factors that affect reaction rates

surface area of a solid reactant

concentration or pressure of a reactant

nature of the reactants

temperature

presence/absence of a catalyst.

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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...

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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

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.

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.

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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.

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.

Here's an analogy.

Imagine that you are baby-sitting a bunch of 6 year olds. You put them in a yard and you let them run around. Every now and then a couple of kids will run into each other. Now imagine that you decide to feed them some sugar. What happens? They run around faster and of course there are many more collisions. Not only that, the collisions are likely to be a lot harder/more intense.

Now, let's look at the effect graphically. Recall that in any sample of matter (the example we used previously was a gas), individual particles have different kinetic energies. Some are moving fast some are moving slowly, and most are moving at some intermediate speed.

Sarah Al Saeed

Individual properties of substances also affect reaction rates. The scope of these properties is broad and there are few generalizations that you can apply consistently. Some of the properties in this category are state of matter, molecular size, bond type and bond strength.

State of Matter

Gases tend to react faster than solids or liquids: It takes energy to separate particles from each other. In order to burn candle wax, the solid wax has to be melted and then vaporized before it reacts with oxygen. Methane gas is already in the gas state so it burns faster than wax.

Aqueous ions tend to react faster than species in other states of matter: Solid lead(II) nitrate will react with solid potassium iodide, but the reaction is really, really slow. That's because the ionic bonding in each reactant is strong and the ions in each compound are hard to separate from each other. When aqueous solutions of these compounds are mixed, the formation of lead(II) iodide is rapid. In aqueous solutions, the ions of each compound are dissociated. When the two the solutions are mixed together, all that is required for a reaction to occur is contact between the lead(II) ions and the iodide ions.

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