Rates and Energy

HOW FAST?

The rate of a reaction measures the speed of a reaction or how fast it is.  The rate can be found by measuring how much of a reactant is used or how much of a product is formed and the time taken.  The rate can also be found by measuring the time taken for a certain amount of reactant to be used or product to be formed.  These methods give the average rate for the time measured.

Rate of reaction = amount of reactant used / time OR amount of product formed / time

An average rate can also be found by measuring the time it takes for a certain amount of solid to appear in a solution.  If a gas is given off in a reaction, its average rate can be found by measuring the time taken to collect a certain amount of gas.

The rate of reaction at any given time can be found from the gradient of the line on a graph of amount of reactant used or product formed against time.  The steeper the gradient, the faster the rate of reaction.  A graph can be produced by measuring the mass of gas released or the volume of gas produced at intervals of time.  You can also measure changes in the colour, concentration or pH of a reaction mixture over time.

COLLISION THEORY AND SURFACE AREA

Collision theory states that reactions can only happen if particles collide.  However, the particles must also collide with enough energy to change into new substances.  The minimum energy they need to react is called the activation energy.

Factors that increase the chance of collisions or the energy of the particles will increase the rate of reaction:

• Temperature
• Concentration of solutions
• Pressure of gases
• Surface area of solids
• Using a catalyst

Breaking large pieces of a solid into smaller pieces exposes new surfaces and so increases the surface area.  This means there are more collisions in the same time.  This means that a powder reacts faster than large chunks of a solid.  The finer the powder, the faster the rate of reaction.

THE EFFECT OF TEMPERATURE

Increasing the temperature increases the speed of the particles in a reaction mixture.  This means they collide more often, increasing the rate of reaction.  They also collide with more energy, also increasing the rate of reaction.  A small change in temperature has a very large effect on reaction rates.  A rise of 10 degrees Celsius will double the rate of most reactions.  A decrease in temperatures will slow reactions down.

THE EFFECT OF CONCENTRATION AND PRESSURE

The particles in a solution are moving around randomly…