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Endothermic and Exothermic Reactions 1

All chemical reactions involve energy in some way. You should know from physics that nothing can happen without energy being involved. Endothermic reactions absorb energy from the surroundings, whereas exothermic reactions release energy into the surroundings. This GCSE Chemistry quiz is the first of two looking at endothermic and exothermic reactions.

Where the energy transferred in a reaction is heat, you can detect the type of reaction (endothermic or exothermic) by using a thermometer or your skin. Heat energy always flows from higher temperatures to lower temperatures. When you carry out an experiment in the lab at room temperature and the reaction vessel (beaker, conical flask or whatever) feels colder, then you have an endothermic reaction. Heat energy will flow from the warmer surrounings into the reaction mixture - it is taking in energy. If the vessel feels warmer, the reaction must be exothermic as the heat energy will flow from the reaction mixture into the surroundings. In practice, it isn't a good idea to just feel the temperature of a reaction vessel as you could end up with a hot or cold burn! It is better to use a thermometer

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But sometimes, temperature can be misleading. When carrying out an electrolysis reaction, the temperature of the electrolyte will sometimes rise. So it must be exothermic? Wrong! Electrolysis reactions are endothermic as they absorb electrical energy. The rise in temperature is actually caused by the heating effect of the current.

For the exam, you need to know some examples of exothermic and endothermic reactions, most of which you should be familiar with from science lessons. Good examples of exothermic reactions are burning (some of the enrgy is released into the surroundings as light), the neutralisation of a strong acid by a strong alkali and the reaction between calcium oxide and water to make calcium hydroxide (quicklime reacting to form slaked lime). Apart from electrolysis, good examples of endothermic reactions are any thermal decomposition (such as the decomposition of limestone in the blast furnace) and the reaction between ethanoic acid and sodium carbonate.

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  1. In an exothermic reaction...
    In some cases this is obvious but in others it would need extremely sensitive equipment to detect the temperature rise
  2. In an endothermic reaction...
    You can ususally feel the decrease in temperature by touching the outside of the reaction vessel
  3. When an exothermic reaction takes place, the reaction mixture gives out heat because...
    When bonds are broken, energy is released and vice-versa
  4. When ammonium nitrate is added to water, an endothermic reaction occurs and...
    This is a key characteristic of endothermic reactions
  5. Which of the following reactions is exothermic?
    Combustion gives out large amounts of energy
  6. Where will the products be in relation to the reactants in an endothermic energy diagram?
    This shows that the products of an endothermic reaction contain a greater amount of energy than the reactants i.e. energy has been taken in by the reaction mix
  7. Energy change in a chemical reaction is represented by the symbol...
    The Ä sign is the Greek capital letter delta and means 'change in'
  8. ΔH is negative for what type of reactions?
    On an energy level diagram, the energy contained in the bonds of the products is less than that of the reactants
  9. ΔH is positive for what type of reactions?
    On an energy level diagram, the energy contained in the bonds of the products is greater than that of the reactants
  10. Energy changes in chemical reactions are measured in...
    kJ/mol = kilojoules per mole

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