A Level Chemistry WJEC

This subject is broken down into 32 topics in 4 modules:

  1. AS Unit 1: The Language of Chemistry, Structure of Matter and Simple Reactions 7 topics
  2. AS Unit 2: Energy, Rate and Chemistry of Carbon Compounds 8 topics
  3. A2 Unit 3: Physical and Inorganic Chemistry 9 topics
  4. A2 Unit 4: Organic Chemistry and Analysis 8 topics
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This page was last modified on 28 September 2024.

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Chemistry

AS Unit 1: The Language of Chemistry, Structure of Matter and Simple Reactions

Formulae and Equations

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Formulae and Equations

General Concepts

  • Chemical formulas are a shorthand way of describing a chemical compound using atomic symbols and numerical subscripts.
  • The molecular formula gives the exact number of atoms of each element in a molecule, while the empirical formula gives the simplest ratio of atoms in a compound.
  • Structural formulas show the arrangement of atoms in a molecule.

Balancing Equations

  • Chemical equations represent the changes that occur during a chemical reaction.
  • A balanced chemical equation obeys the law of conservation of mass, showing that the number of atoms of each element are the same on both sides of the equation.
  • Balancing equations involves adjusting the coefficients of the reactants and products until the number of atoms of each element is balanced.
  • Reactants are the substances that react in a chemical reaction and appear on the left side of the equation, while products are the substances that are formed and appear on the right side.

State Symbols in Equations

  • State symbols are used in chemical equations to indicate the physical state of each reactant and product: (s) for solid, (l) for liquid, (g) for gas, and (aq) for substances dissolved in water (aqueous).

Ions and Ionic Equations

  • An ion is an atom or group of atoms that has lost or gained electrons and so has an electric charge.
  • The charge on an ion is shown after the atomic symbol: e.g., Na<sup>+</sup>, Cl<sup>-</sup>, OH<sup>-</sup>.
  • Ionic equations show only the ions that change in a reaction. Substances not changed by the reaction, known as spectator ions, are not included.

Moles and Stoichiometry

  • The mole is a unit for amount of substance. One mole of a substance contains the same number of entities (such as atoms, molecules, or ions) as there are in 12 grams of carbon-12.
  • Stoichiometry is the study of the quantitative relationships in a chemical reaction, determining the proportions in which elements or compounds react.

Chemical Reactions

  • Chemical reactions may be categorized as combination, decomposition, displacement, or exchange reactions.
  • In combination reactions, two or more substances react to form one product.
  • In decomposition reactions, one substance breaks down into two or more substances.
  • In displacement reactions, one element is replaced by another in an compound.
  • In exchange reactions, the positive ions in the reacting compounds switch places.

Course material for Chemistry, module AS Unit 1: The Language of Chemistry, Structure of Matter and Simple Reactions, topic Formulae and Equations

Chemistry

A2 Unit 3: Physical and Inorganic Chemistry

Chemical kinetics

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Chemical kinetics

Chemical Kinetics Overview

  • Chemical kinetics is the study of rate at which reactions proceed and the factors that influence these rates.
  • It is important in predicting the speed of reactions and determining reaction mechanisms.

Collision Theory

  • Reactions can only occur when particles collide.
  • Only a fraction of collisions lead to a reaction. These are referred to as successful collisions.
  • A successful collision is characterised by particles having sufficient kinetic energy (above the activation energy) and the correct spatial alignment.

Rate of Reaction

  • Measured as the rate of change of concentration of a reactant or product over time, usually in mol dm^-3 s^-1.
  • An increase in concentration of particles can increase the rate of reaction as it increases the likelihood of collisions.

Activation Energy

  • The minimum energy needed for a reaction to proceed.
  • The fraction of particles with sufficient energy can be increased by raising the temperature.
  • A catalyst can lower the activation energy, increasing the rate of reaction.

Temperature

  • An increase in temperature increases the rate of reaction as particles possess more kinetic energy, leading to more successful collisions.
  • Temperature is the single most powerful way of increasing reaction rate.

Pressure

  • For reactions involving gases, an increase in pressure increases the rate of reaction as it leads to a higher concentration of particles, and thus more frequent collisions.

Catalysts

  • Catalysts increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy.
  • They are not consumed in the reaction, and can therefore be used repeatedly.

Rate Equations

  • The rate equation shows how rate is affected by the concentration of each reactant.
  • Rates can be zero order (rate is independent of concentration), first order (rate is directly proportional to concentration) or second order (rate is proportional to the square of the concentration).

Determining Order of Reaction

  • The order of reaction can be determined from concentration-time graphs, initial rates method or the method of half lives.
  • The overall order of reaction is the sum of the individual orders for each reactant.

Reaction Mechanisms

  • A reaction mechanism describes the steps that take place during a reaction on a molecular level.
  • Each step is known as an elementary step or a reaction intermediate.
  • The rate-determining step (RDS) is the slowest step in the whole reaction and governs the rate. Determine the RDS from the experimental rate equation.

Using the Arrhenius Equation

  • The Arrhenius equation quantifies the effect of temperature on the rate of reaction.
  • It relates the energy of activation and the temperature to the rate constant, k.

Remember, it's crucial to understand and apply these concepts when tackling problems in chemical kinetics. Repetitive practice and revision of these core ideas will ensure your overall understanding of chemical kinetics is solid and comprehensive. The more you practice applying these concepts to different problems, the better. Happy revising!

Course material for Chemistry, module A2 Unit 3: Physical and Inorganic Chemistry, topic Chemical kinetics

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