Factors Affecting Rates of Reactions (Leaving Cert Chemistry): Revision Notes

Factors Affecting Rates of Reactions

The speed at which chemical reactions occur can be influenced by several key factors. Understanding these factors is crucial for controlling reactions in laboratory and industrial settings. Through experimentation, chemists have identified six main factors that affect reaction rates, which we'll explore in detail.

Nature of reactants

The type of chemical bonds present in reactants significantly influences how quickly a reaction proceeds. This factor relates to whether compounds are ionic or covalent in nature.

Ionic vs covalent compounds

Ionic compounds generally react much faster than covalent compounds. This occurs because ionic substances are already dissociated into ions when dissolved in solution, meaning they can react immediately without needing to break existing bonds first.

Covalent compounds typically react more slowly because their atoms are held together by shared electron pairs. Before these compounds can participate in reactions, energy must be supplied to break these covalent bonds, which takes additional time.

Practical examples

Consider what happens when you mix solutions containing silver ions (Ag⁺) and chloride ions (Cl⁻). The reaction occurs almost instantaneously because both reactants are already in ionic form:

$\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl} \downarrow$

In contrast, when hexane (a covalent compound) reacts with bromine, the process takes much longer:

$\text{C}_6\text{H}_{14} + \text{Br}_2 \rightarrow \text{C}_6\text{H}_{13}\text{Br} + \text{HBr}$

The stopwatch needs to run for over 16 minutes because covalent bonds in hexane must first be broken before new bonds can form. This demonstrates why covalent reactions are generally slower than ionic reactions.

Surface area

The amount of surface area exposed to reactants plays a crucial role in determining reaction speed. When reactants have more surface area available for contact, collisions between particles become more frequent, leading to faster reactions.

The marble chip experiment

This principle is clearly demonstrated when comparing the reaction of hydrochloric acid with large marble lumps versus small marble chips. Small chips react much faster because they provide a greater surface area for acid particles to collide with the calcium carbonate.

Dust explosions

An extreme example of surface area effects can be seen in dust explosions, which represent serious industrial hazards. When normally combustible materials are ground into very fine particles, they can create explosive conditions.

Laboratory demonstrations using lycopodium powder (a fine plant spore) safely show how dramatically increased surface area can affect combustion rates. When the powder is dispersed through a flame, it creates an impressive fireball effect.

Concentration

The concentration of reactants directly affects how quickly reactions proceed. Higher concentrations mean more reactant particles are available in a given volume, leading to more frequent collisions and faster reaction rates.

Experimental investigation

This relationship can be demonstrated using magnesium ribbon reacting with hydrochloric acid of different concentrations:

$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \uparrow$

When magnesium is placed in more concentrated acid, hydrogen gas is produced much more rapidly. The experimental setup allows us to measure gas production over time, clearly showing the effect of concentration changes.

Mathematical relationship

The relationship between concentration and reaction rate is directly proportional. This means:

• Doubling the concentration doubles the rate
• Halving the concentration halves the rate

Since reaction rate is often measured as the reciprocal of time ($\frac{1}{t}$), we can express this as:

$\text{Rate} \propto \frac{1}{\text{time}}$

The general rate equation can be written as:

$\text{Rate} = \frac{\text{Change in concentration (mol/L)}}{\text{Time (seconds)}}$

Worked example with sodium thiosulfate

This linear relationship is a hallmark of first-order kinetics with respect to concentration.

Temperature

Temperature has a profound effect on reaction rates. We observe this in everyday life - food spoils more slowly when refrigerated because the chemical reactions causing decay proceed more slowly at lower temperatures.

Effect on molecular motion

Higher temperatures increase the kinetic energy of particles, causing them to move faster and collide more frequently. These more energetic collisions are also more likely to overcome the activation energy barrier required for reaction to occur.

Catalysts

Catalysts are special substances that alter the rate of chemical reactions without being permanently consumed in the process. They provide an alternative reaction pathway with lower activation energy.

Definition and properties

Key characteristics of catalysts include:

• They remain chemically unchanged after the reaction
• Only small amounts are needed to be effective
• They provide alternative reaction pathways
• They can be recovered and reused

Types of catalysts

Positive catalysts (or simply "catalysts") speed up reactions. For example, manganese dioxide catalyses the decomposition of hydrogen peroxide:

$\text{H}_2\text{O}_2 \xrightarrow{\text{MnO}_2} \text{H}_2\text{O} + \frac{1}{2}\text{O}_2 \uparrow$

Negative catalysts (also called inhibitors) slow down reactions. Glycerine is sometimes added to hydrogen peroxide solutions to slow down decomposition during storage.