Topic 6: Chemical reactions

Changes that involve the states of matter and energy. They are reversible; there are no new substances made, it is only the appearance of the substance that changes

Changes in state Dissolving Mixing two solids (e.g. sand and soil)

Changes that involve the bonds being broken and new ones being formed. They are often irreversible

Bubbles or releasing gas Absorbing or releasing heat Changing colour Releasing an odour Inability to reverse the change Precipitation of a solid from a liquid solution The formation of a new chemical

Rusting Neutralisation Electrolysis Digestion Respiration Souring of milk Baking a cake

A measure of how long it takes for a reactant to be used up or a product to be formed.

Temperature Concentration Pressure of gases Surface area of reacting solids Use of catalysts

If reactant molecules exist in different phases, the rate of reaction will be limited by the surface area of the phases that are in contact. Powders react faster than blocks - greater surface area and since the reaction occurs at the surface we get a faster rate

Increasing the concentration of one or more reactants will often increase the rate of reaction.

Rate of reactions speed up with increasing temperature

The presence of a catalyst increases the rate of reaction by providing an alternate pathway which has lower activation energy

Pressure increases the concentration of gases which in turn results in the increase of the rate of reaction. The reaction rate increases in the direction of less gaseous molecules and decreases in the reverse direction.

The rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction.

Increase in surface area of a solid reactant results in an increase in the number of collisions and this results in an increase in the number of successful collisions

More particles in a more concentrated solution so collision rate between reactant particles is higher. The more often the particles collide, the greater the chance they have to overcome the activation energy and of a successful collision occurring.

As the temperature increases, the reactant particles increase their kinetic energy and they move faster, resulting in more collisions between the particles. Theis causes the reaction rate to increase.

Provides an alternative reaction path which has a lower activation energy. This causes more of the collisions to become successful, increasing the rate of reaction.

Increase in pressure results in the gas particles being push closer together, increasing the number of particles per unit volume. This means that they collide more frequently, and so the rate of reaction increases.

A substance which can alter the rate of a reaction without being chemically change etself

Because different catalysts catalyse different reactions and not all reactions have suitable catalysts.

Measure the rate at which a reactant is used up Measure the rate at which a product is formed

• They eventually reach an equilibrium • When the rate of the forward and reverse reactions are equal = equilibrium • When the concentrations of the reactants and products remain constant = equilibrium