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# Chapter 3: Reactions - Knowledge Organiser **C1** ## Chemical Reactions A chemical reaction is a process where atoms are rearranged to make new substances with the atoms joined together in different ways. ### Equations * **Reactants:** The substances you start with in a reaction. * **Products...

# Chapter 3: Reactions - Knowledge Organiser **C1** ## Chemical Reactions A chemical reaction is a process where atoms are rearranged to make new substances with the atoms joined together in different ways. ### Equations * **Reactants:** The substances you start with in a reaction. * **Products:** The substances you finish with in a reaction. * **Word Equations:** Represent a reaction using words. Reactants are on the left, products are on the right, separated by an arrow. Example: copper + oxygen → copper oxide * **Balanced Symbol Equations:** Represent a reaction using chemical formulas. Example: 2Cu + O₂ → 2CuO. A balanced equation shows the formula of each substance, how atoms are rearranged, and the relative number of atoms of each substance. ### What Happens During a Chemical Reaction? All chemical reactions involve an energy transfer to or from the surroundings: | Energy Transfer | Temperature of Surroundings | Type of Reaction | Example | |---|---|---|---| | from the surroundings to the reaction mixture | decreases | endothermic | thermal decomposition | | to the surroundings from the reaction mixture | increases | exothermic | combustion | ### Conservation of Mass In a reaction, atoms are not created or destroyed; they are just rearranged. The total mass of the reactants is always equal to the total mass of the products. This is called conservation of mass. * **Increased Mass:** If the mass seems to increase, it's because atoms have been added from a gas. * **Decreased Mass:** If the mass seems to have decreased, it's because atoms have rearranged and formed a gas that has escaped. **Changes of state** (e.g., water to ice to steam) are physical changes, not chemical reactions. No new substance is made; the change is reversible. ### Types of Reactions * **Thermal Decomposition:** A decomposition reaction where a substance breaks down into simpler substances. Usually requires heat. * **Burning Fuels (Combustion):** A type of oxidation reaction where a fuel reacts with oxygen. Transfers energy by heating. Fossil fuels (petrol, diesel, coal) take millions of years to form, cannot be replaced when used, and are considered non-renewable. Fossil fuels produce carbon dioxide and water when combusted, which is harmful to the environment and contributes to climate change. Hydrogen is another fuel that can be combusted, producing only water as a product - potentially a better alternative to fossil fuels. * **Oxidation:** When substances react with oxygen. ### Speed of Reactions Some reactions are very fast, others are very slow. * **Catalysts:** Substances that speed up reactions without being used up in the process. Different reactions require different catalysts. ### Key Terms * balanced symbol equation * catalyst * chemical reaction * combustion * conservation of mass * decomposition * endothermic * exothermic * fossil fuel * fuel * non-renewable * oxidation * physical change * product * reactant * reversible * thermal decomposition * word equation