Question

Constants | Periodic Table

Consider the reaction

A+2B⇌C

whose rate at 25 ∘C was measured using three different sets of initial concentrations as listed in the following table:

Trial	[A] (M)	[B] (M)	Rate (M/s)
1	0.50	0.050	1.5×10−2
2	0.50	0.100	3.0×10−2
3	1.00	0.050	6.0×10−2

Part a

Calculate the initial rate for the formation of C at 25 ∘C, if [A]=0.50M and [B]=0.075M.

Express your answer to two significant figures and include the appropriate units.

View Available Hint(s)

initial rate =

?

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Constants | Periodic Table

Learning Goal:

To calculate average and relative reaction rates.

Reaction rate can be defined either as the increasein the concentration of a product per unit time or as the decrease of the concentration of a reactant per unit time. By definition, reaction rate is a positive quantity.

In the reaction X→2Y, for example, Y is being produced twice as fast as X is consumed and thus

rate of X=12(rate of Y)

Each rate can be expressed as the change in concentration over the change in time, Δt:

−Δ[X]Δt=12(Δ[Y]Δt)

Time (s)	0	10.0	20.0	30.0	40.0	50.0
[P2O5] (mol L−1)	0	2.90×10−3	5.90×10−3	7.70×10−3	8.90×10−3	9.50×10−3

Part e

What is the average rate of consumption of H+ during the same time interval?

Express your answer with the appropriate units.

View Available Hint(s)

Rate of consumption of H+ =

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Answer 1

Part 1)

Here, rate = k[A]² [B]

From experiment (1)

1.5 x 10^-2 = k x 0.5² x 0.05

Therefore, k= (1.5 x 10^-2) / (0.5² x 0.05) = 1.2 M^-2s^-1

Now, initial rate for the formation of C at 25 ^∘C, if [A] = 0.50M and [B] = 0.075M can be calculated as,

Rate = 1.2 x [0.5]² x [0.075] = 0.022 M/s

Part 2)

Rate of formation of P₂O₅ = −Δ[A]/Δt = [(8.90×10⁻³)-( 2.90×10⁻³)] / (40-10)

= (6 × 10⁻³) / 30 = 2 x 10^-4 M/s

Part 3)

Here, this part is incomplete, the average rate of consumption of Br^- is not provided.

If you know the average rate of consumption of Br^-, then according to the following reaction,

5Br^-(aq) + BrO₃^-(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O (l)

Average rate of consumption of H⁺ = average rate of consumption of Br^- x (6/5)

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