Topic 7- Equilibrium

Physical Systems

Chemical Systems

The Concept of Equilibrium Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the ______________. In the figure above, equilibrium is finally reached in the third picture. same rate

The Concept of Equilibrium As a system approaches equilibrium, both the forward and reverse reactions are occurring. At equilibrium, the forward and reverse reactions are proceeding at the same rate. Once equilibrium is achieved, the amount of each reactant and product remains constant.

Writing the Equation for an Equilibrium Reaction N2O4(g) ⇌ 2 NO2(g)

Kc = equilibrium constant

Equilibrium—The Haber Process Consider the Haber Process, which is the industrial preparation of ammonia: N2(g) + 3 H2(g) ⇌ 2 NH3(g) The equilibrium constant depends on stoichiometry:

Magnitude of K If K>>1, the reaction favors products; products predominate at equilibrium. If K<<1, the reaction favors reactants; reactants predominate at equilibrium.

Magnitude of Kc

Is a Mixture in Equilibrium? Which Way Does the Reaction Go? reaction quotient =Q. Q looks like the equilibrium constant, K, but the values used to calculate it are the current conditions, not necessarily those for equilibrium. To calculate Q, one substitutes the initial concentrations of reactants and products into the equilibrium expression.

Comparing Q and K Nature wants Q = K. If Q < K, nature will make the reaction proceed to products. If Q = K, the reaction is in equilibrium. If Q > K, nature will make the reaction proceed to reactants.

Step 1: Write Kc expression Step 2: Use to solve for Q Step 1: Write Kc expression Step 3: Compare Q to Kc

Manipulations of Kc

LeChâtelier’s Principle “If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.”

Change in Volume or Pressure When gases are involved in an equilibrium, a change in pressure or volume will affect equilibrium: Higher volume or lower pressure favors the side of the equation with more moles and vice-versa).

Change in Temperature Is the reaction endothermic or exothermic as written? That matters! Endothermic: Heats acts like a reactant; adding heat drives a reaction toward products. Exothermic: Heat acts like a product; adding heat drives a reaction toward reactants.

Catalysts Catalysts increase the rate of both the forward and reverse reactions. Equilibrium is achieved faster, but the equilibrium composition remains unaltered. Activation energy is lowered, allowing equilibrium to be established at lower temperatures.

Summary with Kc

The Haber Process

The production of methanol

17.1 The equilibrium law Le Chatelier’s principle for changes in concentration can be explained by the equilibrium law. The position of equilibrium corresponds to a maximum value of entropy and a minimum in the value of the Gibb’s free energy The Gibb’s free energy change of a reaction and the equilibrium constant can both be used to measure the position of an equilibrium reaction and are related by the equation, ΔG = -RTlnK

Homogeneous Equilibrium (we are only dealing with reactants and products that are all in the same phase of matter)

RICE BOXES write a balanced chemical equation Write in the values of the concentrations that you know in three rows under each species – initial, change, equilibrium Initial concentration = concentration of reactants originally placed in container (product concentration is usually zero unless otherwise stated) Change = usually a plus sign for products as they are created and a minus sign for reactants as they are used up Equilibrium = calculated from initial ± change in concentration Write the Kc expression and substitute in the EQUILIBRIUM values you calculated to calculate the Kc value

When the Kc value is very small If the Kc value is very small (less than 10-3) At equilibrium, there are much more reactants than products. We can assume therefore that the initial concentration of the reactant is about equal to the reactant concentration at equilibrium.

Free energy and equilibrium

Kc can be calculated from thermodynamic data