Energy & Chemical Reactions

Exothermic Reactions Energy is needed to break bonds apart. Energy is released when bonds are formed. In any chemical reaction, the SAME NUMBER of bonds are broken as are made. So how do exothermic reactions give out energy to the surroundings?

Energy Level Diagrams These are drawn as graphs. They are used to show exothermic and endothermic reactions. The x axis shows time, the y axis shows energy. Energy Time

} Exothermic Reactions An exothermic energy diagram looks like this. Time reactants } Energy released products

} Exothermic Reactions CH4 + 2O2  2H2O + CO2 Combustion and respiration are exothermic reactions. e.g. burning methane (natural gas). CH4 + 2O2  2H2O + CO2 Energy Time CH4 + 2O2 } Energy released 2H2O + CO2

Bond Energy If the SAME NUMBER of bonds are made and broken, how can energy be given out in an exothermic reaction? The answer is that different chemical bonds have different amounts of energy in them. Carbon is joined to a hydrogen by a strong bond. 412 kJ of energy is needed to break this bond, and to separate the atoms The same amount of energy is given out when the bond is formed. H O has a bond energy of 463 kJ C==O has a bond energy of 743 kJ

Calculating Bond Energies CH4 + 2O2 2H2O + CO2 C-H bond = 412 4 of these = 1648 O=O bond = 496 2 of these = 992 Total energy 2640 kJ required H-O bond = 463 4 of these = 1852 C=O bond = 743 2 of these = 1486 Total energy = 3338 kJ released More energy is released by bond making than is required for bond breaking, so 698 kJ of energy is given out to the surroundings in this reaction, so this is an exothermic reaction.

Activation Energy Most exothermic reactions need a bit of energy to get them started, like lighting methane with a match. This energy is known as ACTIVATION ENERGY. This energy breaks the bonds and allows atoms to separate. This changes the shape of the energy diagram to this. Energy Time reactants products } Activation energy

Why is activation energy needed ? Separate atoms (with most energy): 1 C, 4 H, and 4 O Energy Time CH4 + 2O2 2H2O + CO2 Energy is needed to break bonds so atoms can become separate. The separate atoms can then react and rejoin into different compounds. If the new compounds require less energy to form, the reaction is exothermic.

Endothermic Reactions Examples are photosynthesis and most electrolysis reactions. The energy level diagrams look like this. Energy Time products } Energy needed to make the reaction work. reactants

Endothermic Reactions In these reactions, the energy required for bond breaking IS GREATER than the energy released by bond making. This means that to make endothermic reactions take place, extra energy is needed from an external source.

Questions (a) What energy source is needed for these reactions to occur: (i) Photosynthesis? (ii) electrolysis? (b) Which type of reaction are these examples of? (c) What can you say about the number of bonds broken and made during a chemical reaction? (d) Sketch energy level diagrams to show exothermic and endothermic reactions. (e) What is an exothermic reaction? (f) Why are some reactions exothermic? (g) What is Activation Energy, and why is it needed? (h) Add this to the correct energy diagram sketch.