D.I. Mendeleev A.M.Butlerov

The velocity of chemical reactions
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The velocity of chemical reactions. Additional material.

Nuance 1:

This expression

v = Dc/Dt

allows to determine only average velocity of reaction for a given time interval. But scientists, as a rule, are interested in the velocity at a given moment of time, i.e. so-called moment velocity of reaction. It is defined as a derivative of function c(t):

v = dc/dt

graph c(t)

If we determine the velocity of reaction by one of the reagents then derivative c(t) has a negative value since concentration of reagents decrease. But this is impossible, as it does not comply with the physical meaning. Therefore the formula is modified to have physical sense

v = -dc/dt

Nuance 2:

Let's determine the velocity of the same reaction

H2 + I2 = 2HI

Not by decrease of concentrations of reagents, but by increase of concentration of products

v(HI) = dc(HI)/dt

v(H2) is equal to v(I2), but unequal to v(HI), as if the concentrations of reagents decrease in 3 times, the concentration of HI would increase in 2*3=6 times!. To make speaking about the overall velocity of reaction possible, the rate of the reaction should be divided by the coefficient:

v(HI) = dc(HI)/2dt

In general case, for the reaction

aA + bB = eE + fF

the velocity is

v = -dc(A)/adt = -dc(B)/bdt = dc(E)/edt = dc(F)/fdt



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