How will an increase in temperature affect each of the following equilibria? How will a decrease in the volume of the reaction vessel affect each? (a) \(2 \mathrm{H}_{2} \mathrm{O}(g) \rightleftharpoons 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \quad \Delta H=484 \mathrm{kJ}\) (b) \(\mathrm{N}_{2}(g)+3 \mathrm{H}_{2}(g) \rightleftharpoons 2 \mathrm{NH}_{3}(g) \quad \Delta H=-92.2 \mathrm{kJ}\) (c) \(2 \operatorname{Br}(g) \rightleftharpoons \operatorname{Br}_{2}(g) \quad \Delta H=-224 \mathrm{kJ}\) (d) \(\mathrm{H}_{2}(g)+\mathrm{I}_{2}(s) \rightleftharpoons 2 \mathrm{HI}(g) \quad \Delta H=53 \mathrm{kJ}\)

Chemical equilibrium is a state in a reversible chemical reaction where the rate of the forward reaction equals the rate of the backward reaction. Due to this balance, the concentration of the reactants and products remains unchanged over time. This dynamic equilibrium is essential for understanding many chemical processes and can be visually represented by a double-headed arrow in the reaction equation.

It's important to realize that equilibrium does not mean the reactants and products are in equal concentrations but that their ratios remain constant. Applying Le Chatelier's Principle can help predict how changes in conditions such as temperature and pressure can shift this delicate balance.

Endothermic and exothermic reactions are two types of chemical reactions distinguished by how they transfer energy. Endothermic reactions absorb energy from their surroundings, typically as heat; their enthalpy change \( \Delta H \) is positive because the system gains heat. Conversely, exothermic reactions release energy to the surroundings, and their \( \Delta H \) is negative due to the loss of heat.

An easy way to remember this is to associate 'endo-' with 'enter' and 'exo-' with 'exit'. Recognizing the type of reaction is crucial when predicting the effects of temperature changes on chemical equilibrium according to Le Chatelier's Principle.

An equilibrium shift refers to the change in the position of equilibrium following a disturbance in the system's conditions. Le Chatelier's Principle predicts the direction of the shift to counter the change. If the equilibrium is disturbed by an increase in temperature, for instance, the equilibrium will shift toward the endothermic direction to absorb the excess heat; a decrease in temperature would shift it toward the exothermic direction to release heat.

This shifting helps the system to achieve a new state of balance under the altered conditions. By understanding the principles of equilibrium shift, students can predict how changes in external conditions affect chemical reactions.

Pressure and volume changes in gases are also factors that affect chemical equilibrium, especially for reactions involving gaseous substances. According to Le Chatelier's Principle, a decrease in volume will result in an increase in pressure, and the system will shift towards the side with fewer moles of gas to counteract this pressure change. Conversely, an increase in volume (decrease in pressure) favors the side with more moles of gas.

Understanding the proportional relationship between pressure and volume (Boyle's Law) is essential for predicting the effect of volumetric changes on gas-phase equilibria. It's worth noting that this applies only to gases; solids and liquids are virtually incompressible and are not affected by pressure changes in the same way.