Chemical reactions, energy and reaction barriers

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Catalyst action described

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A catalyst provides an alternative reaction pathway with a lower activation energy and increases the reaction rate without being consumed.

Key concepts

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Three fundamental mechanisms of chemical reaction

Chemical reactions proceed by proton transfer, electron transfer, or electron sharing. Proton transfer moves H+ ions between species and defines acid–base reactions. Electron transfer moves electrons from one species to another and defines redox reactions. Electron sharing forms or breaks covalent bonds when atoms share pairs of electrons.

Proton transfer (acid–base)

Proton transfer changes the identity of acids and bases by moving H+ from a donor to an acceptor. Conjugate acid–base pairs form when a proton leaves one species and binds to another. Proton transfer often occurs rapidly in solution when suitable donors and acceptors collide in the correct orientation.

Electron transfer (redox)

Electron transfer involves oxidation (loss of electrons) and reduction (gain of electrons). Oxidation and reduction always occur together so that total charge is conserved. Electron transfer frequently occurs in ionic solutions and in reactions with metals, where changes in oxidation state reflect net electron movement.

Electron sharing (covalent bond changes)

Electron sharing forms or breaks covalent bonds by rearranging shared electron pairs between atoms. Bond formation releases energy as atoms achieve lower-energy electron configurations. Bond breaking requires input of energy to overcome attractive forces between atoms.

Barriers to reaction and activation energy

Reactions face energetic barriers that prevent every collision from producing products. The activation energy is the minimum energy required for reactants to reach a transition state where bonds can break and form. Higher activation energy produces slower reactions at a given temperature because fewer collisions meet the required energy threshold.

Factors that change reaction rates

Reaction rate increases when collisions become more frequent or more energetic. Temperature increase raises the fraction of collisions exceeding the activation energy. Higher concentration or pressure increases collision frequency. Catalysts lower the activation energy and increase the probability that collisions produce products without altering the overall energy change.

Conservation of energy in chemical reactions

Energy is conserved during chemical reactions: the total energy of reactants and surroundings equals the total energy of products and surroundings. Chemical energy converts into thermal energy, light, or work, or the reverse, but no net energy creation or destruction occurs. The measured enthalpy change describes the net energy exchanged with the surroundings at constant pressure.

Key notes

Important points to keep in mind

Three reaction mechanisms: proton transfer (acid–base), electron transfer (redox), electron sharing (covalent).

Activation energy is the energetic barrier that collisions must overcome for reaction to proceed.

Temperature, concentration, pressure, surface area and catalysts change reaction rates by affecting collision energy or frequency.

Energy is conserved: chemical energy converts to other energy forms but total energy remains constant.

Bond breaking requires energy; bond formation releases energy; net enthalpy depends on both.

Transition state represents the highest-energy arrangement along the reaction path.

Catalysts change pathway and activation energy but do not alter overall enthalpy change.

Oxidation and reduction occur together; tracking oxidation states reveals electron transfer.