School of Education

Resources - Science and Environmental Education

Chemical reactions

Introduction

This topic explores the key concepts of chemical reactions as they relate to:

  • physical and chemical change
  • hazardous chemicals
  • separation of mixtures
  • acids and bases
  • oxidation and reduction.

Key concepts of chemical reactions

The activities in this topic are designed to explore the following key concepts:

  • A substance that undergoes a physical change remains that substance even though it may have changed state or shape.
  • A substance that undergoes a chemical change is no longer the original substance.
  • In all chemical reactions, the total mass of the reactants equals the total mass of the products.
  • In all chemical reactions, the total number and types of atoms in the reactants equal the total number and types of atoms in the products.
  • Food atoms do not disappear in digestion and they do not convert to energy in the body.
  • In any chemical or physical change, the total amount of energy remains constant.
  • Chemical reactions will not occur until the reactants receive enough energy (activation energy) to break chemical bonds.
  • In an exothermic reaction, the total energy of the products is less than that of the reactants - heat is released to the surrounding substances.
  • In an endothermic reaction, the total energy of the products is more than that of the reactants - heat is taken from the surrounding substances.
  • Chemical reactions are equilibrium reactions; that is, forward and reverse reactions take place at the same time.
  • Chemical equilibrium can be disturbed by adding or removing any of the products or reactants, by changing the volume, or by changing the temperature.
  • Any change that affects the position of an equilibrium state will cause that equilibrium state to shift, if possible, in such a way as to partially oppose the effect of that change.
  • A catalyst is a substance that can alter the rate of a chemical reaction without being used up in the reaction.
  • For something to combust or burn, fuel, oxygen, and activation energy input are needed.
  • An acid is a proton donor; a base is a proton acceptor.
  • Redox reactions involve the transfer of electrons from one reactant to another.

Students' alternative conceptions of chemical reactions

Research into students' ideas about this topic has identified the following non-scientific conceptions:

  • A chemical is a substance made by humans.
  • Mixing an acid with a base (without regard to quantities) neutralises the base, resulting in a neutral solution.
  • In neutralisation, all the H+ and OH- ions are cancelled.
  • Oxidation is the addition of oxygen in a reaction; reduction is the removal of oxygen.
  • If a reaction includes oxygen, it is an oxidation reaction.
  • Oxidation and reduction reactions can occur independently.
  • If water appears during burning, it was present in the wood.
  • Smoke formed during combustion was already in the wood.
  • Combustion is a change of state of matter: solid or liquid to gas.
  • Oxygen aids, but does not participate in, combustion.
  • A candle burning is endothermic, because heat is needed to initiate the reaction.
  • Chemical reactions are reactions that produce irreversible change.
  • Chemical reactions are caused by mixing substances.
  • Chemical reactions must be driven by external intervention - for example, heat.
  • Energy is used up in a chemical reaction.
  • Chemical bonds store energy.
  • Energy is created in chemical reactions.
  • The products of chemical reactions need not have the same mass as the reactants.
  • Water disappears as it evaporates.

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17th November 2008