Rates of Reaction - Kinetics

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[edit] Chemical Kinetics

When we say "chemical kinetics", we mean that we are studying the rates of reaction, and what affects the rates. Chemists have been able to establish a fairly accurate way of predicting how long it will actually take for a reaction to move to completion (meaning there is no reaction going in the direction of the products.

To visualize this, we look at the decomposition of N₂O₅:

2N₂O₅ (g) → 4NO₂ (g) + O₂ (g)

So when we say "move to completion", we mean how long it takes for all possible NO₂ and O₂ to be formed from the reaction process.

[edit] Reaction Rates

To monitor how fast the above reaction is going we could, for example, look at how much N₂O₅ is left as a function of time. Alternatively, we could keep an eye on how much NO₂ is formed. The stoichiometric coefficients allow us to relate these different approaches by defining the Rate R_t as

R_t =-1/r d[R]/dt = + 1/p d[P]/dt

where R is any reactant and r its coefficient and P is any product and p its coefficient. In the above example we get

R_t =-1/2 d[N₂O₅(g)]/dt =1/4 d[NO₂ (g)]/dt= d[O₂(g)]/dt

In the study of kinetics often one of the concentrations is measured over time, eg, by looking at the optical absorption by one of the species. However, it is also possbile to look at overall effects of the reaction, such as the amount of heat generated or the change in conductivity of the reaction mixture. Provided we use properly balanced equations all these can be related to the rate R_t

[edit] What affects them

So what affects reaction rates? First, we might do well to remember what is required for a chemical reaction to take place:

Useful Not useful

Two or more molecules must collide They must collide with enough kinetic energy to react. We associate this with the activation energy (denoted as Ea). They must collide at a useful angle. A rather crummy example would be water and water (to the left).

To improve the rate of a reaction, these conditions can be altered.

To improve the likelihood of a collision, you increase the concentration, pressure or surface area.

To increase the kinetic energy of the reactants then you can increase the temperature. The problem of overcoming activation energy may also be addressed through the use of a catalyst.