Chemistry 7.4: Chemical Reactions

Chemical Reactions

• A chemical reaction involves a change in which a new substance is formed.

Reactants and Products

• Reactants are substances that react together, shown before the arrow in an equation.
• Products are substances formed in a chemical reaction, shown after the reaction arrow in an equation.

Chemical and Physical Changes

• Physical changes alter physical properties of a substance, but no new substance is formed.
• Chemical changes involve the rearrangement of atoms and molecules in reactants to form products, with the total number of atoms conserved (Law of Conservation of Mass).

Describing Chemical Reactions

• Word equations can be used to describe a chemical reaction.

Energy and Combustion

• Fuels store energy, which can be released as heat during combustion.
• Combustion is a reaction with oxygen, resulting in energy transfer to the surroundings as heat and light.

Types of Reactions

• Thermal decomposition: a single reactant breaks down into simpler products when heated.
• Exothermic reaction: energy is given out, usually as heat or light.
• Endothermic reaction: energy is taken in, usually as heat.

Acids and Alkalis

• Acids and alkalis can be corrosive or irritant and require safe handling.
• Indicators are used to identify whether unknown solutions are acidic or alkaline.

pH and Acid-Base Properties

• pH is a scale of acidity and alkalinity from 0 to 14.
• pH depends on acid strength: strong acids have lower pH values than weak acids.
• Acidic solutions have a pH below 7, neutral solutions have a pH of 7, and alkaline solutions have a pH above 7.

Examples of Acids

• Strong acids: hydrochloric, sulfuric, and nitric acid.
• Weak acids: acetic and citric acid.

Neutralisation

• Mixing an acid and alkali produces a chemical reaction, resulting in neutralisation.

Metals and Acids

• Some metals react with acids, forming a metal salt and hydrogen.