13.3 The Chemical Equilibrium

Some chemical reactions do not react to completion. As soon as the reactants react to produce products (the forward reaction), the products decompose to produce back the reactants (the reverse reaction). When the rate of the forward reaction equals the rate of the reverse reaction, the reaction is said to have reached a state of dynamic equilibrium. Both the reactants and products are present at the end of the reaction. Chemical equilibrium is the state of a chemical reaction when the concentrations of the products and reactants are unchanged over time.

At the equilibrium state, the concentrations of the reactants and products are unchanged as long as no additional stress (temperature change, volume change, pressure change or addition or removal of reactants or products) is applied to the equilibrium reaction. If stress is applied to the equilibrium reaction, the reaction will adjust itself by shifting to the side of the reaction that minimizes or reduces the stress that is applied.

Typical graphs illustrating the chemical equilibrium are given below:

Reference:http://faculty.chem.queensu.ca/people/faculty/mombourqutete/FirstYrChem/equilibrium/index.htm

In a chemical equilibrium the rate of forward reaction of producing the products equals the rate of the reverse reaction of decomposing the products and producing the reactants back.

The chemical equilibrium is considered as a reversible chemical reaction by which the reactants react to produce the products and as soon as they are produced, the products are decomposed to produce back the reactants. At the end of chemical reactions both reactants and products are present. This is quite the opposite in case of irreversible chemical reaction where the reactants are completely into the products and at the end of the chemical reactions there are only the products are present.

The reversible chemical reactions are designated by double arrows while the irreversible chemical reactions are designated by one arrow only

Examples:

N2(g)  +    3 H2(g)          2 NH3(g)                    Reversible chemical reaction (Chemical Equilibrium)

CaCO3(s)     CaO(s)      +     CO2(g)                     Irreversible chemical reaction

The videos illustrate the principle of chemical equilibrium concepts:

The simulation below covers the concept of the reversible chemical reactions

https://phet.colorado.edu/en/simulation/legacy/reversible-reactions

Setup – Search “PhET Reversible Reactions” simulation. Open and run the simulation. https://phet.colorado.edu/en/simulation/legacy/reversible-reactions

• Explore! Click on everything to find the variables and observe how they affect the reaction. (Don’t just try to max out the computer’s memory chip.)

• Reaction Conditions: Move the position of the reactants, transition state, and products wherever you wish and choose a temperature. Be reasonable!!

Reactants                                             Transition state                        

Products                                                           Temp                          

• Design! You will run three trials. Each one should have 100 total molecules. Start with different amounts of A and B for each trial. Place the starting amounts in the table at time 0. Record the amount of A and B in the chamber every 20 seconds for 5 minutes.

• Which side of the reaction is favored (are there more reactants or products) for the experiment you set up? Why is that so?

• Graph the concentration (number of molecules) of both molecules A and B vs time. You should have two separate curves (A and B).

• What is happening to the concentrations at the beginning of the experiment? How does that differ from what is happening at the end of the experiment? Mark a vertical line on the graph at the point where equilibrium is established.

• All three trials started at different amounts. How did the final ratios of A to B compare?

• Did the reaction ever stop?

1. Define activation energy and graphically determine the activation energy requirements that take place during a reaction.

1. Sketch and analyze graphs showing changes in reactant and product concentrations as reactions proceed towards equilibrium

1. State the characteristics of a system at equilibrium.

1. Use a potential energy diagram to determine whether reactants or products are favored during a reaction

Physical changes can be considered as chemical equilibria such as physical changes of the states of the molecules or the compound. There are several examples of such changes:

Solid iodine phase diagram reveals the following data based on the reversible chemical equilibrium:

I2(s)           I2(l)      I2(g)

https://uwaterloo.ca/chem13-news-magazine/october-2015/feature/sublimation-iodine-rise-and-fall-misconception

Triple point: 113.5 ^oC at 12.1 kPa (point 1 on phase diagram) Melting point: 113.7 ^oC at 101.3 kPa (point 2)

Boiling point: 184.3 ^oC at 101.3 kPa (point 3) Critical point: 546 ^oC at 11,700 kPa

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The figure below shows iodine solid on hot plate being melted and evaporated/

https://brainly.in/question/1198038

Another example of physical reversible change is phase diagram of water: Solid (ice) water ßà  liquid water ßà                 steam (vapor) water H2O(s)     ßà     H2O(l)                 H2O(g)

Water phase diagram is given below:

https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_ChemPRIME_(Moore_et_al.)/10%3A_Solids_Li quids_and_Solutions/10.14%3A_Phase_Diagrams

Triple point: 0.01 ^oC at 0.006 atm (point T on phase diagram) Melting point: 0.01 ^oC at 0.006 atm (point T on phase diagram) Boiling point: 100 ^oC at 1 atm (point β)

Critical point: 374 ^oC at 218.3 atm (point C)

The figure below depicts the 3 phases of water coexist together at open system at pressure of 1 atm and temperature of

0.00 ^oC (figure a) and at closed system at pressure of 0.006 atm and temperature of 0.01 ^oC (figure b).

https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_ChemPRIME_(Moore_et_al.)/10%3A_Solids_Li quids_and_Solutions/10.14%3A_Phase_Diagrams