8.1 Exothermic Reactions

• • a chemical reaction that releases energy into the surroundings in the form of heat
• Burning (combustion)

  • when a substance burns, it reacts and combines with the oxygen in the air the substance reacts with oxygen is called a fuel which stores chemical energy
  • then the fuel (made up of carbon element) burns, the chemical energy is changed into thermal, light and sound energy
  • the energy dissipates (spreads out) into the surroundings
• • Burning Hydrogen
  • Hydrogen is a fuel in a model rocket. The combustion of hydrogen is an exothermic reaction
  • when the atoms of hydrogen and oxygen rearrange themselves and combine together, energy is given out
  • this chemical energy is changed into kinetic, thermal, sound and light energy
  • word equation: hydrogen (h) + oxygen (o) – water (h2o)
• Burning Magnesium

  • burning magnesium produces magnesium oxide (white powder)
  • energy is given out as heat and light as the magnesium and oxygen atoms rearrange themselves
  • any reaction in which a substance combines with oxygen is an oxidation reaction
  • word equation: magnesium + oxygen – magnesium oxide
• Reaction of Metal with Water

  • water is made up of particles containing atoms of hydrogen and oxygen
  • the atoms rearrange to form potassium hydroxide and hydrogen
  • stored chemical energy is changed to thermal energy, which dissipates into the surroundings
  • word equation: potassium + water – potassium hydroxide + hydrogen
• Reaction of Metal with Acid

  • when magnesium reacts with hydrochloric acid, the test tube gets hot (temp increases)
  • word equation: magnesium + hydrochloric acid – magnesium chloride + hydrogen
• Applications

  Using exothermic reaction

  • self-heating cans of food and drink apply the concept of exothermic reaction
  • when the can is opened, calcium oxide reacts with water
  • this reaction releases heat energy into the surroundings which warms up the food or drink
  • word equation: calcium oxide + water - calcium hydrogen + energy