Factors Affecting Rates of Reaction and Catalysis

Temperature and Reaction Rate

• Increasing the temperature causes reactant particles to move faster and collide more frequently.

• Particles collide with greater kinetic energy, meaning a higher proportion of particles possess energy greater than the activation energy.

• The frequency of effective collisions increases, which increases the rate of reaction.

• As temperature increases, the time taken for a reaction to complete decreases.

• Indigestion tablet investigation: Sodium hydrogencarbonate and citric acid react in water to release $CO_2(g)$; safety precautions require maintaining the temperature at or below $50\,^\circ\text{C}$.

Concentration and Pressure

• Concentration in Solutions: Increasing the concentration increases the number of reactant particles in a given volume of solution, increasing collision frequency and the reaction rate.

• Gas Pressure: Increasing the pressure squashes gas particles closer together into a given space, increasing collision frequency and the reaction rate.

• Rate Proportionality: Reaction rate is directly proportional to reactant concentration:

  • In the reaction between $0.167\,g$ of calcium and dilute hydrochloric acid, doubling acid concentration from $0.500\,mol\,dm^{-3}$ to $1.00\,mol\,dm^{-3}$ doubles the number of $H^+(aq)$ ions per unit volume.

  • Initial rate at $t = 0\,s$ for $0.500\,mol\,dm^{-3}$ acid: $\frac{80\,cm^3}{60\,s} = 1.33\,cm^3/s$.

  • Initial rate at $t = 0\,s$ for $1.00\,mol\,dm^{-3}$ acid: $\frac{80\,cm^3}{30\,s} = 2.67\,cm^3/s$.

• Environmental Impact: Higher concentrations of acid in rainwater accelerate the erosion and chemical weathering of limestone and marble ($CaCO_3$).

Catalysts and Activation Energy

• A catalyst increases the rate of a chemical reaction without being chemically changed or used up, allowing small amounts to be reused repeatedly.

• Catalysts do not increase collision frequency or particle kinetic energy; they provide an alternative reaction pathway with a lower activation energy.

• A higher proportion of reactant particles have sufficient energy to react, increasing the frequency of effective collisions.

• Catalysts change only the rate of reaction, not the products, and are written above reaction arrows rather than as reactants or products in balanced equations.

• Catalyst Poisoning: Impurities in reaction mixtures can chemically bind to a catalyst, blocking its active surfaces and rendering it inactive.

Industrial and Biological Applications of Catalysts

• Transition Metal Catalysts: Industrial processes frequently utilize transition metals, including iron for synthesizing ammonia, platinum for making nitric acid, and platinum or palladium in vehicle catalytic converters.

• Surface Area: Solid catalysts are prepared as powders, pellets, fine gauzes, or coated onto honeycombed supports to maximize their surface area to volume ratio for reactions occurring on their surfaces.

• Economic and Environmental Advantages: Catalysts enable reactions to proceed rapidly at lower temperatures and pressures, significantly lowering industrial energy costs, conserving non-renewable fossil fuels, and reducing carbon dioxide emissions to combat climate change.

• Biological Catalysts: Enzymes are large protein molecules folded into intricate shapes that accommodate reactant substrates, operating effectively at relatively low temperatures.

Experimental Methods for Measuring Rates

• Gas Volume Collection:

  • Decomposition of hydrogen peroxide: $2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$, using catalysts such as manganese(IV) oxide ($MnO_2$) or potassium iodide ($KI$).

  • Acid-carbonate reaction: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + CO_2(g) + H_2O(l)$.

  • Gas is collected and measured over time using either a gas syringe or an inverted burette/measuring cylinder inverted over a water trough.

  • Safety requirements: Wear eye protection; $MnO_2$ is harmful; ensure gas syringe pistons do not detach and break.

• Precipitate and Turbidity Monitoring:

  • Sodium thiosulfate reaction: $Na_2S_2O_3(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + SO_2(aq) + S(s) + H_2O(l)$.

  • Reaction rate is monitored by measuring the time taken for insoluble sulfur ($S(s)$) precipitate to obscure a mark through the solution.