Science term 3 and 4

Seite 1

16.07.19

Reaction Rate

Rate of a chemical reaction is how fast a chemical reaction proceeds.

Some chemical reactions are very fast and proceed over a very small time span, for example, explosions, burning gas.

Some chemical reactions are very slow and proceed over a very large time span, for example, decomposition of concrete, corrosion (rusting) of steel.

Some chemical reactions are moderate and proceed at a moderate pace, for example, souring of milk.

We use the changing of rates of reactions a lot in our daily lives. Think about the storage of items and the prevention of undesirable chemical reactions like rusting. There is a range of factors that we can change that affect the rate of a chemical reaction.

Reactions take place when particles collide with a certain amount of energy. The minimum amount of energy needed for the particles to react is called the activation energy and is different for each reaction. The rate of reaction depends on two things: the frequency of collisions between particles (the more collisions that happen the more likely they are to succeed) the energy with which particles collide.

If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

Anything the increases the number of successful collisions between reactant particles will speed up a reaction.

What factors affect the rate of reactions?

Increased temperature.

The increased concentration of dissolved reactants, and increased pressure of gaseous reactants.

The increased surface area of solid reactants.

Use of a catalyst.

Reactants do not proceed at a constant rate. They start off at a certain speed, then get slower and slower and slower until they stop.

As the reaction progresses, the concentration of reactants decreases.

This reduces the frequency of collisions between particles and so the reaction slows down.

Temperature- increasing the temperature will cause the particles to move faster with more energy. They will, therefore, collide more often and with greater energy. These two things mean there are more successful collisions per second and therefore a faster rate of reaction.

Concentration- Increasing the concentration of a reactant simply means there are more particles which may collide and so react. More collisions mean a faster reaction. The amount of a solute in a solvent. (Solute- dissolved thing. The thing that will react.) g/L or M. In solution every particle is available for a collision. Hence more particles in higher concentration mean more chances for collisions.

Surface area- using a powder instead of a lump means the surface area is greater, which means a greater area of reactant is exposed and so available for a collision. Surface area is the part of a solid you can touch. One large particle has a smaller surface than two smaller particles. More surfaces that can be touched means more places a collision can happen. Using different sized particles at different times can control the rate of a reaction.

Catalysts- Reduces the amount of energy needed to have a successful collision. Some catalysts work because one particle is fixed to a surface, making a collision more likely.