Over the time interval 300 to 400 seconds, the rate of reaction with respect to A is Δ[A]/Δt = 3.7 × 10-5 M/s. Over the same time interval what is the rate of reaction with respect to B, Δ[B]/Δt? Group of answer choices i) Δ[B]/Δt = (1/4)(Δ[A]/Δt) = (1/4)(3.7 × 10-5 M/s) = 9.2 × 10-6 M/s ii) Δ[B]/Δt = Δ[A]/Δt = 3.7 × 10-5 M/s iii) Δ[B]/Δt = (1/2)(Δ[A]/Δt) = (1/2)(3.7 × 10-5 M/s) = 1.8 × 10-5 M/s iv) Δ[B]/Δt = -(1/2)(Δ[A]/Δt) = -(1/2)(3.7 × 10-5 M/s) = -1.8 × 10-5 M/s

Change in concentration = (concentration) = concentrationfinal – concentrationinitial
change in time = t = tfinal – tinitial
For instance, as the reaction A + B C progresses, the concentration of C increases. The rate is expressed as the change in the molar concentration of C, [C], during the time interval t. Concentration of C = [C ]
For a specific reaction we need to take into account the stoichiometry; that is, we need the balanced equation. For example, let's express the rate of the following reaction in terms of the concentrations of the individual reactants and products.
2NO(g) + O2(g) 2NO2(g)
These concentrations can be monitored experimentally as a function of time. Notice from the balanced equation, that 2 mol NO reacts with 1 mol O2; therefore, the concentration of NO will decrease twice as fast as that of O2.
Since the rates of change of individual reactants and products may differ the convention is to make the reaction rate come out to be the same no matter which reactant or product is used to calculate it. First we divide each concentration change by the coefficient from the balanced equation
Second, a negative sign is inserted before terms involving reactants. The change in NO concentration, [NO], is negative because the concentration of NO decreases with time. Inserting a negative sign in the expression makes the rate of reaction a positive quantity.
For a general equation:
aA + bB cC
the rate can be expressed in terms of any individual reactant or product.
No matter which reactant or product we use, the reaction rate will be positive and have the same value. 1. Write expressions for the rate of the following reaction in terms of each of the reactants and products.
2N2O5(g) 4NO2(g) + O2(g)
Rate of reaction =
2. In the reaction 2NO(g) O2(g) + N2(g)
If the rate of formation of O2 is 0.054 M/s, what is the rate of change of NO concentration?
Average Rate. The average rate of reaction over any time interval is equal to the change in the concentration of a reactant of a product [C] divided by the time interval, t, during which the change occurred.
[A], or

Instantaneous Rate: The slope of the tangent to the curve at any given time or instant when you plot or graph [reactant] vs. time.

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