Chemical Reactions

The One Thing

Chemical reactions rearrange atoms to form new substances, while reaction rates depend on successful collisions and reversible systems respond predictably to changes in conditions. Redox reactions involve simultaneous electron transfer, and reversible reactions may reach dynamic equilibrium.

Reactions, Processes and Experiments

Key Terms

• Physical change: A change in which no new substance is formed; it is often reversible, such as melting, boiling, or dissolving.
• Chemical change: A change that forms one or more new substances with different chemical properties, often shown by colour change, gas production, a precipitate, temperature change, or light.
• Chemical reaction: A process in which atoms are rearranged to form new substances. Reactants are changed into products.
• Activation energy: The minimum energy that reacting particles must have for a successful collision to occur.
• Collision theory: Particles react only when they collide with enough energy and with the correct orientation.
• Oxidation: Loss of electrons, gain of oxygen, or loss of hydrogen. It can be remembered as OIL: Oxidation Is Loss of electrons.
• Reduction: Gain of electrons, loss of oxygen, or gain of hydrogen. It can be remembered as RIG: Reduction Is Gain of electrons.
• Redox reaction: A reaction in which oxidation and reduction occur simultaneously because electrons lost by one substance are gained by another.
• Oxidising agent: A substance that causes another substance to be oxidised; it accepts electrons and is itself reduced.
• Reducing agent: A substance that causes another substance to be reduced; it donates electrons and is itself oxidised.
• Rate of reaction: The change in amount or concentration of a reactant or product per unit time.
• Catalyst: A substance that increases reaction rate by providing an alternative pathway with lower activation energy and remains chemically unchanged overall.
• Reversible reaction: A reaction in which products can react to reform the original reactants, shown by the symbol ⇌.
• Dynamic equilibrium: The state in a closed system where the forward and reverse reactions occur at equal rates, so concentrations remain constant.
• Le Chatelier's principle: If conditions affecting an equilibrium are changed, the system shifts in the direction that reduces the effect of the change.

Easily Confused

• Physical change vs chemical change: A physical change forms no new substance, whereas a chemical change forms one or more new substances with different chemical properties.
• Oxidation vs reduction: Oxidation is loss of electrons, whereas reduction is gain of electrons.
• Oxidising agent vs reducing agent: An oxidising agent accepts electrons and is reduced, whereas a reducing agent donates electrons and is oxidised.
• Exothermic vs endothermic: An exothermic reaction releases energy and warms the surroundings, whereas an endothermic reaction takes in energy and cools them.
• Rate of reaction vs equilibrium position: Rate describes how quickly a reaction proceeds, whereas equilibrium position describes the relative amounts of reactants and products at equilibrium.
• Dynamic equilibrium vs static equilibrium: Dynamic equilibrium involves continuing forward and reverse reactions at equal rates; it is not a state in which reactions have stopped.
• Catalyst vs change in equilibrium position: A catalyst allows equilibrium to be reached more quickly but does not change its position or the final equilibrium amounts.
• Activation energy vs overall energy change: Activation energy is the minimum energy needed for a successful collision, whereas the overall energy change compares the energy levels of reactants and products.

What Gets Asked

• Identifying whether a change is physical or chemical: State whether a new substance forms and use the specified signs of chemical change, such as gas formation, a precipitate, permanent colour change, temperature change, or light. The common error is treating a reversible state change as a chemical reaction.
• Balancing chemical equations: Ensure that the number of each type of atom is the same on both sides because atoms are conserved. The common error is failing to conserve atoms.
• Calculating or interpreting reaction rate: Use rate = quantity of reactant used or product formed ÷ time taken, and identify a steeper product-against-time graph as the faster reaction. The common error is confusing graph steepness with the final amount of product.
• Explaining factors affecting reaction rate: Relate temperature, concentration, pressure, and surface area to collision frequency and the number of particles with energy equal to or greater than the activation energy. The common error is giving the factor without explaining successful collisions.
• Identifying oxidation and reduction in redox reactions: Use electron transfer and oxidation states, remembering that electrons lost must equal electrons gained. The common error is identifying oxidation without identifying the simultaneous reduction.
• Predicting equilibrium shifts: Apply Le Chatelier's principle to concentration, pressure, and temperature changes, including the endothermic direction and the side with fewer or more gas molecules. The common error is stating that a catalyst changes the equilibrium position; it only makes equilibrium occur more quickly.