Chemical Reactions Overview

Chemical Reactions

• A chemical reaction is a process where atoms are rearranged to form new substances.
• Reactants are the starting substances, and products are the resulting substances.
• Reactions can be represented by word equations (e.g., copper + oxygen → copper oxide) or balanced symbol equations (e.g., 2Cu + O₂ → 2CuO).
• A balanced symbol equation shows the formula of each substance, how atoms rearrange, and the relative number of atoms in each substance.

What Happens During a Chemical Reaction?

• All chemical reactions involve the transfer of energy to or from the surroundings.
• Energy Transfer: Reactions can either absorb energy (endothermic) or release energy (exothermic).
  • Endothermic reactions absorb energy from their surroundings, causing a decrease in temperature.
  • Exothermic reactions release energy to their surroundings, causing an increase in temperature.
• Conservation of Mass: In a chemical reaction, atoms are rearranged, but not created or destroyed. The total mass of reactants equals the total mass of products.
• Physical Changes: Changes of state (e.g., water to ice to steam) are not chemical reactions but are physical changes—they are reversible.
• Signs that a chemical reaction is happening: Seeing flames or sparks, noticing a smell, hearing fizzing, or feeling temperature changes.

Types of Reactions

• Thermal Decomposition: A decomposition reaction where a substance breaks down into simpler substances when heat is applied.
• Combustion: A type of oxidation reaction where a fuel reacts with oxygen to release energy in the form of light and heat. Fossil fuels (petrol, diesel, coal) are non-renewable sources of energy, producing carbon dioxide when burned, which is harmful. Hydrogen can also be combusted, releasing only water as a byproduct.
• Oxidation: A reaction where a substance reacts with oxygen.

Speed of Reactions

• Some reactions are fast, others slow. Catalysts can speed up reactions without being used up in the reaction. Different reactions require different catalysts.

Key Terms

• Balanced symbol equation: Shows the chemical formulas and the number of atoms of each element involved in a reaction.
• Catalyst: A substance that speeds up a reaction without being consumed.
• Chemical reaction: A process where atoms rearrange to form new substances.
• Combustion: A reaction that releases energy, usually in the form of heat and light.
• Conservation of mass: The total mass of reactants is always equal to the total mass of products.
• Decomposition: A reaction where a substance breaks down into simpler substances.
• Endothermic: A reaction that absorbs energy from its surroundings.
• Exothermic: A reaction that releases energy to its surroundings.
• Fossil fuel: A fuel formed from the remains of ancient plants and animals.
• Fuel: A substance that reacts to release energy when burned.
• Non-renewable: Resources that cannot be replenished over a short time, such as fossil fuels.
• Oxidation: A reaction where a substance combines with oxygen.
• Physical change: A change of state, where the substance remains the same.
• Product: The new substance(s) formed during a chemical reaction.
• Reactant: A substance that undergoes changes during a reaction.
• Reversible: A change that can be returned to its original state.
• Thermal decomposition: A decomposition reaction that requires an input of heat.
• Word equation: A description of a chemical reaction using words.