Chap 6 Chemical Reactions

• In science we encounter two types of changes: physical and chemical
• Physical changes

  Alter the form or appearance of a substance without changing its chemical composition
• Physical changes

  • Melting ice
  • Dissolving sugar in water
  • Mixing two or more substances (e.g. sand and salt)
• Physical changes do not make any new chemical substances
• Physical changes can usually be reversed because they don't involve any change in the chemical composition of the substances involved
• Chemical changes


  Involve the formation of new substances with different chemical compositions
• Chemical changes


  • Burning wood
  • Rusting iron
• Unlike physical changes, most chemical reactions cannot be reversed to return to the original reactants
• Signs that a chemical reaction has taken place

  • Formation of a solid
  • Formation of gas (bubbling or fizzing)
  • Change in color
  • Release or absorption of heat (temperature change)
  • Formation of a new odor
• Collision Theory


  For a chemical reaction to happen, particles must collide with each other and they must do so with enough energy
• Collisions are essential for breaking bonds and forming new substances
• Successful collisions lead to a reaction occurring
• Unsuccessful collisions (where particles collide but don't have enough energy) do not lead to a reaction
• Factors that influence the rate of reaction

  • Number of particles per unit volume
  • Frequency of collisions between particles
  • Kinetic energy of particles
  • Activation energy
• The more particles in a volume, the higher the chance of collisions
• Higher kinetic energy increases the likelihood of successful collisions and thus the reaction rate
• Activation energy is the minimum amount of energy required for a chemical reaction to occur
• Only collisions with energy greater than or equal to the activation energy lead to a successful reaction
• The rate of reaction is determined by the number of successful collisions occurring in a given time
• Rate of reaction


  How quickly reactants are consumed and products are formed in a chemical reaction
• Factors affecting the rate of reaction

  • Concentration of reactants in solution
  • Pressure of reacting gases
  • Surface area of solid reactants
  • Temperature
  • Using a catalyst including enzymes
• Increasing the concentration of reactants in a solution

  Leads to a higher rate of reaction
• In Reactions where reactants are gases, increasing the pressure of reacting gases


  Leads to a higher rate of reaction
• Increasing the surface area of solid reactants

  Leads to a higher rate of reaction
• Increasing the temperature


  Leads to a higher rate of reaction
• Catalyst


  A substance that speeds up a chemical reaction without being changed or consumed in the process
• Enzymes


  Biological catalysts that accelerate biochemical reactions in living organisms
• Without a catalyst


  The reaction requires more energy input to start
• With a catalyst


  The reaction proceeds more readily and at a faster rate
• Investigating the rate of a reaction

  1. Measure the change in mass of a reactant or a product

  2. Measure the amount of gas formed