HS - PS1 - Chemical equilibrium - FINAL CJT (4-15-14)

Module: Chemical Equilibrium

DCI: PS1.B (B) In many situations, a dynamic and condition-dependent balance between a reaction and the reverse reaction determines the numbers of all types of molecules present.

 PE: HS-PS1-6 Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.

[Clarification Statement: Emphasis is on the application of Le Chatelier’s Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products.] [Assessment Boundary: Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations.]

The basic content of the standard is as follows.

- Any chemical reaction will reaction a point of equilibrium between the reaction and the reverse reaction.

- The point of equilibrium is affected by factors that can shift it, resulting in the reaction or the reverse reaction to take over, and return the system to a state of equilibrium again.

These are the factors.

CONCENTRATION:

Increasing the concentration of a chemical will increase the rate of reaction that will reduce that concentration. If a system is in equilibrium, increasing the concentration of the reactants will increase the rate of the reaction, reducing the concentration of reactants. If the concentration of products is increased, the rate of the reverse reaction will go up, reducing the concentration of product.

TEMPERATURE:

In an exothermic reaction, heat can be treated as a product, thus an increase in temperature would result in a higher rate for the reverse reaction. In an endothermic reaction, heat can be treated as a reactant, so an increase in heat will drive the reaction to more product.

PRESSURE:

Changes in pressure are attributable to changes in volume. The equilibrium concentrations of the products and reactants do not directly depend on the pressure subjected to the system. However, a change in pressure due to a change in volume of the system will shift the equilibrium.

Considering the reaction of nitrogen gas with hydrogen gas to form ammonia:

N₂ + 3 H₂ ⇌ 2 NH₃    ΔH = -92kJ mol^-1

4 volumes ⇌ 2 volumes

Note the number of moles of gas on the left-hand side and the number of moles of gas on the right-hand side. When the volume of the system is changed, the partial pressures of the gases change.

Thus, an increase in system pressure due to decreasing volume causes the reaction to shift to the side with the fewer moles of gas. A decrease in pressure due to increasing volume causes the reaction to shift to the side with more moles of gas. There is no effect on a reaction where the number of moles of gas is the same on each side of the chemical equation.