14.1.2 Rate Laws: How the Reaction Rate Depends on Concentration

Question 1

Rate Laws: How the Reaction Rate Depends on Concentration

Answer 1

• The overall reaction rate can be expressed in terms of the rate of change of any component of the reaction.
• The rate law for a reaction is an equation relating the rate of production of products (or consumption of reactants) to the concentrations of starting materials.

Question 2

note

Answer 2

• The overall reaction rate can be expressed in terms of the rate of change of any component of the reaction. Since reactants are disappearing over the course of a reaction, the rate is expressed as negative for these components of the reaction.
• For example, in the decomposition of dinitrogen pentoxide (N 2 O 5 ) to form nitrogen dioxide (NO 2 ) and oxygen (O 2 ), the rate of production of oxygen is equal to 1/4 the rate of production of nitrogen dioxide, and –1/2 the rate of disappearance of dinitrogen pentoxide.
• The rate law for a reaction is an equation relating the rate of production of products to the concentrations of starting materials.
• As an analogy, consider a junior high school dance. The rate of production of dancing couples is proportional to the number of non-dancing boys and girls at any given time. As couples form, and the number of non-dancing boys and girls decreases, the rate of production of dancing couples decreases. The rate law for this situation would be expressed as rate = k · [boys][girls], where k is the rate constant for this reaction.
• In a general rate law, rate = k · [A] m [B] n , the constants m and n are the orders of the reaction. The reaction is said to be m the order with respect to A and nth order with respect to b. The reaction has an overall order of m + n. The values of m and n usually have no relation to the stoichiometric coefficients a and b—they must be determined experimentally.

Question 3

Chlorine gas(Cl2 ) reacts with fluorine gas (F2 ) according to the following equation.

Cl2(g) + 3F2(g) → 2ClF3(g)

Suppose you combine one mole of Cl2 with three moles of F2, and then made a graph of the concentration of the reactants and the products versus time. Which of the following statements would be true of the graph at time = t ?

Answer 3

The slope of the curve for ClF3 will equal twice the rate of the reaction.

Question 4

Dicyclopentadiene (C10H12 ) forms cyclopentadiene (C5H6 ) according to the following equation.

C10H12 → 2C5H6

This reaction is first order overall, and the rate constant is equal to 1.92 × 10^−4 s^−1 at 170°C. What is the rate of formation of C5H6 when the concentration of C10H12 is 0.162 M?

Answer 4

6.22 × 10^−5 M / s

Question 5

The graph shows the concentrations of the reactants and products of a reaction over time.

Based on this graph, which of the following conclusions can you draw about the reaction?

Answer 5

The reaction can be written as 2C + A → B

Question 6

Nitrogen dioxide (NO2 ) reacts with carbon monoxide (CO) according to the following equation.

NO2(g) + CO(g) → NO(g) + CO2(g)

If this reaction is second order in [ NO2 ] and second order overall, what are the units of k in the rate law of this reaction?

Answer 6

M ^−1s^−2

Question 7

Nitrogen dioxide (NO2 ) reacts with ozone(O3 ) according to the equation:

2NO2(g) + O3(g) → N2O5(g) + O2(g)

If the rate law for this reaction is rate = k[ NO2 ] [O3 ], what is the overall order of the reaction?

Answer 7

second

Question 8

Borohydride ions (BH4− ) react with water according to the following equation.

BH4− (aq) + 4H2O(l) → B(OH)4− (aq) + 4H2(g)

The following graph shows the concentrations of the reactants and products of this reaction over time. Which curve best represents the concentration of B(OH)4−?

Answer 8

C

Question 9

Mercury(II) chloride (HgCl2 ) reacts with oxalate ions (C2O42− ) according to the following equation.

2HgCl2(aq) + C2O42− (aq) → 2Cl − (aq) + 2CO2(g) + Hg2Cl2(s)

If Δ[CO2 ] / Δt = 5.2 × 10−7 M / s,
what is the value of Δ[C2O42− ] / Δt at the
same instant?

Answer 9

−2.6 × 10^−7 M / s

Question 10

Iodomethane reacts with hydroxide ions according to the following equation.

CH3I(aq) + OH − → CH3OH(aq) + I − (aq)

The dependence of the reactant concentration on the rate can be written as,
Rate = k [CH3I] [OH − ].
What can you conclude about this reaction based on this information?

Answer 10

The reaction is second order overall.

Question 11

Suppose that the following reaction is first order in [B] and second order overall.

2A + B → 2C

Which of the following equations represents the rate law for this reaction?

Answer 11

−d [B] / dt = k[A] [B]

Question 12

Disulfur dichloride (S2Cl2 ) reacts with carbon tetrachloride (CCl4 ) according to the following equation.

S2Cl2(g) + CCl4(g) → CS2(g) + 3Cl2(g)

This reaction is first order in [S2Cl2 ] and second order in [CCl4 ]. Which of the following changes would increase the initial rate of the reaction?

Answer 12

decreasing [S2Cl2 ] by a factor of 2 and doubling [CCl4 ]