Rates of reaction
The rate of a reaction is measured as either the amount of reactant used up in a given time or the amount of product formed in a given time.
In order for a reaction to occur, the particles must collide.
If more collisions can be made to take place in a given time, the rate of the reaction will increase.
Some collisions don’t lead to a reaction taking place. Only collisions with sufficient energy can lead to a successful reaction. So, if we can increase the success rate of the collisions, this will also lead to an increase in the rate of reaction.
The minimum energy that must be possessed by the particles for the collision to be successful is called the activation energy.
Increases in concentration, pressure of gases and surface area of solids all lead to more collisions taking place per second.
Increase in temperature leads to more collisions per second but also collisions that have, on average, a greater energy so the success rate is higher too.
Catalysts lower the activation energy by providing an alternative route and so the success rate is higher.
Catalysts are usually specific to a particular reaction. They are very important in a number of industrial processes.
Concentrations are usually measured in units of mol dm-3 (moles per litre).
Equal volumes of gases at the same temperature and pressure contain the same number of molecules (and moles) of gas.
Energy and reactions
When chemical reactions occur there is an energy exchange with the surroundings.
If heat is given out to the surroundings, it is said to be an exothermic reaction, if heat is absorbed from the surroundings it is said to be an endothermic change. Beware, if a reaction “gets hot” it may seem like it is absorbing heat from outside but really it is the chemicals reacting, giving their heat energy to the water or container etc and then the heat spreads out to the surroundings.
It is an exothermic change. If a reaction can go in both forward and backward directions it is called a reversible reaction. If the forward direction is exothermic, the reverse direction must be equally endothermic. If the forward reaction is endothermic, the reverse must be equally exothermic.
If the system is closed (i.e. substances can’t escape or be added), a reversible reaction can come to equilibrium.
At equilibrium, the forward and reverse reactions keep going but at the same speed so that there is no overall change.
The equilibrium position can be adjusted by changing conditions of temperature and pressure. This can be used to our advantage in industry.
An increase in temperature favors the endothermic direction.
An increase in pressure favors the direction with a decrease in the number of moles of gas.
The conditions for reversible reactions in industry also take into account the needs to have a good rate and a minimal cost. A compromise is sometimes needed.
Energy use (and waste) must be minimized for the sake of maximum profit and environmental concern.
Electrolysis
Electrolysis is the passage of electricity to cause a chemical reaction.
Only ionic substances can undergo electrolysis.
Ionic substances must be either molten (liquid) or