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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:

Δmol NH 3 Δ t × 1 mol N 2 2 mol NH 3 = Δmol N 2 Δ t

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

1 2 Δmol NH 3 Δ t = 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:

1 2 Δ [ NH 3 ] Δ t = 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 :

1 3 Δ [ H 2 ] Δ t = 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:

2.91 × 10 −6 M /s 9.71 × 10 −6 M /s 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:

1 4 Δ [ NH 3 ] Δ t = 1 5 Δ [ O 2 ] Δ t = 1 4 Δ [ NO ] Δ t = 1 6 Δ [ H 2 O ] Δ t

Check your learning

The rate of formation of Br 2 is 6.0 × 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:

1 5 Δ [ Br ] Δ t = Δ [ BrO 3 ] Δ t = 1 6 Δ [ H + ] Δ t = 1 3 Δ [ Br 2 ] Δ t = 1 3 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 × 10 −2 mol/L/h, that is:

Δ [ H 2 O 2 ] Δ t = 3.20 × 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:

1 2 Δ [ H 2 O 2 ] Δ t = 1 2 Δ [ H 2 O ] Δ t = Δ [ O 2 ] Δ t

Therefore:

1 2 × 3.20 × 10 −2 mol L −1 h −1 = Δ [ O 2 ] Δ t

and

Δ [ O 2 ] Δ t = 1.60 × 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 × 10 −6 mol/L/s, what is the rate of production of nitrogen and hydrogen?

Answer:

1.05 × 10 −6 mol/L/s, N 2 and 3.15 × 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 = 1 a Δ [ A ] Δ t = 1 b 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 = + 1 2 Δ [ CIF 3 ] Δ t = Δ [ Cl 2 ] Δ t = 1 3 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 × 10 −2 5.04 × 10 −3 3.37 × 10 −3 2.53 × 10 −3 2.08 × 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 × 10 −4 M s −1 , what is the rate of appearance of Br 2 ( aq ) at that moment?

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Questions & Answers

who are the alchemist?
Victor Reply
alchemy science of transmutation. typically it is aim at tranforming lead to or other base metals to gold and the creation of the philosophers stone which in reality isn't a stone it's something priceless something we all need for coming times. don't be fooled
Kendrick
read Corinthians 5 verses 50 to the end of the chapter then read revelations chapter 2 verse 17
Kendrick
The word "Alchemy" comes from the forgotten name for Ancient Egypt, Khemmet. Khem was the name for the Egyptian Empire, but the actual land of Egypt was called Khemmet because the "T" on the end of a word denoted a physical location on Earth and not just an idea.
Michael
Wow!
mendie
What's the mass number of carbon
Charlie Reply
first Faraday's law
Akinbola
mass number of carbon is 12.
Nnenna
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safiya
atomic number of oxygen is 8
Nnenna
which quantum number divides shell into orbitals?
Tomiwa Reply
azimuthal
Emmanuel
hi
Charlie
azimuthal
reinhard
azimuthal
Charlie
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Desmond Reply
an atom is a smallest indivisible part of an element
Henry
an atom is the smallest part of an element that takes part in a chemical reaction
Nana
wat is neutralization
Dubem Reply
when any acid reacts with base to decrease it's acidity or vice-versa to form salt and solvent.. which is called neutralization
Santosh
explain buffer
Organic
buffer is a solution which resists changes in pH when acid or alkali added to it..
Santosh
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UTHMAN
buffer is the solution which resist the change in pH by addition of small amount of acid or alkali to it
KAUSIK
neutralisation is the process of mixing of a acid and a base to form water and corresponding salt
KAUSIK
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Princewill Reply
what are the elent of ionic and covalent bonding
Princewill
what is gases
Wesike Reply
Its one of the fundamental sate of matter alone side with liquid, solid and plasma
John
What is chemical bonding
John
To my own definitions. It's a unit of measurement to express the amount of a chemical substance.
Ozoaniehe Reply
What is mole
TAMIL Reply
It's the unit of measurements used to express the amount of chemical substance.
Ozoaniehe
What is pressure
Stellamaris Reply
force over area
Jake
force applied per unit area
john
force applied per unit area
Prajapati
Why does carbonic acid don't react with metals
Aditya Reply
Why does carbonic acid don't react with metal
Aditya
Some metals will react depending on their Standard Electrode Potential. Carbonic acid is a very weak acid (i.e. a low hydrogen ion concentration) so the rate of reaction is very low.
Paul
sample of carbon-12 has a mass of 6.00g. How many atoms of carbon-12 are in the sample
Emokiniovo Reply
a sample of carbon-12 has a mass of 6.00g. How many atoms of carbon-12 are in the sample
Sharmin Reply
an object of weight 10N immersed in a liquid displaces a quantity of d liquid.if d liquid displaced weights 6N.determine d up thrust of the object
ugonna Reply
how human discover earth is not flat
Jason Reply
We don't fall off. If set off in any direction in a straight line and keep going. You'll end up back where you started.
Adelle
earth is spherical
Unique
Also, every other planet is spherical as that is the most energy efficient shape. gravity pulls equally on all areas. Sphere.
Adelle
Practice Key Terms 5

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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