Chemical equations

• Chemical reactions involve the breaking and making of bonds between atoms to produce new substances
• A chemical change occurs when a chemical reaction takes place
• Observations that help determine a chemical reaction has taken place include:

  • Change in state
  • Change in color
  • Evolution of a gas
  • Change in temperature
• Word-equation for burning magnesium in oxygen: Magnesium + Oxygen → Magnesium oxide
• Chemical equation for burning magnesium in oxygen: Mg + O2 → MgO
• Balanced chemical equations are necessary to follow the law of conservation of mass
• Balancing a chemical equation involves ensuring the number of atoms of each element is the same on both sides of the equation
• In a combination reaction, calcium oxide reacts with water to form slaked lime
• Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat
• In the reaction CaO(s) + H2O(l) → Ca(OH)2(aq), calcium oxide and water combine to form calcium hydroxide
• This reaction is an example of a combination reaction, where a single product is formed from two or more reactants
• Combination reactions involve two or more substances combining to form a single product
• Exothermic reactions release heat along with the formation of products
• Examples of exothermic reactions include the burning of natural gas and respiration
• Slaked lime (calcium hydroxide) reacts with carbon dioxide in air to form calcium carbonate, which gives a shiny finish to walls
• The chemical formula for marble is CaCO3, which is formed when calcium hydroxide reacts with carbon dioxide
• Decomposition reactions involve a single reactant breaking down to give simpler products
• Thermal decomposition reactions occur when a decomposition reaction is carried out by heating
• An example of a thermal decomposition reaction is the decomposition of calcium carbonate to calcium oxide and carbon dioxide
• Displacement reactions involve one element displacing or removing another element from a compound
• Examples of displacement reactions include iron displacing copper from copper sulphate solution and zinc displacing copper from copper sulphate
• Double displacement reactions involve the exchange of ions between two compounds to form two new compounds
• In a double displacement reaction between sodium sulphate and barium chloride, barium sulphate and sodium chloride are formed
• When mixing sodium sulfate (Na2SO4) and barium chloride (BaCl2) solutions, a white insoluble substance, known as a precipitate, is formed
• A precipitation reaction occurs when a reaction produces a precipitate
• The reaction between Na2SO4(aq) and BaCl2(aq) forms BaSO4(s) and 2NaCl(aq)
• In the reaction between copper powder and oxygen, copper is oxidised to form black copper(II) oxide (CuO)
• When hydrogen gas is passed over heated CuO, the black coating turns brown as copper is obtained
• During the reaction, CuO is reduced as it loses oxygen, while hydrogen is oxidised as it gains oxygen
• Redox reactions involve oxidation and reduction, where one reactant gets oxidised while the other gets reduced
• Corrosion is the process where metals like iron get coated with a reddish-brown powder over time
• Rancidity occurs when fats and oils oxidise, changing their taste and smell
• In a displacement reaction, an element displaces another element from its compound
• Double displacement reactions involve the exchange of ions between reactants
• Precipitation reactions produce insoluble salts
• Oxidation involves the gain of oxygen or loss of hydrogen, while reduction involves the loss of oxygen or gain of hydrogen