Exothermic and Endothermic Reaction

• Endothermic reactions absorb energy.
• Exothermic reactions release energy.
• The temperature of the surroundings increases during an exothermic reaction, while it decreases during an endothermic reaction.
• Burning 4g of zinc (Zn) in oxygen (O2) to produce 6g of zinc oxide (ZnO)

  1. Zinc + oxygen --> zinc oxide

  2. 2Zn + O2 --> 2ZnO
• The mass of the oxygen that reacted with the zinc was 2g
• Exothermic reaction


  Energy is EXITING the reactants and being transferred to the surroundings as heat
• Endothermic reaction


  Energy is ENTERING the reactants from the surroundings
• Exothermic reaction


  Temperature increases
• Endothermic reaction


  Temperature decreases
• Exothermic changes


  • Freezing
  • Condensing
• Endothermic changes


  • Melting
  • Boiling
• All chemical reactions involve energy transfers
• Melting and boiling are examples of endothermic changes because they involve a gain of energy
• Freezing and condensing are examples of exothermic changes because they involve a loss of energy
• For an endothermic reaction, you are looking for a gain of energy or a decrease in temperature
• For an exothermic reaction, you are looking for a loss of energy or an increase in temperature
• Endothermic reactions: Potassium chloride and sodium hydrogen carbonate
• Exothermic reactions: calcium chloride and sodium carbonate
• Melting ice in water is an endothermic change
• Dissolving baking soda in vinegar is an exothermic reaction
• In an exothermic reaction, temperature increases
• An endothermic reaction includes a gain of energy
• In a decomposition reaction, you write the products on the right and the reactants on the left
• You should always use --> instead of = when writing chemical reactions
• A balanced symbol equation tells you the relative amounts of reactants and products, which a word equation does not
• Physical change

  • Melting
• Dissolving

  Physical change
• A thermal decomposition reaction is different from a normal decomposition reaction because it involves the breakdown of a substance due to heat
• Variable you change


  Independent variable