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Mathematical learning outcomes:
CM3.2i interpretation of charts and graphs when dealing with reaction profiles
CM3.2ii arithmetic computation when calculating energy changes
Assessable content statements:
C3.2a distinguish between endothermic and exothermic reactions on the basis of the temperature change of the surroundings
C3.2b draw and label a reaction profile for an exothermic and an endothermic reaction to include activation energy, energy change, reactants and products
C3.2c explain activation energy as the energy needed for a reaction to occur
C3.2d calculate energy changes in a chemical reaction by considering bond making and bond breaking energies
Learners need to understand that when chemical reactions are a result of first breaking followed by making bonds. Any differences in the energies involved result in either the emission or absorption of energy. An exothermic reaction gives out energy while an endothermic reaction draws in energy (from their surroundings). Thus exothermic reactions raise the temperature and endothermic reactions lower the temperature (of their surroundings).
The graphical representation of these energy changes is done using energy profile diagrams. Carefully done in stages drawing the lines for reactants and products with the ΔH indicated with the subsequent addition of the activation energy curve enable the learner to grasp both ideas from the same diagram. It needs to be made clear here that the top of the curve represents the point at which all the bonds are broken in preparation for bond making.
Common misconceptions or difficulties learners may have
Learner’s misconceptions stem from difficulties with the idea of conservation of energy and they think that energy is lost or used up. The fact that energy can only be transferred is lost due to the number of ways such energy can be transferred. Learners also misunderstand the changes involved in bond making (exothermic) and bond breaking (endothermic). Reactions requiring an initial input of energy such as burning hydrocarbons may be thought to be endothermic.
A short video demonstrating an endothermic reaction.
This is a solid/solid reaction rather than the usual dissolution used to demonstrate endothermic processes.
A short video demonstrating an exothermic reaction. The second link is instructions on how to demonstrate the reaction between aluminium and iodine in the classroom using a fume cupboard.
Aluminium iodide formation is a reaction that doesn’t need an initial heat input.
Approaches to teaching the content
Exothermic reactions can be illustrated by means of some of the most exciting demonstrations in chemistry e.g. burning magnesium (white hot approx. 3000 °C and almost as hot as the sun!), exploding can (methane and air mixture). Endothermic reactions are rarer though the fact that solids can react with each other by simple agitation is worth demonstrating.
A presentation including animations to show the development of energy profile diagrams.
An omission here is the activation energy required in an endothermic reaction and therefore this needs teasing out. It could be a very useful AfL tool.
A useful short video covering bond energy calculations.
Useful for self-learning with some other useful links.
A practical covering the exothermic displacement of copper from copper sulphate solution by a number of metals.
Could be used as a simple introduction to calorimetry. Details are sufficiently complete.
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