Chemistry Ch3 reactions

• Chemical reaction


  A change in which atoms rearrange and join together differently to make new substances
• Chemical reactions


  • Make new substances
  • Transfer energy to or from the surroundings
• Most chemical reactions are not easily reversible
• Physical change


  A change that does not make new substances and is reversible
• Evidence that a chemical reaction is happening

  • Seeing huge flames or tiny sparks
  • Noticing a sweet smell or foul stink
  • Feeling the substances getting hotter or colder
  • Hearing a loud bang or gentle fizzing
• Catalyst


  A substance that speeds up a slow chemical reaction without being used up
• Useful chemical reactions


  • Making medicines like paracetamol
  • Making fabrics like polyester
  • Making building materials like cement
  • Making tissues in your body like muscle and bone
• Chemical reactions transfer energy

  • Burning gas cooks food
  • Burning petrol makes vehicles go
  • Reactions in your cells provide energy for biological processes
• Unwanted chemical reactions


  • Rusting damages bicycles, boats, and bridges
  • Chemical reactions make food rot
• Physical changes

  • Warming chocolate to make it melt
  • Changes of state
  • Dissolving
• Reactant

  Starting substance in a chemical reaction
• Product

  Substance made in a chemical reaction
• Word equation

  Representation of a chemical reaction using words
• When substances burn, a chemical reaction happens where new substances are formed
• Charcoal

  • It is a form of carbon
  • At 20°C it is a black solid
• When carbon burns

  1. It reacts with oxygen from the air

  2. The chemical reaction makes a new substance, carbon dioxide, which forms as an invisible gas
• In this chemical reaction, two elements join together to make a compound
• Word equation

  • Shows reactants on the left of the arrow, products on the right of the arrow
  • The arrow means 'reacts to make', different to an equals sign (=) used in a maths equation
• Word equation for the reaction of iron and sulfur

  • iron + sulfur → iron sulfide
• In every chemical reaction, atoms are rearranged to make new substances
• The numbers of each type of atom do not change in a chemical reaction
• When nitrogen reacts with oxygen at high temperatures

  1. Nitrogen molecules split up

  2. The atoms join together differently to make nitrogen monoxide molecules
• Figure 4 shows how the atoms rearrange and join together differently in the chemical reaction of nitrogen and oxygen
• Combustion


  Burning of a substance in the presence of oxygen, releasing heat and light
• Oxidation reaction


  Any reaction in which a substance reacts with oxygen
• Combustion reaction


  1. Substance reacts with oxygen

  2. Energy transferred to surroundings as heat and light
• Fuels


  • Materials that burn to transfer energy by heating
  • Examples: methane, petrol, diesel, hydrogen, waste cooking oil
• Methane molecule has one atom of carbon joined to four atoms of hydrogen
• Methane is a compound of carbon and hydrogen
• Combustion of methane

  1. Methane reacts with oxygen

  2. Products: carbon dioxide and water
• Petrol is a mixture of compounds made up of carbon and hydrogen atoms
• Petrol, diesel, and methane from under the ground or sea are fossil fuels
• Fossil fuels are non-renewable
• Products of combustion of petrol

  • Carbon dioxide
  • Water
• Electric cars do not make carbon dioxide directly
• Hydrogen vehicles burn hydrogen in their engines, with water as the only product
• Combustion of hydrogen

  1. Hydrogen reacts with oxygen

  2. Product: water
• Combustion reactions are examples of oxidation reactions
• Decomposition reaction


  A reaction where one compound breaks down into two or more products
• Decomposition reaction


  1. Reactants

  2. Products