Solved: What factors influence reaction rates? How?

Answer :

Chemical reactions proceed at different rates. The factors that affect reaction rates are:

• surface area of a solid reactant
• concentration or pressure of a reactant
• temperature
• nature of the reactants
• presence/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...

however, ...

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 zinc
• coal dust with oxygen gas
• grain dust with oxygen gas

Concentration

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.