FACTORS AFFECTING RATE OF REACTION

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.

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.

COLLISON THEORY

• 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.

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

NATURE OF REACTANTS

• Nature of Reactants 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.

STATES 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.

CATALYST

• A catalyst is a species that speeds up a chemical reaction without being chemically changed upon completion of the reaction. In other words, the mass of a catalyst is the same before and after a reaction occurs. Common examples of catalysts include: MnO2 in the decomposition of H2O2 Fe in the manufacture of NH3 Pt in the conversion of NO and CO to N2 and CO2

SURFACE AREA

• 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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