Unit 6: Equilibria — Short Questions

Dynamic equilibrium and static equilibrium are different in how they function.

Dynamic Equilibrium
Dynamic equilibrium happens when reactions are still occurring but the rate of the forward reaction equals to the rate of the reverse reaction. This means that the concentrations of reactants and products remain constant, but they are constantly changing and interconverting. Dynamic equilibrium is all about ongoing reactions maintaining balance.

Static Equilibrium
Static equilibrium, happens when all the forces are balanced, when everything is at rest. There is no movement or change happening in the system. The static equilibrium is a state of complete stillness with no changes taking place and are not interacting or converting into one another. It occurs in physical processes.

If the temperature is increased in a reversible reaction, it will favor the endothermic process. In this case, the reaction is breaking down water into hydrogen and oxygen gas, which requires energy. By raising the temperature, the system will shift to absorb that extra heat, promoting the formation of more hydrogen and oxygen gases. This means that the concentrations of H₂ and O₂ will increase, while the concentration of H₂O will decrease.

We can get the maximum yield in a reversible reaction, by following factors:
1. Adding the Reactant: Increasing the concentration of the reactants will shift the reaction in forward direction, lead to more products.
2. Temperature: If the reaction is exothermic (releases heat), lowering the temperature at equilibrium can increase the yield of products. Conversely, if the reaction is endothermic (absorbs heat), increasing the temperature will favor the formation of products.
3. Pressure: For reactions involving gases, increasing the pressure will favor the side with fewer moles of gas. This can lead to a higher yield of products if they are on that side.
4. Removal of Products: Continuously removing products as they are formed can drive the reaction forward, increasing the yield of products.

Use a Catalyst Adding the catalyst can decrease the time to attain the position of equilibrium in a reversible reaction. Adding a catalyst provides an alternative pathway for the reaction with a lower activation energy, which speeds up both the forward and reverse reactions without being consumed in the process.

Increasing the pressure on the reaction N₂₍g₎ + O₂₍g₎ ⇌ 2NO₍g₎ will have an effect based on the number of moles of gas on each side of the equation. In this reaction, there are 2 moles of gas on the reactant side (1 mole of N₂ and 1 mole of O₂) and 2 moles of gas on the product side (2 moles of NO). Since the number of moles of gas is the same on both sides of the reaction, increasing the pressure will not favor either the forward or reverse reaction.
Therefore, there will be no significant shift in the position of equilibrium due to a change in pressure in this specific reaction.

A simple example of dynamic equilibrium is the balance between ice, water, and water vapor. In a closed container, ice can melt into water and water can evaporate intovapouron heating. At the same time, water vapour can condense back into liquid water and liquid water can freeze into ice.
When the rates of these processes are equal, the amounts of ice, liquid water, and water vapor remain constant, even though they are constantly changing states.

When dinitrogen tetra oxide (N₂O₄) is kept in a sealed flask at 100°C, it will start to convert into nitrogen dioxide (NO₂). The reaction is represented as follows:
N₂O₄₍g₎ ⇌ 100°C → 2NO₂₍g₎

At this temperature, the equilibrium will shift towards the production of more NO₂ which is brown in color. As a result, the color of the mixture will gradually change from colorless (due to N₂O₄) to a brown due to NO₂ is formed.
If the system reaches equilibrium, the mixture will have a brown color due to the presence of nitrogen dioxide, although the exact shade will depend on the concentrations of both gases.

One effective method to withdraw water from the reaction mixture of ethyl acetate production is by using a drying agent or a dehydrating agent. Common drying agents include substances like magnesium sulphate (MgSO₄) or sodium sulphate (Na₂SO₄).
You can add the drying agent to the reaction mixture and it will absorb the water that forms. Once the water is absorbed, you can then filter out the drying agent leaving behind a mixture with a higher concentration of ethyl acetate.

The reactions in which products do not recombine to form reactants are called irreversible reactions.

• They are supposed to be completed.
• They are represented by single arrow (→) between reactants and products.
• It proceeds in only one direction.

Example NaCl₍aq₎ + AgNO₃₍aq₎ → AgCl₍s₎ + NaNO₃₍aq₎

"The reactions in which products react to produce reactants are called reversible reactions."

• These reactions never go to completion.
• They are represented by a double arrow (⇌) between reactants and products.
• Reversible reaction moves in both the forward and backward reaction under the same conditions.

Example N₂₍g₎ + 3H₂₍g₎ ⇌ 400°C, 200 atm → 2NH₃₍g₎
Fe

A reversible reaction, goes to completion if either one of the products is withdrawn from the reaction mixture or being a gas, it escapes into the atmosphere.
For example, calcium carbonate is decomposed by heating at a particular temperature.

