thal-pee]

Back

Enthalpy

---

A thermodynamic quantity equivalent to the total heat content of a system, representing the energy available as heat.

---

Example: This icon shows energy (arrows) moving into a chemical system (test tube), representing an endothermic reaction where the system's enthalpy increases.

Front

Enthalpy Change (ΔH) Noun

---

[en-thal-pee cheynj]

Back

Enthalpy Change (ΔH)

---

The measure of the overall change in heat energy of a chemical system during a reaction at constant pressure.

---

Example: This diagram shows an endothermic reaction, where the products have higher enthalpy (energy) than the reactants, resulting in a positive enthalpy change (+ΔH).

Front

System Noun

---

[sis-tuhm]

Back

System

---

In chemistry, the specific portion of the universe being studied, such as the reactants and products in a reaction.

---

Example: This diagram shows a 'system' (the area of interest), its 'surroundings' (everything outside), and the 'boundary' that separates them, where energy exchange occurs.

Front

Surroundings Noun

---

[suh-roun-dingz]

Back

Surroundings

---

Everything in the universe that is not part of the system being studied and can exchange energy with it.

---

Example: This diagram shows that the 'surroundings' is everything outside a defined 'system,' separated by a 'boundary.' Energy can be exchanged between the system and its surroundings.

Front

Reactants Noun

---

[ree-ak-tuhnts]

Back

Reactants

---

The initial substances that enter into and are altered during the course of a chemical reaction.

---

Example: This diagram shows that reactants, like methane (CH₄) and oxygen (O₂), are the starting materials on the left side of a chemical reaction.

Front

Products Noun

---

[prod-ukts]

Back

Products

---

The new substances that are formed as the result of a chemical reaction taking place.

---

Example: In this chemical reaction, the substances on the right (carbon dioxide and water) are the products, which are formed from the reactants on the left.

Front

Potential Energy Noun

---

[po-ten-shuhl en-er-jee]

Back

Potential Energy

---

The stored energy within a chemical system, related to the arrangement of atoms and the strength of chemical bonds.

---

Example: This image uses a cyclist on a hill as an analogy for potential energy changes. Going uphill requires energy input to increase potential energy (like an endothermic reaction), while going downhill releases that stored energy (like an exothermic reaction).

Front

Bond Energy Noun

---

[bond en-er-jee]

Back

Bond Energy

---

The measure of energy required to break a chemical bond or the energy released when a bond is formed.

---

Example: This image shows a chemical reaction and a table of bond energies, which are the amounts of energy associated with specific chemical bonds.