What factors influence reaction rates? How?
Chemical reactions proceed at different rates. The factors that affect reaction rates are:surface area of a solid reactantconcentration or pressure of a reactanttemperaturenature of the reactantspresence/absence of a catalyst.
A change in one or more of these factors may alter the rate of a reaction. In this lesson, you will define these factors, and describe and predict their effects on reaction rates.Surface Area
Surface area is the exposed matter of a solid substance.
Imagine that you are holding a perfect cube of magnesium. The surface area is the sum of the area of all six sides of the cube. The surface area of the cube can be increased by dividing the cube into smaller cubes. Surface area is maximized when a single large cube is crushed to fine powder.
The rate of reaction of a solid substance is related to its surface area. In a reaction between a solid and an aqueous/liquid/gas species, increasing the surface area of the solid-phase reactant increases the number of collisions per second and therefore increases the reaction rate.
In a reaction between magnesium metal and hydrochloric acid, magnesium atoms must collide with the hydrogen ions. When the magnesium atoms form one big lump...
the number of collisions per second between magnesium and hydrogen is higher, and the rate of reaction is faster.
Increasing the surface area of a solid reactant increases the reaction rate.
By increasing surface area, there are more collisions per unit of time. That's why many solids are powdered using a mortar and pestle before being used in a reaction.
Examples of other reactions where surface area is important are:active metals with acids, e.g. HCl with zinccoal dust with oxygen gasgrain dust with oxygen gasConcentration
The concentration of a substance can be expressed in a variety of ways depending on the nature of a substance. Aqueous solutions typically have their concentrations expressed in mol/L. For example, a solution made by dissolving sodium hydroxide in water has its concentration expressed as moles of NaOH per litre of solution. Gases can also have their concentrations expressed in mol/L.
In terms of the collision theory, increasing the concentration of a reactant increases in the number of collisions between the reacting species per second and therefore increases the reaction rate.
Consider the reaction between hydrochloric acid and zinc metal.
In one beaker, 6.00 mol/L HCl is reacted with 2.00 g of Zn.
In another, 1.00 mol/L HCl is reacted with 2.00 g of Zn.
Which reaction should occur at the faster rate?
In terms of the collision theory, collisions between zinc atoms and hydrochloric acid are more frequent in the beaker containing 6.0 M HCl - there is more acid per unit of volume.
You can change the concentration of an aqueous species by simply adding more solute (to make it more concentrated) or adding more solvent (to make it more dilute).
You can change the concentration of a gas by adding more gas to a fixed volume or by decreasing the volume of the container. Conversely, the concentration of a gas can be decreased by removing (evacuating) a gas from a fixed volume or by increasing the volume of the container.Pressure
The concentration of a gas is a function of the pressure on the gas. Increasing the pressure of a gas is exactly the same as increasing its concentration. If you have a certain number of gas molecules, you can increase the pressure by forcing them into a smaller volume.
Under higher pressure or at a higher concentration, gas molecules collide more frequently and react at a faster rate. Conversely, increasing the volume of a gas decreases pressure which in turn decreases the collision frequency and thus reduces the reaction rate.
It is important to note however that there are reactions involving gases in which a pressure change does not affect the reaction rate. For this reason, the rates of reactions involving gases have to be determined by experiment.
Also note that solids and liquids are not affected by pressure changes.
Need a good analogy for the effect of concentration on the rate of a chemical reaction?Temperature
With the exception of some precipitation reactions involving ionic compounds in solution, just about all chemical reactions take place at a faster rate at higher temperatures. The question is why?
Temperature (in Kelvin degrees) is proportional to the kinetic energy of the particles in a substance. For example, if the Kelvin temperature of a substance is doubled, then the average kinetic energy of the particles in that substance is doubled.
At higher temperatures, particles collide more frequently and with greater intensity.
Increasing the temperature by say 10°C causes some of the intermediate speed molecules to move faster. The result is more molecules with sufficient kinetic energy to form an activated complex upon collision!
Now consider the relationship between threshold kinetic energy...