12.1 Chemical reaction rates (Page 3/8)

Chemistry Page 3 / 8

Relative rates of reaction

The rate of a reaction may be expressed in terms of the change in the amount of any reactant or product, and may be simply derived from the stoichiometry of the reaction. Consider the reaction represented by the following equation:

{2NH}_{3} (g) ⟶ N_{2} (g) + {3H}_{2} (g)

The stoichiometric factors derived from this equation may be used to relate reaction rates in the same manner that they are used to related reactant and product amounts. The relation between the reaction rates expressed in terms of nitrogen production and ammonia consumption, for example, is:

- \frac{{Δmol NH}_{3}}{Δ t} \times \frac{1 mol N_{2}}{2 mol {NH}_{3}} = \frac{Δmol N_{2}}{Δ t}

We can express this more simply without showing the stoichiometric factor’s units:

- \frac{1}{2} \frac{Δmol {NH}_{3}}{Δ t} = \frac{Δmol N_{2}}{Δ t}

Note that a negative sign has been added to account for the opposite signs of the two amount changes (the reactant amount is decreasing while the product amount is increasing). If the reactants and products are present in the same solution, the molar amounts may be replaced by concentrations:

- \frac{1}{2} \frac{Δ [{NH}_{3}]}{Δ t} = \frac{Δ [N_{2}]}{Δ t}

Similarly, the rate of formation of H ₂ is three times the rate of formation of N ₂ because three moles of H ₂ form during the time required for the formation of one mole of N ₂ :

\frac{1}{3} \frac{Δ [H_{2}]}{Δ t} = \frac{Δ [N_{2}]}{Δ t}

[link] illustrates the change in concentrations over time for the decomposition of ammonia into nitrogen and hydrogen at 1100 °C. We can see from the slopes of the tangents drawn at t = 500 seconds that the instantaneous rates of change in the concentrations of the reactants and products are related by their stoichiometric factors. The rate of hydrogen production, for example, is observed to be three times greater than that for nitrogen production:

\frac{2.91 \times 10^{−6} M /s}{9.71 \times 10^{−6} M /s} \approx 3

A graph is shown with the label, “Time ( s ),” appearing on the x-axis and, “Concentration ( M ),” on the y-axis. The x-axis markings begin at 0 and end at 2000. The markings are labeled at intervals of 500. The y-axis begins at 0 and includes markings every 1.0 times 10 superscript negative 3, up to 4.0 times 10 superscript negative 3. A decreasing, concave up, non-linear curve is shown, which begins at about 2.8 times 10 superscript negative 3 on the y-axis and nearly reaches a value of 0 at the far right of the graph at the 2000 marking on the x-axis. This curve is labeled, “[ N H subscript 3].” Two additional curves that are increasing and concave down are shown, both beginning at the origin. The lower of these two curves is labeled, “[ N subscript 2 ].” It reaches a value of approximately 1.25 times 10 superscript negative 3 at 2000 seconds. The final curve is labeled, “[ H subscript 2 ].” It reaches a value of about 3.9 times 10 superscript negative 3 at 2000 seconds. A red tangent line segment is drawn to each of the curves on the graph at 500 seconds. At 500 seconds on the x-axis, a vertical dashed line is shown. Next to the [ N H subscript 3] graph appears the equation “negative capital delta [ N H subscript 3 ] over capital delta t = negative slope = 1.94 times 10 superscript negative 6 M / s.” Next to the [ N subscript 2] graph appears the equation “negative capital delta [ N subscript 2 ] over capital delta t = negative slope = 9.70 times 10 superscript negative 7 M / s.” Next to the [ H subscript 2 ] graph appears the equation “negative capital delta [ H subscript 2 ] over capital delta t = negative slope = 2.91 times 10 superscript negative 6 M / s.” — This graph shows the changes in concentrations of the reactants and products during the reaction $2 {NH}_{3} ⟶ 3 N_{2} + H_{2} .$ The rates of change of the three concentrations are related by their stoichiometric factors, as shown by the different slopes of the tangents at t = 500 s.

Expressions for relative reaction rates

The first step in the production of nitric acid is the combustion of ammonia:

4 {NH}_{3} (g) + 5 O_{2} (g) ⟶ 4 NO (g) + 6 H_{2} O (g)

Write the equations that relate the rates of consumption of the reactants and the rates of formation of the products.

Solution

Considering the stoichiometry of this homogeneous reaction, the rates for the consumption of reactants and formation of products are:

- \frac{1}{4} \frac{Δ [{NH}_{3}]}{Δ t} = - \frac{1}{5} \frac{Δ [O_{2}]}{Δ t} = \frac{1}{4} \frac{Δ [NO]}{Δ t} = \frac{1}{6} \frac{Δ [H_{2} O]}{Δ t}

Check your learning

The rate of formation of Br ₂ is 6.0

\times

10 ⁻⁶ mol/L/s in a reaction described by the following net ionic equation:

{5Br}^{−} + {BrO}_{3}^{−} + {6H}^{+} ⟶ {3Br}_{2} + {3H}_{2} O

Write the equations that relate the rates of consumption of the reactants and the rates of formation of the products.

