Exothermic and Endothermic Reactions

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Exothermic Reaction Noun

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

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Exothermic Reaction

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A chemical reaction or process that releases energy from the system into the surroundings, typically in the form of heat.

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Example: This diagram shows heat energy (represented by arrows) being released from a central source, making the surroundings hotter, which defines an exothermic process.

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Endothermic Reaction Noun

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

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Endothermic Reaction

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A chemical reaction or process in which the system absorbs energy from its surroundings, typically in the form of heat.

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Example: This diagram shows the inside of an instant cold pack. When the inner bag of water breaks, it mixes with ammonium nitrate, causing a reaction that absorbs heat.

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System Noun

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[sis-tuhm]

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System

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The specific part of the universe under study, such as the reactants and products in a chemical reaction.

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Example: This diagram shows a 'system' (the area being studied), its 'surroundings' (everything else), and the 'boundary' that separates them for tracking energy changes.

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Reactant Noun

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[ree-ak-tuhnt]

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Reactant

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A substance that is present at the start of a chemical reaction and is consumed during the reaction.

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Example: This image shows the substances on the left side of a chemical reaction, methane (CH4) and oxygen (O2), which are clearly labeled as the reactants.

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Product Noun

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[prod-uhkt]

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Product

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A substance that is formed as the result of a chemical reaction.

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Example: This diagram shows that in a chemical reaction, the new substances formed, like carbon dioxide and water, are called the products.

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Potential Energy Noun

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

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Potential Energy

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The energy stored within a chemical system, often in its chemical bonds, based on the arrangement of its particles.

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Example: A soccer player's raised leg has stored potential energy due to its position, which is converted to kinetic energy when the leg swings to kick the ball.

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Chemical Bond Noun

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[kem-i-kuhl bond]

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Chemical Bond

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A lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds.

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Example: This diagram shows two types of chemical bonds in water: strong covalent bonds holding atoms together within a molecule and weaker hydrogen bonds attracting different molecules.

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Enthalpy Noun

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[en-thal-pee]

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Enthalpy

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A thermodynamic quantity representing the total heat content of a system, where its change indicates the reaction type.

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Example: Adding heat energy increases the enthalpy of water, causing it to change from a solid (ice) to a liquid and then to a gas (vapour).

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Activation Energy Noun

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[ak-tuh-vey-shuhn en-er-jee]

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Activation Energy

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The minimum quantity of energy that reacting species must possess in order to undergo a specified chemical reaction.

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Example: This graph shows that a chemical reaction needs a push of energy, called activation energy, to get started, like pushing a ball over a hill.

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Catalyst Noun

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[kat-l-ist]

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Catalyst

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A substance that increases the rate of a chemical reaction by lowering the activation energy, without being consumed.

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Example: This diagram shows how a catalyst lowers the activation energy (the 'energy hill') of a reaction, providing an easier pathway for reactants to become products.