Chapter 8-Chemical and Physical Changes

• Physical Change 


  A change that does not alter the chemical nature of the substance, no new chemical substances are formed.
• A Physical Change is accompanied by:

  • Change in the arrangement of particle
  • Release or absorption of energy
• Chemical Change


  A change in which the chemical nature of the substances involved changes; new chemical substances are formed
• Chemical Change


  A change in which the chemical nature of the substances involved changes; new chemical substances are formed
• Chemical Changes involve:

  • A colour change
  • Formation of a gas
  • The formation of a precipitate
  • A change in temperature
• The products have a different chemical composition to the original substances (reactants)
• A chemical change involves the breaking of existing bonds
• The bonds between particles are broken
• Catalyst


  A substance that increases the rate of the reaction but remains unchanged at the end of the reaction
• Catalysts


  • They lower the activation energy of the reaction
  • They only speed up the reaction or lower the activation energy but do not affect the heat of formation or enthalpy (AH) of the reaction
• Catalysts do not affect the heat of formation or enthalpy (AH) of the reaction
• Activation energy is the energy required to start a reaction
• Types of Chemical Change Reactions

  • Simple
  • Synthesis
  • Decomposition
• Many different types of chemical reactions exist including synthesis and decomposition reactions
• State symbols


  • (g)
  • (s)
  • (l)
  • (aq)
• (g)

  Gaseous substances
• (s)

  Solids
• (l)

  Liquids
• (aq)

  Species in solution in water
• Synthesis reaction includes combustion reactions
• The crystal is completely broken apart as all of the ions become solvated, and a solution of solvated (aqueous) ions results
• Electrolyte

  A compound that dissolves in water to form an electrically conducting solution
• Non-electrolyte


  A compound that dissolves in water to form a non-conducting solution
• Strong electrolyte


  A compound that dissolves in water to give a solution that conducts strongly
• Weak electrolyte

  A compound that dissolves in water to give a solution that conducts weakly
• Factors affecting conductivity of a solution

  • The type of solution
  • Temperature of the solution
  • Solubility of a particular substance
  • The concentration of the ions in solution
• Electric Current

  The rate of flow of electric charge
• Hydration

  The process where ions become surrounded with water molecules in a water solution
• Activated complex []

  A high energy,unstable,temporary transition state between reactants
• Hydration

  The process where one becomes surrounded with water molecules in a water solution
• When an ionic substance dissolves in water

  The positive ends of the water molecule are attracted to the negatively charged anions and the negative ends of the water molecule are attracted to the positively charged cations
• Formation of an ionic solution from a crystal

  1. The ionic crystal is immersed in a solvent (e.g., water)

  2. Ions become solvated and break away from the crystal

  3. The crystal is completely broken apart as all of the ions become solvated, and a solution of solvated (aqueous) ions results
• Electrolyte

  A compound that dissolves in water to form an electrically conducting solution
• Ions in solution are able to conduct electricity
• Non-electrolyte

  A compound that dissolves in water to form a non-conducting solution
• Strong electrolyte

  A compound that dissolves in water to give a solution that conducts strongly
• Weak electrolyte


  A compound that dissolves in water to give a solution that conducts weakly
• The conductivity of a solution depends on various factors
• Ion Exchange Reactions

  Reactions where there is an exchange between the cations (positive ions) and the anions when the reactants react with each other
• General reaction of ion exchange

  AB + CD → AD + CB