Answer:

$- \frac{1}{5} \frac{Δ [{Br}^{−}]}{Δ t} = - \frac{Δ [{BrO}_{3}^{−}]}{Δ t} = - \frac{1}{6} \frac{Δ [H^{+}]}{Δ t} = \frac{1}{3} \frac{Δ [{Br}_{2}]}{Δ t} = \frac{1}{3} \frac{Δ [H_{2} O]}{Δ t}$

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Reaction rate expressions for decomposition of h ₂ O ₂

The graph in [link] shows the rate of the decomposition of H ₂ O ₂ over time:

{2H}_{2} O_{2} ⟶ {2H}_{2} O + O_{2}

Based on these data, the instantaneous rate of decomposition of H ₂ O ₂ at t = 11.1 h is determined to be
3.20 $\times$ 10 ⁻² mol/L/h, that is:

- \frac{Δ [H_{2} O_{2}]}{Δ t} = 3.20 \times 10^{−2} {mol L}^{−1} h^{−1}

What is the instantaneous rate of production of H ₂ O and O ₂ ?

Solution

Using the stoichiometry of the reaction, we may determine that:

- \frac{1}{2} \frac{Δ [H_{2} O_{2}]}{Δ t} = \frac{1}{2} \frac{Δ [H_{2} O]}{Δ t} = \frac{Δ [O_{2}]}{Δ t}

Therefore:

\frac{1}{2} \times 3.20 \times 10^{−2} mol L^{−1} h^{−1} = \frac{Δ [O_{2}]}{Δ t}

and

\frac{Δ [O_{2}]}{Δ t} = 1.60 \times 10^{−2} mol L^{−1} h^{−1}

Check your learning

If the rate of decomposition of ammonia, NH ₃ , at 1150 K is 2.10

\times

10 ⁻⁶ mol/L/s, what is the rate of production of nitrogen and hydrogen?

Answer:

1.05 $\times$ 10 ⁻⁶ mol/L/s, N ₂ and 3.15 $\times$ 10 ⁻⁶ mol/L/s, H ₂ .

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Key concepts and summary

The rate of a reaction can be expressed either in terms of the decrease in the amount of a reactant or the increase in the amount of a product per unit time. Relations between different rate expressions for a given reaction are derived directly from the stoichiometric coefficients of the equation representing the reaction.

Key equations

$relative reaction rates for a A ⟶ b B = - \frac{1}{a} \frac{Δ [A]}{Δ t} = \frac{1}{b} \frac{Δ [B]}{Δ t}$

Chemistry end of chapter exercises

What is the difference between average rate, initial rate, and instantaneous rate?

The instantaneous rate is the rate of a reaction at any particular point in time, a period of time that is so short that the concentrations of reactants and products change by a negligible amount. The initial rate is the instantaneous rate of reaction as it starts (as product just begins to form). Average rate is the average of the instantaneous rates over a time period.

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Ozone decomposes to oxygen according to the equation ${2O}_{3} (g) ⟶ {3O}_{2} (g) .$ Write the equation that relates the rate expressions for this reaction in terms of the disappearance of O ₃ and the formation of oxygen.

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In the nuclear industry, chlorine trifluoride is used to prepare uranium hexafluoride, a volatile compound of uranium used in the separation of uranium isotopes. Chlorine trifluoride is prepared by the reaction ${Cl}_{2} (g) + {3F}_{2} (g) ⟶ {2ClF}_{3} (g) .$ Write the equation that relates the rate expressions for this reaction in terms of the disappearance of Cl ₂ and F ₂ and the formation of ClF ₃ .

$rate = + \frac{1}{2} \frac{Δ [{CIF}_{3}]}{Δ t} = - \frac{Δ [{Cl}_{2}]}{Δ t} = - \frac{1}{3} \frac{Δ [F_{2}]}{Δ t}$

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A study of the rate of dimerization of C ₄ H ₆ gave the data shown in the table:
${2C}_{4} H_{6} ⟶ C_{8} H_{12}$

Time (s)	0	1600	3200	4800	6200
[C ₄ H ₆ ] ( M )	1.00 $\times$ 10 ⁻²	5.04 $\times$ 10 ⁻³	3.37 $\times$ 10 ⁻³	2.53 $\times$ 10 ⁻³	2.08 $\times$ 10 ⁻³

(a) Determine the average rate of dimerization between 0 s and 1600 s, and between 1600 s and 3200 s.

(b) Estimate the instantaneous rate of dimerization at 3200 s from a graph of time versus [C ₄ H ₆ ]. What are the units of this rate?

(c) Determine the average rate of formation of C ₈ H ₁₂ at 1600 s and the instantaneous rate of formation at 3200 s from the rates found in parts (a) and (b).

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A study of the rate of the reaction represented as $2 A ⟶ B$ gave the following data:

Time (s)	0.0	5.0	10.0	15.0	20.0	25.0	35.0
[ A ] ( M )	1.00	0.952	0.625	0.465	0.370	0.308	0.230

(a) Determine the average rate of disappearance of A between 0.0 s and 10.0 s, and between 10.0 s and 20.0 s.

