OCR GCSE Chemistry C7 Topic Two - Energy Changes, Collisions, Equilibrium

OCR Higher Tier GCSE Chemistry Topic Two - 'How much? How fast? How far?' Notes on energy changes in chemical reactions, energy level diagrams, activation energy, equilibrium, and dynamic equilibrium.

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  • Created on: 09-04-11 19:09

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C7 Chemistry for a sustainable
Topic 2 ­ How much? How fast? How
far?…read more

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Energy changes and chemical
Exothermic reactions
All chemical changes give out or take in energy.
Many reactions give our energy to their surroundings, and this type of reaction is called exothermic.
Respiration is an example of an exothermic change. During respiration, oxygen and glucose change to carbon
dioxide and water in ways that provide the energy for growth, movement and warmth in living things.
The reaction of magnesium with dilute hydrochloric acid is also exothermic
The change can be described with the help of an energy level diagram. The energy of the product is less than
the energy of the reactants.…read more

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Energy changes and chemical
Endothermic reactions
There are some changes that take in energy from their surroundings. These are endothermic reactions.
Photosynthesis is endothermic: plants take in energy from the Sun to convert carbon dioxide and water to
The change can be described with the help of an energy level diagram. The energy of the product is greater
than the energy of the reactants.…read more

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How much energy? How fast?
In all reactions, regardless of whether they are exothermic or endothermic, some of the chemical bonds in the
reactants have to be broken before new chemical bonds can be formed. The strength of the chemical bonds
that break and form during a reaction determine the size of the overall energy change, and whether it is
exothermic or endothermic.
It is not enough for the atoms of two reactants to collide. Bonds between atoms must break before new
molecules can form. This needs energy. For every reaction, there is a certain minimum energy needed before
the process can happen. This minimum energy is called the activation energy.
In chemical reactions, if the activation energy is low, a high proportion of collisions have enough energy to
break bonds, and the reaction is fast even at low temperatures.
Reactions in which the activation energy is high are very slow at room temperature, because only a small
fraction of collisions have enough energy to cross the activation energy barrier.
Heating the mixture to raise the temperature gives the molecules more energy. In the hot mixture, more
molecules have enough energy to react when they collide.…read more

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How far?
Reversible changes often reach a state of balance, or equilibrium. Melting ice and water are at equilibrium at
0C. At this temperature, the two states of water coexist with no tendency for all the ice to melt or all the water
to freeze.
Chemists use a special symbol in equations for reactions at equilibrium:
·At equilibrium, the concentrations of reactants and products do not change.
·An equilibrium state can be approached from either the `reactant side' or the `product side' of a reaction.
In reactions such as that of iodine and organic solvent with aqueous potassium iodide, there comes a point in
which the molecule movement rate between reactants and products are equal. At this point, both forward and
backward reactions continue, but there is no overall change because each layer is gaining and losing molecules
at the same rate. This is a dynamic equilibrium.
The diagram to the right shows an example of a dynamic equilibrium.…read more


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