Chemical Reactions and Equation-Chemistry

• Observations that help determine a chemical reaction


  1. Evolution of a gas

  2. Change in temperature

  3. Formation of a precipitate

  4. Change in color

  5. Change of state
• Chemical change example

  • Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
• Writing chemical equations

  Representation of a chemical reaction in terms of symbols and chemical formulae of the reactants and products
• Word equation


  Chemical reaction expressed in words rather than chemical formulas to identify the reactants and products
• Physical change


  Change in color or state occurs but no new substance is formed
• Symbol of elements

  Chemical code for an element, abbreviated form of its name
• Writing chemical equation example

  • Zn(s) + dil. H2SO4(aq) → ZnSO4(aq) + H2(↑)
• Chemical reactions


  Chemical changes in which reactants transform into products by making or breaking of bonds between different atoms
• Physical change example

  • Water changes to steam on boiling but no new substance is formed
• Types of chemical reactions

  • Combination
  • Decomposition
  • Single Displacement
  • Double displacement
  • Redox
  • Endothermic
  • Exothermic
  • Precipitation
  • Neutralisation
• Chemical change


  One or more new substances with new physical and chemical properties are formed
• Conservation of mass is maintained in balancing a chemical reaction
• Valency


  Combining capacity of an element, number of electrons lost, gained, or shared when forming a molecule
• Balanced chemical equation

  The number of atoms of each element in the reactants side is equal to that in the products side
• Displacement reaction

  More reactive element displaces a less reactive element from its compound or solution
• The number of atoms for each element in the reactants side has to balance the number of atoms in the products side
• Types of chemical reactions

  • Combination
  • Decomposition
  • Single Displacement
  • Double displacement
  • Redox
  • Endothermic
  • Exothermic
  • Precipitation
  • Neutralisation
• Combination reaction

  Two elements or compounds combine to give one single product
• According to the law of conservation of mass, no atoms can be created or destroyed in a chemical reaction
• Representations for different states of matter

  • Liquids: (l)
  • Gases: (g)
  • Aqueous solutions: (aq)
  • Gas produced in the reaction: (↑)
  • Precipitate formed in the reaction: (↓)
• Steps for balancing chemical equations

  Hit and trial method: Change the coefficients so that the number of atoms of each element is the same on each side of the chemical equation
• Conservation of mass
• The total mass of the products formed in a chemical reaction is equal to the total mass of the reactants participated in a chemical reaction
• Short-cut technique for balancing a chemical equation

  Set up a series of simultaneous equations for each element and solve for the coefficients
• Decomposition reaction


  A single reactant decomposes to give two or more products