Chemical Reactions and Equations

Chemical Reactions

  • Definition: Chemical reactions occur when the bonds between atoms are broken and new bonds are formed, resulting in a new arrangement of atoms and at least one new substance.

  • Representation: Chemical reactions can be represented through various models that illustrate the arrangements of atoms in molecules or compounds. They can also be expressed in different forms:
      - Worded equations: Verbally describe the reaction.
      - Chemical equations: Show reactants and products using chemical symbols.

  • Structure of Chemical Equations:
      - Reactants are located on the left-hand side of the equation.
      - Products are located on the right-hand side of the equation.

  • Categories of Reactions: Chemical reactions can be categorized into types, which include:
      - Acid-base reactions
      - Neutralisation
      - Displacement
      - Synthesis
      - Combustion
      - Oxidation
      - Decomposition
      - Exothermic reactions
      - Endothermic reactions

Balancing Chemical Equations

  • The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction.

  • The Law of Constant Proportions indicates that a compound always contains the same relative amounts of each element, regardless of how it is formed.

  • Balancing Chemical Equations: Chemical equations must be balanced to ensure that the same number of each type of atom is present in both the reactants and products.
      - Only coefficients (the numbers in front of the compounds) may be altered to balance the equation, whereas the chemical formulas must remain unchanged.

Acid and Base Reactions

  • Acids: Substances with a pH below 7 that are corrosive.

  • Bases: Substances with a pH above 7 that are also corrosive.
      - Alkalis: Soluble bases in water, which also have a pH above 7.

  • pH Scale: Ranges from 0 to 14, with 7 being neutral. The scale indicates how acidic or basic a substance is.

  • Neutralisation Reactions: Reaction between an acid and a base producing water and a salt, often exemplified by treating indigestion (excess hydrochloric acid) with antacids.

Combustion Reactions

  • Definition: Combustion reactions occur when a fuel burns in oxygen gas, producing heat along with water vapour and carbon dioxide.

  • Fossil Fuels: E.g., octane, derived from crude oil, serve as common fuels.

  • Example of Respiration: A biological combustion reaction where glucose reacts with oxygen, producing carbon dioxide and water.

  • Characteristics of Combustion: These reactions are examples of oxidation reactions as they involve electron transfer.

Green Chemistry

  • Definition: Green chemistry focuses on designing chemical processes that minimize energy use and waste production while synthesizing products for societal use.

  • Environmental Impact: Reactions can lead to unwanted by-products and damage the environment, for instance:
      - Sulfur dioxide and nitrogen oxides from industrial activity combining with atmospheric water to produce acid rain.

  • Objective: By understanding the underlying chemical reactions, it is possible to optimize processes to be more energy-efficient and sustainable.

Key Terms

  • Acid Rain: Rainwater, snow, or fog that contains dissolved chemicals (e.g., sulfur dioxide) causing acidity.

  • Acids: Chemicals that react with bases to form salts and water; they have a sour taste.

  • Alkalis: Bases that dissolve in water.

  • Aqueous Solutions: Mixtures with substances dissolved in water.

  • Corrosive: Chemicals that react with and wear away substances, especially metals.

  • Crude Oil: Liquid formed from the remains of ancient marine organisms, a fossil fuel from which various products are derived.

  • Decomposition Reaction: A reaction where a single reactant yields two or more products (e.g., AB<br>ightarrowA+BAB <br>ightarrow A + B).

  • Diatomic Molecules: Substances made of two bonded atoms.

  • Enhanced Greenhouse Effect: Increased atmospheric heating due to excessive greenhouse gases produced by human activity.

  • Equation: A statement that describes a chemical reaction, detailing reactants and products.

  • Fossil Fuels: Fuels like coal, oil, and natural gas derived from ancient organisms.

  • Hydrocarbons: Compounds containing only hydrogen and carbon atoms, such as methane (CH4CH_4).

  • Neutralisation: The reaction between acids and bases producing salt and water.

  • Octane: A hydrocarbon (C88H1818) and the primary component of petrol.

  • Open System: A system where both energy and matter can be exchanged with the surroundings.

  • Oxidation: A chemical reaction involving the loss of electrons.

  • pH Scale: A numerical scale from 0 (acidic) to 14 (basic) for measuring acidity.

  • Pickling: The preservation of food in vinegar (ethanoic acid).

  • Products: Resulting chemical substances from a chemical reaction.

  • Reactants: Original substances in a chemical reaction.

  • Respiration: The metabolic process where glucose decomposes in the presence of oxygen, releasing water, carbon dioxide, and energy.

  • Synthesis Reaction: A reaction in which two or more species combine to form a more complex product (e.g., A+B<br>ightarrowABA + B <br>ightarrow AB).