Chemistry - Rate of Reactions

Define Rate of Reaction

The Change in amount of reactants or product per unit time. It can be seen from the gradient of a reactant-time or product-time graph.

Describe a regular reactant-time/product-time graph.

Initially, the gradient is the steepest because the concentration of (reactant) is at the highest at the start, leading to the fastest rate of reaction

As the reaction progresses, the gradient becomes more gentle as the concentration of (reactant) is decreasing, leading to a decreasing rate of reaction.

Eventually, the gradient becomes zero and the graph plateaus. This is because the (limiting reactant) is all used up, and hence the rate of reaction is zero.

Why does an ‘S’-shaped graph occur?

An oxide layer on a metal strip takes time to dissolve, exposing the metal underneath . An exothermic reaction releases heat in the first few minutes, speeding up the reaction. One of the products of the reaction acts as a catalyst so the reaction speeds up as the first trace of products are formed.

Define the collusion theory.

For two reactant particles to successfully react, they must first collide with sufficient energy (activation energy) and in the correct orientation. Such collisions are known as effective collisions.

Explain temperature as a factor.

At higher temperatures, the rate of reaction increases. Reacting particles absorb heat and gain kinetic energy to move faster. These reactant particles collide into each other more frequently and with more force. A greater proportion of particles possess energy equal to or greater than the activation energy,

Explain presence of catalyst as a factor.

In the presence of a catalyst, the rate of reaction increases. The catalyst lowers the activation energy via an alternative reaction pathway. A greater proportion of particles possess energy equal to or greater than the Activation Energy. This leads to an increase in frequency of effective collisions.

Explain particle size/surface area as a factor.

With a smaller particle size, the rate of reaction increases. The solid has a greater total surface area for collisions to take place. These reacting particles collide into each other more frequently. This leads to an increase in frequency of effective collisions.

Explain pressure/concentration as a factor.

At higher pressures/concentrations, the rate of reaction increases. Reacting particles are closer together as there are more reacting particles per unit volume. These reactant particles collide into each other more frequently. This leads to an increase in frequency of effective collisions.

Catalyst

Substances that increase the rate of a chemical reaction, but it itself is recovered chemically unchanged at the end of the reaction.

Characteristics of catalysts

• Only a small amount of catalyst is needed to speed up a reaction

• Catalysts increase the rate of reaction but do not affect the yield.

• Catalysts are regenerated at the end, although it may be chemically changed in the middle of the reaction. It may be physically changed at the end of the reaction.

Catalyst for decomposition of hydrogen peroxide

MnO2 (Manganate (IV) oxide)

Catalyst for Haber Process of preparing ammonia

Fe (Iron)

Catalyst for industrial manufacture of margarine

Ni (Nickel)

Catalyst for catalytic cracking of crude oil

SiO2 (Silicon dioxide), Al2O3 (Aluminum oxide)

Catalyst for catalytic converters in car exhausts

Rh (Rhodium), Pd (Palladium), Pt (Platinum)