Rates of Reactions What is Reaction Rate The Collision Theory Factors Affecting Reaction Rate

Reaction Rate Reactions occur when atoms break and/or make bonds. Some reactions are very slow, taking years to complete, some reactions are explosively fast. The speed that a reaction progresses at is called a rate of reaction. The rate of reaction can be determined by how fast the reactants (the chemicals reacting) disappear or how quickly the products are created.

Examples Mg + 2HCl → MgCl2 + H2 You could time the Mg metal disappearing or the H2 gas being produced 2Mg + O2 → 2MgO You could time the Mg burning until the white flare disappears

Particle Theory This states that all matter is made up of tiny particles which are in constant motion. Remember this……

The Collision Theory It is believed that two particles need to come in contact with each other at a certain velocity in order to react. When this occurs the bonds break and/or are made. This is called the Collision Theory. Everything that affects the rate of reaction has some affect on the collision of the particles involved. Some factors speed up the particle (give them more energy), some factors increase their chances of contacting each other, one factor even holds the particles at the right angle for an effective collision.

Factors Affecting Rates - Temperature Temperature is a way of measuring the vibration of the atoms/molecules of a substance: Hot substances have fast moving particles Cold substances have slow moving particles The particles in a cold substance react slowly because they move slowly and are therefore less likely to collide. When they do impact they collide with less energy and are therefore less likely to break/make bonds. The particles in a hot substance are more likely to collide and when they do, they collide with more energy.

Factors Affecting Rate: Concentration Higher concentrations of reactants mean that the chemicals are more likely to collide with something they can react with. More collisions = more reactions = faster rate of reaction

Factors Affecting Rate – Surface Area Chemicals can only react at their surfaces, because that’s the only place where two chemicals can meet. More surface area = greater chance of contact = more collisions = more reactions = greater rate of reaction

Factors Affecting Rate - Catalysts Catalysts are chemicals that will speed up a rate of a reaction but they are not consumed by the reaction. (they remain unchanged) This makes them useful because they can be re-used over and over again. Some catalysts provide a specific site for the collisions to occur. Other catalysts lower the amount of energy required to start the reaction (known as activation energy).