Chemical Equilibria (Video Required)

irreversible reactions

When reactions proceed to completion, it is represented in a chemical reaction using the unidirectional arrow. Such reactions are called ______, and the reactants will be converted to products until the depletion of the limiting reagent.

Reversible (or equilibrium) reactions

These are depicted using bidirectional harpoons (⇌), denoting that both forward and reverse reactions occur simultaneously.

chemical equilibrium

When the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of the reactants and products remain constant, then the state of _______ is achieved.

Le Chatelier’s principle

This principle states that if an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches the new equilibrium position.

affect

Changes in concentration, temperature, volume, pressure, and the presence of a catalyst ____ a chemical system at equilibrium.

aqueous and gaseous

Only these species can affect a system at equilibrium. Solids and liquids, regardless of amount, do not cause a change in equilibrium position.

decreasing the volume

In gaseous systems at equilibrium, increasing the pressure is synonymous with _______ and vice versa. This also implies that changes in pressure (and volume) only affect gaseous systems simply because aqueous solutions, liquids, and solid reagents are incompressible.

higher number of moles

During expansion (reduced pressure and higher volume), the reaction favors the formation of gas with a ________ since there is a larger space that the gas can occupy.

gas with fewer moles

The opposite is true during compression. The increase in pressure reduces the volume the gas occupies, and the equilibrium reaction shifts toward the formation of ________.

product, reactant.

Another parameter that affects equilibrium reaction is temperature. Changes in temperature, however, only affect heat-releasing (exothermic) and heat-absorbing (endothermic) reactions. For such cases, the heat released is treated as if it is a ____ of the chemical reaction, while the heat absorbed is treated as if it is a _____.

endothermic, exothermic

Since we are talking about equilibrium reactions, if the forward reaction is ______ (ΔH° > 0), then the reverse is ______ (ΔH° < 0). Take note, however, that there is no such thing as negative heat. When you encounter a negative ΔH°, the negative sign only implies that the heat was released (or the net lose of enthalpy in a system). On the other hand, if you encounter positive ΔH°, it means that heat was absorbed by the system.

providing an alternative reaction pathway with a lower energy of activation

There is a common misconception that catalysts increase the speed of chemical reactions by lowering the activation energy. In reality, a catalyst increases the rate of a chemical reaction by ___________.

does not alter

When applied in equilibrium reactions, the catalyst increases the rate of the forward reaction. However, it also increases the rate of the reverse reaction. Hence, the catalyst ______ the equilibrium constant nor shift the equilibrium reaction

aqueous and gaseous

Take note that only ________ species must be included in the equilibrium constant expression. Solids and liquids are not included because their concentrations are so large that it is almost unchanged in any equilibrium reactions.

(SO₃ )² / (O₂ )³

Write the equilibrium constant expression for the following reaction:

1. S_(s) + O_2(g) ⇌ SO_3(g)

(NH₄ +) (-OH) / (NH₃ )

Write the equilibrium constant expression for the following reaction:

2. NH_3(aq) + H₂O_(l) ⇌ NH₄⁺_(aq) + ^—OH_(aq)

(H₃ O+) (-OH)

Write the equilibrium constant expression for the following reaction:

1. H₂O_(l) ⇌ H₃O⁺_(aq) + ^—OH_(aq)