The rate of formation or the reaction rate of hydrochloric acid depends partially upon the concentration of HCl in the solution that it is mixed with. Generally speaking, the rate increases with a higher level of concentration. However, it may be tempting to assume that rate of formation is directly proportional to the increased concentration rate when, in fact, this is not necessarily the case. Instead, other factors also play a role in the process. Therefore, you cannot assume, for example, that doubling the concentration of HCl will double the reaction rate.
Another factor that plays a role in determining the formation rate of HCl is the specific chemicals or substances it is mixed with or reacts with. While a higher concentration of HCl will generally result in a faster reaction rate, this rate can be diminished somewhat by the substance it reacts with, which will, in some cases, slow the rate of reaction down. This partly depends upon the concentration of the chemical. For example, a diluted sodium thiosulphate solution mixed with HCl will slow the rate of formation.
The physical properties of HCl provide further indication of the expected rate of reaction. Hydrochloric acid often comes in various concentrations. Each level of concentration is typically more reactive that the previous. For example, a 20 percent solution is actually more than double the concentration of a 10 percent solution in terms of its kg HCl/m3 level. This means that reaction rate of a 20 percent solution is likely to be more than double the 10 percent solution. HCl generally comes in concentrations up to 38 percent.
Determining the average rate of formation for HCl is no simple matter given all of the factors that affect the rate at which the solution reacts. The average rate can only be determined by the specific chemical reaction. For instance, in the chemical equation H2 + 2ICl----> 2HCl + I2, the rate of formation for HCl is twice what it is for the I2. For the sake of argument, assume that the I2 has a formation rate of 0.03 M/s; the average rate of formation for HCl will be twice that at 0.06 M/s.