Chapter 6 - Chemical reactions

• Law of conservation of mass

  During a chemical reaction, atoms cannot be created or destroyed. They can be rearranged to form new substances.
• The mass of reactants must equal the mass of the products
• Ion

  When an atom has lost/gained an electron and now has a charge
• Types of ions

  • Cation (positive ions because they have lost an electron)
  • Anion (negative ions because they have gained an electron)
• Metals form cations, non-metals form anions
• Cations and anions can bond to form ionic compounds, which are neutral - the positive and negative charge balance each other
• Chemical potential energy

  The source of energy we get from fossil fuels, batteries and food. It is stored within the chemical bonds in the molecules.
• Some reactions absorb energy while others release energy
• Exothermic reactions

  Reactions that release energy in the form of heat or light. This happens if the amount of energy stored in the reactants is greater than the energy stored in the products.
• Reaction types involving oxygen

  • Combustion reactions
  • Respiration reactions
  • Corrosion reactions
• Combustion reactions

  • Substance burns in oxygen to produce light/heat, all exothermic
• Types of combustion reactions

  • Complete combustion (excess oxygen, forms carbon dioxide)
  • Incomplete combustion (limited oxygen, forms different carbon product)
• Hydrocarbon

  Compound containing hydrogen and carbon atoms
• Complete combustion of hydrocarbons

  Hydrocarbon + oxygen -> carbon dioxide + water
• Respiration reactions

  • Occurs inside cells of all living things, Combines glucose with oxygen, Produces energy for living things to survive
• Respiration

  Glucose + oxygen -> carbon dioxide + water
• Corrosion reactions

  • Most metals combine with oxygen gas in the air, Makes metal oxides, Other chemicals can be involved e.g. water
• Endothermic reactions

  Reactions that absorb energy from the environment to form products. This happens if the amount of energy stored in the reactants is less than the amount of energy stored in the products.
• Photosynthesis

  Carbon dioxide + Water -> Glucose + Oxygen
• Decomposition reaction

  When single reactants break into several products
• Thermal decomposition

  • Some chemicals only decompose when heated, Metal carbonates and metal hydrogen carbonates both undergo thermal decomposition, This process can be used to extract metals from their ores
• Combination reaction

  Occurs when two reactants combine to form a single product
• Precipitation reaction

  When two clear solutions are mixed together, they form an insoluble solid (precipitate). Precipitation occurs when two soluble reactants combine to form an insoluble product.
• Ionic compounds

  Made up of cations (positive ions) and anions (negative ions). When dissolved, the cations and anions break away from each other and spread throughout the solvent. Ionic compounds have no overall charge - the total charge from the cations is balanced by the total charge of the anions.
• Reactions with acids

  • Acid + Base = Salt + Water
  • Acid + Metal = Salt + Hydrogen gas
  • Acid + Carbonate = Salt + Water + Carbon dioxide
• Factors affecting rate of chemical reactions

  • Temperature
  • Concentration of reactants
  • Surface area
  • Agitation (mixing or stirring)
  • Catalysts
• Temperature effect on reaction rate

  Increasing temperature increases the speed of particles, making them collide more frequently. Increasing temperature also gives the particles more energy, making it easier for them to rearrange.
• Concentration effect on reaction rate

  Increasing concentration increases the rate of reaction because particles are more likely to collide. Collision is necessary for reactants to rearrange to form products.
• Agitation effect on reaction rate

  Stirring reactants can increase the rate of reaction as it ensures the reactants are kept in contact.
• Surface area effect on reaction rate

  A larger surface area allows for more reactant particles to be available, which increases the rate of reaction.
• Catalyst

  Chemicals that speed up the rate of reaction but are not consumed (used up) during the reaction. They work by lowering the amount of energy needed for the reaction and making it easier for the reactant molecules to combine.
• Catalysts

  • Enzymes