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Chemistry – Key Topic 6
4.6 The rate and extent of chemical change
4.6.1 Rate of reaction
4.6.1.1 Calculating rates of reaction
The rate of a chemical reaction can be found by measuring the quantity of a reactant
used or the quantity of the product formed over time. It is how fast the reactants are
changed into products
- One of the slowest is the rusting of iron
- A moderate speed reaction is magnesium reacting with acid to produce a gentle
steam of bubbles
- Burning is a fast reaction
Mean rate of reaction = quantity of reactant used/time or quantity of product
made/time
The quantity of reactant or product can be measured by the mass in grams or by a
volume in cm^3
- The units of rate of reaction may be given as g/s or cm^3/s or mol/s
- If there is a curve on a graph, remember to use a tangent to calculate the mean
rate of reaction at that point
Reaction rate is very important in the chemical industry. Any industrial process has to
make money by producing useful products. This means the right amount of product
needs to be made as cheaply as possible. The rate of reaction must be fast enough to
make it quickly but safely.
4.6.1.2 Factors which affect the rates of chemical reactions
Factors which affect the rates of chemical reactions include: the concentrations of
reactants in a solution, the pressure of reacting gases, the surface area of solid
reactants, the temperature and presence of catalysts
The rate of a chemical reaction depends on the collision frequency of reacting
particles and the particles colliding with enough energy. If you increase the chances
of the particles reacting, you increase the rate of reaction.
4.6.1.3 Collision theory and activation energy
, Collision theory explains how various factors affect rates of reactions. According to
this theory, chemical reactions can occur only when reacting particles collide with
each other and with sufficient energy. The minimum amount of energy that particles
must have to react is called the activation energy (need this much energy to break the
bonds in the reactants and start the reaction)
- At the start there is the maximum number of reactant particles which collide with
energy equal or greater than activation, so the rate is faster
- As the reaction proceeds, there are less particles, so less collisions, so the rate is
slower than at the start
Increasing the concentration of reactants in a solution, the pressure of reacting gases,
and the surface area of solid reactants increases the frequency of collisions and so
increases the rate of reaction
- Concentration is the number of particles per unit volume. As we increase
concentration, the number of particles in a given volume is increased and
therefore the frequency of collisions increases. There will be more successful
collisions with or higher than the activation energy taking place place per unit of
time, increasing the rate of reaction. The concentration of reactants is most at the
beginning, and decreases during the reaction
- Low pressure is when the particles have more room to collide with each other and
high pressure is when the particles have less room to collide with each other. As
the pressure increases, the particles are squashed closer together which means
that there are more particles in a given space. This increases the number of
successful collisions per second, and more particles have the required activation
energy for a reaction to take place. So the rate of reaction increases
- Reactions involving solids will only take place at the surface. Breaking up the solid
into smaller pieces will increase the SA:V ratio. This means that for the same
volume of solid , the particles around it will have more area to work on – so there
will be more collisions more frequently
Increasing the temperature increases the frequency of collisions and makes the
collisions more energetic, and so increases the rate of reaction
- as the temperature is increased, an increase in the kinetic energy provided to the
particles will be observed. This means there is a higher chance of successful collisions,
and that there will be a greater proportion of molecules that match or have a higher
energy than the required activation energy for a reaction to take place, increasing the
rate of reaction.
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