Unit 2 - Controlling The Rate Flashcards
Reaction Rates
Reaction rates can be controlled by chemists. If the rate of the reaction is too low, the manufacturing process will not be economically viable. If the rate is too high, there is a risk of explosion.
Relative Rate of Reactant
For all reactants, the rate of reaction, is indirectly proportional to the time taken i.e if rate of reaction is high, the time taken is small.
When time is measured in seconds, relative rate is measured in s-1.
Collision Theory
Reactions occur when particles of reactants collide, provided they collide with a certain minimum kinetic energy. The more collisons between particles in a given time, the faster the reation. The higher the energy of the colliding particles, the greater the chance of an effective collision (one that results in a reaction).
Collisions have to have the correct energy (activation energy) and correct genometry/orrientation to occur.
Concentration
Increasing the concentration, increases the rate of the reaction. At higher oncentrations, there are more reacting particles present in a given volume so more collisions take place between these reactant particles so the reaction is faster.
Particle Size
In any reaction involving solids, decreasing the particle size i.e increasing the surface area, increases the rate of reaction. Reactants with a smaller particle size have a larger number of particles on the surface of the solid able to take part in the collisions.
Because it is so finey divided (very small particle size), dust in a flour mill can present a high explosion risk (an explosion is a very fast reaction). For this teason, extractors remove dust from the air.
Pressure
The pressure of a gas is a measure of how often the gas particles collide with the walls of the container. The pressure of a gas can be increased by compressing the same number of particles into a smaller volume.
For reactions, involving gases, increasing the pressure means in more reacting particles in a given volume which results in more collisions will take place between these reacting particles. Increasing the pressure increases the rate of reaction.
Collision Geometry
The orientation of the molecules colliding can also determine whether a reaction takes place.
Successful Collision: Only one collision may possibly lead to a reaction.
Temperature
Temperature is a measure of the average kinetic energy of the particles of a substance. At a low temerature, particles are moving slowly so have a low knietic energy. At a high temperature, particles are moving quickly so have a high kinetic energy.
Increasing the temperature causes a very marked increase in the rate of reaction. If the temperature of a reaction is raised the particles have a higher kinetic energy so move faster and collide more often. A chemical reaction can only occur when reacting particles collide with enough kinetic energy.
At higher temperature, as the particles have more energy, the collisions are more likely to result in a reaction.
Increased T increases:
* frequency of collisions
* energy of collisions
Activation Energy
Not all collisions result in a chemical reaction. For a reaction to occur the colliding particles must have a minimum kineti energy.
The activation energy is the minimum kinetic energy requireed by colliding particles before a reaction may occur.
In general, the lower the activation energy, the faster the reaction.
Temperature Effect
The distribution of energy changes when the temperature changes. When the temperature is increased, the kinetic energy is increased so the distribution graph moves to the right.
The avergae energy increases so at a higher temperature, there are many more molecules with energy greater than the activation energy. Therefore, more collisions are capable of producing a reaction will occur and the reaction will be faster.
Catalytic Effect
A catalyst speeds up a chemical reaction. Although the catalyst takes part in the reaction, it is regenerated at the end of the reaction. A catalyst lowers the activation energy (by providing an alternative pathway.) Therefore more particles now have enough energy to react when they collide so the rate of reaction increases greatly.
Reaction Pathways
Energy changes
During an exothermic reaction )e.g burning methane) some of the chemical potential energy in the reactants is released as heat energy. This means that the products contain less potential energy than the reactants.
During an endothermic reaction heat energy is absorbed from the surroundings and so the products have more potential energy than the reactants.
Enthalpy Change
The enthalpy change is the energy difference between products and reactants. The units are normally in kilojoules (kJ)
ΔH is always negative for an exothermic reaction.
ΔH is always positive for an endothermic reaction.
Activated Complex
An activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier during a reaction. The activate energy is the energy required by colliding particles to form an activated complex.
Values for activation energyare usualy given in kJ mol-1
Reaction Pathways
Effect of a Catalyst
A catalyst provides an alternative reaction pathway with a lower activation energy. The activation energy is lowered for both the forward and reverse reactions. Catalysts have no effect on the potential energy of the reactants or products i.e enthalpy change.