(b) Estimate the instantaneous rate of disappearance of A at 15.0 s from a graph of time versus [ A ]. What are the units of this rate?

(c) Use the rates found in parts (a) and (b) to determine the average rate of formation of B between 0.00 s and 10.0 s, and the instantaneous rate of formation of B at 15.0 s.

(a) average rate, 0 − 10 s = 0.0375 mol L ⁻¹ s ⁻¹ ; average rate, 12 − 18 s = 0.0225 mol L ⁻¹ s ⁻¹ ; (b) instantaneous rate, 15 s = 0.0500 mol L ⁻¹ s ⁻¹ ; (c) average rate for B formation = 0.0188 mol L ⁻¹ s ⁻¹ ; instantaneous rate for B formation = 0.0250 mol L ⁻¹ s ⁻¹

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Consider the following reaction in aqueous solution:
${5Br}^{−} (aq) + {BrO}_{3}^{−} (aq) + {6H}^{+} (aq) ⟶ {3Br}_{2} (aq) + {3H}_{2} O(l)$

If the rate of disappearance of Br ^– ( aq ) at a particular moment during the reaction is 3.5 $\times$ 10 ⁻⁴ M s ⁻¹ , what is the rate of appearance of Br ₂ ( aq ) at that moment?

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Questions & Answers

Three charges q_{1}=+3\mu C, q_{2}=+6\mu C and q_{3}=+8\mu C are located at (2,0)m (0,0)m and (0,3) coordinates respectively. Find the magnitude and direction acted upon q_{2} by the two other charges.Draw the correct graphical illustration of the problem above showing the direction of all forces.

Kate Reply

To solve this problem, we need to first find the net force acting on charge q_{2}. The magnitude of the force exerted by q_{1} on q_{2} is given by F=\frac{kq_{1}q_{2}}{r^{2}} where k is the Coulomb constant, q_{1} and q_{2} are the charges of the particles, and r is the distance between them.

Muhammed

What is the direction and net electric force on q_{1}= 5µC located at (0,4)r due to charges q_{2}=7mu located at (0,0)m and q_{3}=3\mu C located at (4,0)m?

Kate Reply

what is the change in momentum of a body?

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what is a capacitor?

Raymond Reply

Capacitor is a separation of opposite charges using an insulator of very small dimension between them. Capacitor is used for allowing an AC (alternating current) to pass while a DC (direct current) is blocked.

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A motor travelling at 72km/m on sighting a stop sign applying the breaks such that under constant deaccelerate in the meters of 50 metres what is the magnitude of the accelerate

Maria Reply

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Sharon

8m/s²

Aishat

What is Thermodynamics

Muordit

velocity can be 72 km/h in question. 72 km/h=20 m/s, v^2=2.a.x , 20^2=2.a.50, a=4 m/s^2.

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A boat travels due east at a speed of 40meter per seconds across a river flowing due south at 30meter per seconds. what is the resultant speed of the boat

Saheed Reply

50 m/s due south east

Someone

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I believe temperature being an intensive property does not change for any amount of boiling water whereas heat being an extensive property changes with amount/size of the system.

Someone

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Someone

temperature for any amount of water to boil at ntp is 100⁰C (it is a state function and and intensive property) and it depends both will give same amount of heat because the surface available for heat transfer is greater in case of the kettle as well as the heat stored in it but if you talk.....

Someone

about the amount of heat stored in the system then in that case since the mass of water in the kettle is greater so more energy is required to raise the temperature b/c more molecules of water are present in the kettle

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field is a region of space under the influence of some physical properties

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a=v/t. a=f/m a

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pratica A on solution of hydro chloric acid,B is a solution containing 0.5000 mole ofsodium chlorid per dm³,put A in the burret and titrate 20.00 or 25.00cm³ portion of B using melting orange as the indicator. record the deside of your burret tabulate the burret reading and calculate the average volume of acid used?

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Two bodies attract each other electrically. Do they both have to be charged? Answer the same question if the bodies repel one another.

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No. According to Isac Newtons law. this two bodies maybe you and the wall beside you. Attracting depends on the mass och each body and distance between them.

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Are you really asking if two bodies have to be charged to be influenced by Coulombs Law?

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like charges repel while unlike charges atttact

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What is specific heat capacity

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Specific heat capacity is a measure of the amount of energy required to raise the temperature of a substance by one degree Celsius (or Kelvin). It is measured in Joules per kilogram per degree Celsius (J/kg°C).

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specific heat capacity is the amount of energy needed to raise the temperature of a substance by one degree Celsius or kelvin

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12.1 Chemical reaction rates (Page 3/8)

Relative rates of reaction

Expressions for relative reaction rates

Solution

Check your learning

Answer:

Reaction rate expressions for decomposition of h 2 O 2

Solution

Check your learning

Answer:

Key concepts and summary

Key equations

Chemistry end of chapter exercises

Questions & Answers

Read also:

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Reaction rate expressions for decomposition of h ₂ O ₂