Ggiven the reaction 4nh3(g) 5o2(g) --> 4no(g) 6h2o(g), what is the numerical value and sign in front of the change in [ammonia] over time for the rate expression of ammonia

Sign = Negative, since ammonia is a reactant.

Value = 1 / 4 (Inverse of it's coefficient in the equation)

Explanation:

4NH3(g) + 5O2(g) --> 4NO(g) + 6H2O(g)

The Reaction Rate for a given chemical reaction is the measure of the change in concentration of the reactants or the change in concentration of the products per unit time.

For a reaction of the form A+B→C

the rate can be expressed in terms of the change in concentration of any of its components;

rate = −Δ[A] / Δt

rate = − Δ[B] / Δt

rate = Δ[C] / Δt

Considering a reaction in which the coefficients are different:

A + 2B → 2D

It is clear that [B] decreases two times as rapidly as [A], so in order to avoid ambiguity when expressing the rate in terms of different components, it is customary to divide each change in concentration by the appropriate coefficient:

rate = − Δ[A] / Δt =  − Δ[B] / 2Δt= Δ[D] / 2Δt

Things to Note;

- For products, the sign is always positive and for reactants, the sign is always negative.

- The Numerical value is usually the inverse of the coefficient.

Rate Expression  of change in [ammonia] over time for the rate expression of ammonia:

Sign = Negative, since ammonia is a reactant.

Value = 1 / 4 (Inverse of it's coefficient in the equation)