5. The Rate of Chemical Reactions & Chemical Equilibria

proceeds very rapidly, sometimes even explosively.

Rate of reaction of combustion of hydrocarbons (e.g. isooctane in gasoline)

very slowly

Rate of rusting of iron

the change in concentration (or amount) of the reactants participating in particular chemical reaction, or the change in concentration of the products, per unit time (interval during which this change is observed)

The reaction rate (V) for a given chemical reaction may be expressed as:

*a*A + *b*B → *y*Y + *z*Z

general case of a chemical reaction:

concentration change (ΔC) per unit time (Δt) of any substance participating in that reaction

reaction rate may be expressed by measuring the:

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\[INSERT LONG EQUATION FROM SLIDE 1, TOPIC 5, GENERAL CHEM\]

the concentration of one substance (either reactant or product) is measured, at different time intervals

In the experimental studies of reaction kinetics, the reaction rate is calculated by:

**Nature of reactants**

The same type of reaction may proceed at different rate depending on substances involved. For example, addition of HCN is a typical reaction for all carbonyl compounds, but under the same conditions reaction rates are higher for aldehydes than for ketones. Also, the physical states of reacting substances are important in determining their reactivities (e.g. liquid gasoline can burn smoothly, but gasoline vapours can burn explosively)

**Temperature**

As a rule of thumb, temperature increase of 1__0 K (10°C) increases reaction rates approximately 2-3 times (____**van’t Hoff’s rule**____)__. Exact dependence of reaction rate on temperature for a particular reaction may be obtained by studying reaction rates at different temperatures

**Concentration of reactants**

As the concentration of reactants change at constant temperature, the rate of reaction changes.

Rate dependence on concentrations may be described by **rate-law expression** (often called simply the **rate law**).

* The rate law is experimentally determined and can be used to predict the relationship between the rate of a reaction and the concentrations of reactants. For a reaction in which A, B . . . are reactants rate law has general form

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*v* = *k*\[A\]x\[B\]y..

* The values of exponents, x and y, and of the rate constant, k, bear no necessary relationship to the coefficients in the balanced chemical equation for the overall reaction and must be determined experiment all

\[NCLUDE IMG OF THE ABOVE EQUATION. BASICALLY GHE SAME BUT Y AND X ARE SUPERSCRIPT\]

constant *k*

(rate-law expression) symbol for **specific rate constant** (or just the **rate constant**) for the reaction at a given temperature

the order of the reaction with respect to A

The value of *x*:

the order of reaction with respect to B

The value of *y*:

The overall **order of the reaction**

the sum of the reactant orders

__the orders of the reaction with respect to particular reactants__

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*a*A + *b*B → *c*C (one step homogenous reaction)

*v* = *k*\[A\]*a*\[B\]*b*

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\[GET EXQUATION FROM BTTOM OF PAGE 2, TOPIC 5, THE SMALL A AND B ON THE SECOND EQUATION IS SUPERSCRIPT. THATS THE ONLY DIFFERENCE \]

__for one step homogenous reactions the coefficients in the balanced chemical equation for the overall reaction may be used as:__

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The correct answer is 4)

Q1. What is correct expression for the reaction rate of reaction 2A + B → 2C measured by the concentration change of substance A?

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a) 0.006

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C`1`(A`2`) = 0.264mol/dm3; t1 = 35 s

C`2`(A`2`) = 0.246mol/dm3; t2 = 38 s

v = -(0.246mol/dm3-0.264mol/dm3)/(38 s–35 s) =(0.018mol/dm3)/(3 s)

v = 0.006mol/dm3.s

Q2. What is the reaction rate (mol/dm3s) for a reaction: A2+ B2 → 2 AB, if 35 seconds after the start of reaction concentration of A2 was 0.264mol/dm3, and 38 seconds after the start of reaction its concentration was 0.246mol/dm3.

a) 0.006

b) 0.018

c) 0.012

d) 0.003

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