Chemistry C7.3

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  • Created by: keeels
  • Created on: 18-06-13 17:07
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  • C7.3
    • Exothermic reactions
      • Reactions such as burning and respiration are exothermic. They give out heat to the surroundings.The energy of the product is less than than the energy of the reactants.
    • Endothermic reactions
      • Reactions such as photosynthesis take in energy from the surroundings.The energy of the product is greater than the energy of the reactant.
    • there is an interest in hydrogen as a fuel as water is the only product of its reaction with oxygen. This is a exothermic reaction but does not work at room temp. It takes a hot flame or an electric spark to heat up the mixture enough for the reaction to start.
      • some of the bonds in the reactants have to be broken before new chemical bonds in the products can be formed.
        • ITwo H-H bonds and one O=O bond break when the hydrogen reacts with oxygen. The atoms recombine to make water as four new O-H bonds form
        • Think of chemical bonds as tiny springs. In order to get hydrogen to react with oxygen, the tiny springs joining the atoms in the molecules have to be stretched and broken. This takes energy.
          • The product is water. This is created as new bonds form between oxygen atoms and hydrogen atoms. bond formation releases energy - just like relaxing a spring.
            • What decides whether a chemical reaction is exothermic or endothermic?It is the difference between the energy taken in to break bonds and the energy given out as new bonds form
              • The strength of the chemical bonds that break and form during a reaction determines the size of the overall energy change, and whether it is exothermic or endothermic.
              • More energy is given out when the bonds form than is taken in when the bonds break, and so the reaction overall is exothermic.The energy given out keeps the mixture hot enough for the reaction to continue
      • The overall energy change that takes place during a chemical reaction can be calculated if the strength of all the chemical bonds in the reactants are known and the product are known.
        • To calculate the energy change that takes place during the formation of steam from hydrogen and oxygen, the following data is needed. The units of bond strength are kilojoules. H2`(g)+O2`(g)=2H2`O(g).
    • A mixture of hydrogen and oxygen molecules. The molecules that are colliding may react to form new molecules, but only if they have enough energy to start breaking bonds.
      • Bonds between atoms must break before new molecules can form. This needs energy, for every reaction there is a certain minimum amount needed before the process can happen, This is called activation energy.
        • Activation energy is like an energy hill that the reactants have to climb before a reaction will start. The higher the hill , the more difficult it is to get the reaction started.
          • The collisions between molecules have a range of energies, Head-on collisions between fast- moving particles are the most energetic. If the colliding molecules have enough energy, the collision is successful and a reaction occurs.


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