Reaction rate
The reaction rate of a chemical reaction, that is, the speed of production of products or reactants, may generally be expressed as rate = k [a]^x where [a] is the molar concentration of reactant (there may be several), x is the order for that reactant, and k is a constant. Order establishes the mathematical relation of concentration to rate; eg. for an order of 2, if the concentration doubles, the rate quadruples.
Rate is often expressed in moles per liter-second.
There are many factors that affect on the rate of reaction:
Reaction rate for the chemical reaction
a -> products
can be expressed, providing that the reaction is elementary (i.e., the mechanism of the reaction is exactly the manner in which it is written):
-r = k [a]
Rate r is negative, because reaction consumes A.
A pair of forward and reverse reactions may define an equilibrium process. For example:
a + b <=> x + y
The Reaction rate expression for the above reactions (assuming they each are elementary) can be expressed:
-r = k1 [a][b] - k2 [x][y]
, where k1 is rate constant for the reaction which consumes a and b. "k2" is rate constant for the backwards reaction, which consumes x and y and which products are a and b.
The constants k1 and k2 are related to the equilibrium constant for the reaction (K) by the following relationship:
K = k1/k2
The constant K can also be expressed:
K = [a][b]/[x][y] = k1/k2
Each reaction rate constant k(i.e., k1 and k2) has temperature dependedancy, which is usually presented as:
k = A*exp(Ea/RT)
where A is the frequency factor and Ea is the reaction's activation energy. T is the absolute temperature and R is the ideal gas constant. Ea is strictly non-negative, and so reaction rates are non-decreasing with temperature. The formula shown above is known as the Arrhenius equation. The values for A and Ea are dependent on the reaction (so, for example, they may differ between k1 and k2). There are also more complex possible, which describe temperature dependance of rate constant.
The equilibrium constant K is dependent on temperature. This equation is known as the van't Hoff equation.
Referenced By
Biology is soil study of micro-organisms | Chemical kinetics | Chemicals | Chemistry
|
