Calculate the value the equilibrium constant

In the previous graph, note that between t = 0 s and completion:

(i)	Δ[A] = -1.00M	and
(ii)		and	[B]comp. = 0.50M (≠ 0M as it is the excess reagent)
(ii)		and

Open and Closed Systems:

To express it simply in this context, a system is a set of related reactants and products within a container.

• At least one reactant concentration ends up being 0 M.

• Reactions that go to completion typically happen in open systems from which one or more of the products can escape.

Phase equilibrium: State (i.e. phase) changes are physical changes, not chemical changes; however, they still reach equilibrium in closed systems. For example, when liquid water is placed in a sealed container with an airspace above the water, some of the water will evapourate forming a vapour. Eventually the evapouration rate of the water and the condensation rate of the vapour become equal. This leaves a situation with liquid water and an airspace above it containing water vapour of a pressure that thereafter remains constant over time unless further changes are made to the system (e.g. opening the container or changing the temperature). This is called phase equilibrium as it involves two phases of the same substance. For example:

H2O(l)		H2O(g)

aA + bB

where A, B, C, and D are chemical species and a, b, c, and d are stoichiometric numbers, the following expression known as an equilibrium expression is equal to Kc:

𝐾! =

2 NO2(g) 2 NO(g) + O2(g) 𝐊𝐜 =𝐍𝐎𝟐𝐎𝟐 𝟏

𝐍𝐎𝟐 𝟐
________________________________________________________________________

			Kc127°C = 3.8×104 dm6 mol-2

K c500°C = 6.0×10-2 dm6 mol-2• The size of Kc relates to the tendency of a reaction to occur, but it does not relate to the rate of the reaction. When Kc >> 1, the reaction proceeds almost to completion before equilibrium is reached though not necessarily quickly. When Kc << 1, the reaction proceeds only slightly before equilibrium is reached. This slight progression may happen slowly or quickly.

• Kc can be used to determine the final equilibrium position resulting from given [ ]o’s.

R(g)

K! =

!!
!!

Calculating Kc Values of Related Reactions:

1 N2(g) + 3 H2(g) 2 NH3(g)

-this doubles all of the exponents -so square the equilibrium constant

-this inverses the expression

!"! ! = 16.6 M2

________________________________________________________________________

#2.1 Write the equilibrium expression (equal to Kc) for each of the following gas-phase

reactions.

(a)
(b)
(c)	SiH4(g) + 2 Cl2(g)			SiCl4(g) + 2 H2(g)
(d)

		2 NO(g) + Cl2(g)				Kc = 1.6×10-5 mol dm-3
			N2(g) + O2(g)		Kc = 1×1031
For each separate reaction, some quantity of the reactant was placed in a container

#4.1 Is the following statement true or false? “Reactions with large equilibrium

constants are very fast.” Explain.

#8.1	What is the equilibrium concentration of SO3 in the following system if the equilibrium concentrations of SO2 and O2 are both 0.0500 M and Kc = 85.0 M-1?
	2SO2(g) + O2(g)

	H2(g) + F2(g)		2 HF(g)
	is 2.1×103 at a particular temperature. When the system is analyzed at equilibrium at this temperature, the concentrations of H2(g) and F2(g) are both found to be 0.0021 M. What is the concentration of HF(g) in the equilibrium

SO2(g) + NO2(g) SO3(g) + NO(g)

occurs under these conditions. Calculate the value of the equilibrium constant,

(a) 6 SO2(g) + 3 O2(g) 6 SO3(g)

(b) SO2(g) + ½ O2(g) SO3(g)

2 NO(g) N2(g) + O2(g)

A 1.0 L flask contains 0.024 mol NO, 2.0 mol N2, and 2.6 mol O2. For these

question #10(a). Keep extra significant figures through the calculations.

1Note: From Chemistry, Sixth Edition, 2003, Steven Zumdahl and Susan Zumdahl.