CaCO₃₍s₎ ⇌ CaO₍s₎ + CO₂₍g₎
Heat

If the above reaction is carried out in an open container, the carbon dioxide gas will escape into the atmosphere as soon as it is formed and the reaction is forced to go to completion.

N₂₍g₎ + 3H₂₍g₎ ⇌ 400°C, 200 atm → 2NH₃₍g₎
Fe

In this reaction, one mole of nitrogen gas reacts with three moles of hydrogen gas under the conditions of reaction in a closed container to give two moles of ammonia gas. After its formation, the ammonia gas decomposes to give the reactants back. The reaction never goes to completion. At any time, all the three species are simultaneously present in the reaction mixture.

When cobalt chloride hexahydrate (CoCl₂.6H₂O) which is pink in colour is heated, it is converted to anhydrous CoCl₂ which is blue in color.
When anhydrous cobalt chloride absorbs less moisture, it is first converted into a dihydrate which is purple in colour.

CoCl₂ · 6H₂O → Heat → CoCl₂ · 6H₂O
Hydrated Cobalt Anhydrous Cobalt
chloride(Pink) chloride(Blue)

When the rate of the forward reaction takes place at the rate of reverse reaction, the composition of the reaction mixture remains constant; it is called a chemical equilibrium state.

At the stage of dynamic equilibrium, the rates of both forward and backward reactions are equal but takes place in opposite direction.

Vast deposits of coal are available in Thar, Sindh. This coal can be used to generate electricity. When coal is made to react with steam then CO and H₂ are produced. These products then react by a reversible reaction called catalytic methanation to yield methane.

C₍s₎ + H₂O₍g₎ → CO₍g₎ + H₂₍g₎
water gas

CO₍g₎ + 3H₂₍g₎ → CH₄₍g₎ + H₂O₍g₎
Catalyst

The following physical conditions for equilibrium is:
i. The concentration of reactant or product remains unchanged.
ii. The temperature of the system remains constant.
iii. The pressure or volume of the system remains constant.
iv. Reaction must happen in closed system.

When fizzy drinks are made, CO₂ is dissolved in the liquid drink under pressure and sealed. When you remove lid of the bottle, bubbles of CO₂ of suddenly appear. When you put the lid back on the bottle, the bubbles stop. This is due to the following equilibrium.

CO₂₍g₎ ⇌ CO₂₍aq₎

The forward reaction happens during manufacturing and the reverse reaction happens on opening of bottle.

A catalyst increases both the rates of forward and back reaction of a reversible reaction. So if a reversible reaction is carried out in the presence of a catalyst it will decrease the time taken by the reaction to attain the state of equilibrium.

A reversible chemical system may be disturbed in the following possible ways.
1. Adding or withdrawing one or more of the reacting species
2. Adding or withdrawing one or more of the product species
3. Changing the temperature of the reaction
4. Effect of the presence of a catalyst on a reversible reaction
5. Changing the pressure of the reaction if it involves reactants or the products in the gaseous state.

Consider the following reversible reaction at equilibrium.

N₂₍g₎ + 3H₂₍g₎ ⇌ 400°C, 200 atm → 2NH₃₍g₎
Fe

The concentrations of all the participating chemicals will be constant at the state of equilibrium. At this stage if we add more N₂ gas in the mixture, its concentration will increase and the reaction will no longer maintain its state of equilibrium. To restore the equilibrium state again, nitrogen will react with hydrogen to produce more ammonia. This change will go on until a new state of equilibrium is reached at which the concentration of all the species will again become constant. These new concentrations will, however, be different from the concentrations of the earlier equilibrium state.

Now let us disturb the equilibrium again by withdrawing some of the ammonia gas formed. As a result, its concentration will decrease. To restore the equilibrium state, more nitrogen and hydrogen will react to produce ammonia. When the state of equilibrium is reached again, the concentrations of all the species shall again become constant.

The formation of ammonia is exothermic in the forward direction and hence this reaction will be endothermic in the reverse direction.

N₂₍g₎ + 3H₂₍g₎ ⇌ 400°C, 200 atm → 2NH₃₍g₎
Fe
ΔH = -92.4kJ/mole

If this reaction is at equilibrium and its temperature is increased, the state of the equilibrium will be disturbed again. The ΔH of this reaction is negative. This means the total energy of the system containing N₂ and H₂ is higher than that of ammonia. The increase in temperature of this reaction at equilibrium will push the reaction in the backward direction i.e. the reactants side. Decreasing the temperature will drive the equilibrium to the forward direction.

