The Progress of Chemical Reactions

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Collision Theory Noun

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[kuh-lizh-un thee-uh-ree]

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Collision Theory

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A theory stating that reactant particles must collide with sufficient energy and correct orientation for a chemical reaction to occur.

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Example: For a chemical reaction to happen, particles must collide with the correct orientation and enough energy, as shown in the 'successful collision' where new molecules form.

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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 amount of energy required for colliding reactant particles to overcome an energy barrier and initiate a chemical reaction.

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Example: This graph shows that for a chemical reaction to start, reactants need a boost of energy, called activation energy, to get over an energy 'hill'.

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Combustion Reaction Noun

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[kuhm-buhs-chuhn ree-ak-shuhn]

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Combustion Reaction

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A rapid chemical reaction between a substance and an oxidant, usually oxygen, that produces both heat and light.

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Example: This diagram shows a combustion reaction where a hydrocarbon (fuel) reacts with oxygen to produce heat, light, water, and carbon dioxide.

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Activated Complex Noun

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[ak-tuh-vey-tid kom-pleks]

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Activated Complex

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A temporary, unstable arrangement of atoms that forms momentarily at the peak of the activation energy barrier during a reaction.

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Example: This graph shows that the activated complex is the temporary, high-energy state at the peak of the energy hill that reactants must overcome to become products.

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Transition State Noun

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[tran-zish-uhn steyt]

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Transition State

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The highest-energy state along a reaction pathway as reactants are converted into products, corresponding to the activated complex.

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Example: This image shows pushing a ball to the top of a hill, which is like the high-energy, unstable transition state a chemical reaction must pass through.

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Endothermic Reaction Noun

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[en-doh-thur-mik ree-ak-shuhn]

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Endothermic Reaction

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A chemical reaction that absorbs energy from its surroundings, resulting in products having a higher energy than the reactants.

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Example: This image shows arrows pointing into a test tube, representing an endothermic reaction absorbing heat energy from its surroundings.

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Exothermic Reaction Noun

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[ek-soh-thur-mik ree-ak-shuhn]

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Exothermic Reaction

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A chemical reaction that releases energy into its surroundings, resulting in products having a lower energy than the reactants.

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Example: This diagram shows a combustion reaction (burning wood), a common example of an exothermic reaction, where fuel (CₓHᵧ) and oxygen (O₂) release energy as heat and light.

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Reaction Intermediate Noun

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[ree-ak-shuhn in-ter-mee-dee-it]

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Reaction Intermediate

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A molecular entity that is formed from reactants in one step and consumed in a subsequent step of a reaction.

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Example: In a multi-step chemical reaction, a reaction intermediate is a temporary molecule that is created from the reactants and then used up to form the final products.

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Rate-determining step Noun

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[reyt di-tur-min-ing step]

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Rate-determining step

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The slowest step in a multistep chemical reaction, which consequently controls the overall rate of the entire reaction mechanism.

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Example: This diagram shows liquid being poured through a funnel. The narrow funnel is the slowest part of the process, controlling how fast the beaker fills.

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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 a reaction's rate by lowering the activation energy, without being consumed in the overall process.

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Example: A catalytic converter uses a catalyst to speed up the chemical reaction that changes harmful exhaust gases into safer substances like carbon dioxide and water.