Courtesy: Nearing Zero.net

Applications of chemistry focus mainly on chemical reactions and their commercial use. Commercial use requires knowledge of stoichiometry, energetics, and rate. To be useful, reactions must occur at a reasonable rate. To produce the 20 million tons of ammonia needed for fertilizer each year, it is not enough to simply mix nitrogen and hydrogen gases and wait for them to react. We must understand the factors that govern the rate of a reaction. This area of chemistry is called chemical kinetics.

One of the main goals of chemical kinetics is to understand the steps by which a reaction occurs. This series of steps is called the reaction mechanism. Understanding the mechanism allows us to find ways to speed up the reaction. For example, the Haber process for the production of ammonia requires high temperatures to get commercially feasible rates. But higher temperatures (and more cost) would be required without the use of iron oxide, which speeds up the reaction. This exploration of chemical kinetics includes rate laws, reactions mechanisms, and simple models for reactions.

Reaction Rates Kinetics – how fast does a reaction proceed? Reaction rate is the change in the concentration of a reactant or a product with time (M/s). We can consider rate in terms of reactants or products. A B rate = - Δ[A] ΔtΔt rate = Δ[B] ΔtΔt Δ[A] = change in concentration of A over time period Δt Δ[B] = change in concentration of B over time period Δt Because [A] decreases with time, Δ[A] is negative.

A B rate = - Δ[A] ΔtΔt rate = Δ[B]Δ[B] ΔtΔt time

Consider the decomposition of nitrogen dioxide, a gas that causes air pollution. 2NO 2 (g) → 2NO(g) + O 2 (g) The results from an experiment are below. Notice that the concentration of the reactant (NO 2 ) decreases with time and the concentration of products (NO and O 2 ) increase with time.

The average rate for NO 2 (reactant) [NO 2 ] decreases with time – negative sign Average Rates over 50 s time intervals Note that the rate is not constant but decreases with time.

To determine the rate at a particular time (the instantaneous rate) the slope of a line tangent to the curve at that particular time must be computed. The figure shows a tangent drawn at t = 100 seconds. The slope of the line give the rate at t = 100 s. But

Remember the rate can also be defined in terms of the products but we must take into account the coefficients in the balanced equation. The stoichiometry determines the relative rates of consumption of reactants and production of products. 2NO 2 (g) → 2NO(g) + O 2 (g) Both NO 2 and NO have a coefficient of 2, so NO is produced at the same rate NO 2 is consumed. The product O 2 has a coefficient of 1, so it is produced half as fast as NO (coefficient of 2). Therefore, the rate of NO production is twice the rate of O 2 production.

The rate information can be summarized as follows: Rate of consumption of NO 2 = Rate of production of NO = 2(rate of production) of O 2 = = Because the rate is different, dependent on which reactant or product is being studied, we must be very specific when the rate is described for a chemical reaction. 2NO 2 (g) → 2NO(g) + O 2 (g)

Factors that Affect Reaction Rate 1.Temperature Collision Theory: When two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place. Kinetic Theory: Increasing temperature means the molecules move faster. 2.Concentrations of reactants More reactants mean more collisions if enough energy is present 3.Catalysts Speed up reactions by lowering activation energy 4.Surface area of a solid reactant More area for reactants to be in contact 5.Pressure of gaseous reactants or products Increased number of collisions