Change of pressure will disturb the equilibrium state of only those gaseous reactions in which the number of moles of the reacting gases will be different from the number of moles of the gases being produced. Since the formation of ammonia gas meets such a condition, the state of its equilibrium will be disturbed by changing the pressure exerted on the reaction mixture.

N₂₍g₎ + 3H₂₍g₎ ⇌ 2NH₃₍g₎
1 mole 3 moles 2 moles
4 moles 2 moles

In this reaction, 4 moles of reacting gases are producing two moles of product gas. 4 moles of gases at say S.T.P will occupy 4 x 22.414 = 89.656 dm³ of volume. 2 moles of NH₃ will occupy 2 x 22.414 = 44.828 dm³ of volume. If this reaction is at equilibrium and the pressure is increased, the equilibrium will be disturbed. To restore this, the reaction will move to that side in which the number of moles are less i.e. forward direction. The formation of ammonia gas is thus favoured at high pressure.

Some reactions are irreversible while others are reversible due to following reasons:

• In many irreversible reactions, the products formed are more stable than the reactants. For example, combustion reactions often produce gases and heat, making it energetically unfavorable for the products to return to the original reactants.
• Certain conditions, such as temperature and pressure, can influence whether a reaction is reversible. For instance, reactions that occur at high temperatures or pressures may favor the formation of products, making them less likely to return back to reactants.
• If a reaction produces a gas or a precipitate, this can drive the reaction to completion. For example, if a gas is produced, it can escape from the reaction mixture, preventing the reverse reaction from occurring.
• In reversible reactions, the equilibrium can be established where both reactants and products coexist. In contrast, irreversible reactions tend to proceed to completion.
• The speed of the forward and reverse reactions can also play a role. If the reverse reaction has very high activation energy, it may not occur under normal conditions, making the process effectively irreversible.

Combustion reactions are generally irreversible for several reasons:

• Combustion reactions release a significant amount of energy in the form of heat and light. This energy release indicates that the products formed (like carbon dioxide and water) are more stable than the reactants .(like hydrocarbons and oxygen).
• Many combustion reactions produce gaseous products. For example, burning hydrocarbons typically produces carbon dioxide and water vapor. These gases can escape into the atmosphere, which prevents the reverse reaction from occurring since the products are no longer available in the reaction mixture.
• Combustion reactions usually proceed to completion, meaning that all the reactants are converted into products. Overall, the formation of gaseous products and the tendency to go to completion makes combustion reactions effectively irreversible.

You can alter the conditions of a reaction to make a reversible reaction irreversible or an irreversible reaction reversible, but this often involves significant changes to the system.

• Making a Reversible Reaction Irreversible: You can achieve this by removing one of the products from the reaction mixture which drives the reaction to completion. For example, in the synthesis of ammonia,

N₂₍g₎ + 3H₂₍g₎ ⇌ 400°C, 200 atm → 2NH₃₍g₎
Fe

If you continuously remove ammonia as it forms, the reaction will shift to the right produce more ammonia, making it effectively irreversible under those conditions.

• Making an Irreversible Reaction Reversible: This is more challenging but can be done under specific conditions. For example, in the combustion of hydrocarbons, if you were to introduce a process that captures carbon dioxide and water vapor and converts them back to hydrocarbons (like using a chemical reaction in a closed system), you could theoretically create a reversible cycle. However, this typically requires significant energy and specific catalysts.

To determine if a reaction is reversible or irreversible, you can consider several factors:

• Many reactions are inherently reversible, such as those involving weak acids and bases or equilibrium reactions. In contrast, reactions like combustion or certain oxidation-reduction processes are typically irreversible.
• If a reaction releases a significant amount of energy (exothermic), it is more likely to be irreversible. Reactions that absorb energy (endothermic) may be reversible, especially if they can reach an equilibrium state.
• If a reaction produces gases or involves a change in the number of moles of gas, it may favor reversibility. For example, if a gas is produced and escapes, the reaction may not easily reverse.
• The conditions under which a reaction occurs (temperature, pressure, concentration) can influence its reversibility. Changing these conditions can shift the equilibrium and affect whether the reaction can be reversed.

The phase changes in water, such as solid to liquid (melting) and liquid to vapor (evaporation), are reversible processes.

• Melting (Solid to Liquid): When ice is heated, it absorbs energy and melts into liquid water. This process can be reversed by cooling the liquid water, which will freeze back into ice.
• Evaporation (Liquid to Vapor): When liquid water is heated, it turns into water vapor (gas). This process is also reversible; when the vapor cools, it can condense back into liquid water. Both of these phase changes can occur in a closed system, where the water can transition between solid, liquid, and gas states without any loss of mass.