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Summary The rate and extent of chemical change
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The rate of chemical reaction is how fast the reactant are changed into products. One of the slowestis the rusting of iron and chemical weathering. A moderate speed reaction would be magnesium
reacting with acid, burning and explosions.
You can find the speed of a reaction by recording the amount of product formed or the amount of
reactant used up over time. The steeper the line of the graph, the faster the reaction time. Over time
the line becomes less steep as the reactants are used up. The quickest reaction time have the
steepest line and become flat in the least time.
Particles must collide with enough energy in order to react. The rate of a chemical reaction depends
on:
the collision frequency of reacting particles. The more collisions there are the faster the
reaction is.
the energy transferred during a collision. Particles have to collide with enough energy for the
collision to be successful.
The minimum amount of energy that particles need to react is called the activation energy. Particles
need this much energy to break the bonds in the reactants and start the reaction. A successful
collision is a collision that ends in the particles reacting to form products.
The rate of reaction depends on:
Temperature - when the temperature is increased the particles gain kinetic energy, which
causes them to move faster. As they move faster, they collide more frequently. This
increases the rate of reaction.
Concentration - the more concentrated a solution is the more crowded the particles are
going to be, which causes more collisions.
Pressure - when the pressure of a gas is increased, it means that the same number of
particles occupies a smaller space.
Surface area - if one of the reactants is a solid then breaking it up into smaller pieces will
increase its surface area to volume ratio. This means that for the same volume of solid, the
particles around it will have more area to work on. This will cause more collisions.
Catalyst - a catalyst is a substance that speeds up a reactions, without being used up in the
reaction itself. Catalysts work by decreasing activation energy needed for the reaction to
occur. They do this by providing an alternative reaction pathway with a lower activation
energy. Enzymes are biological catalysts.
Rate of Reaction = amount of reactant used or amount of product formed / time
As the reactants react, their concentration falls so the forward reaction will slow down. But as more
and more product are made their concentration rises again. After a while the forward reaction will
be going at exactly the same rate as the backward one- the system is at equilibrium. At equilibrium,
both reactions are still happening, but there’s no overall effect (it’s a dynamic equilibrium). This
means the concentrations of reactants and products have reached a balance and won’t change.
Equilibrium is only reached if the reversible reaction takes place in a ‘closed system’. A closed system
just means that none of the reactants or products can escape and nothing else can get in.
When a reaction’s at equilibrium it doesn’t mean the amounts of reactants and products are equal.
If the equilibrium lies to the right, the concentration of products is greater than that of the reactants.
If the equilibrium lies to the left, the concentration of the reactants is greater than that if the
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