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concentration vs time

(Con Con...concentrations are constant)

Rates vs time

(REqual...rates are equal)

What is equilibrium?

Phase equilibrium-The state of balance between 2 opposing phases (dissolving and crystallization). This occurs when a solution is saturated.

REGENTS NOTE***These processes occur at the SAME RATES. The Concentration remains constant.

Notice after equilibrium is established, the crystallization and dissolving rates equalize. This is called a DYNAMIC EQUILIBRIUM. This implies that there is constant change in the particles but overall the concentration and the mass of the solid remains constant.

Chemical Equilibrium-The state of balance between 2 opposing reactions. Once equilibrium is established the Concentrations Remain CONSTANT and the Rates are EQUAL. "CON CON REqual"

Walking on an escalator example....you are moving but still in the same place because of an opposing force or reaction in chemistry.

Regents Questions-

Jan 2007-43 Which balanced equation represents a phase equilibrium?
(1) H_2(g) + I_2(g)<==> 2HI_(g)
(2) 2NO_2(g) <==> N₂O_4(g)
(3) Cl_2(g) <==> Cl_2(l)
(4) 3O_2(g)<==>  2O_3(g)

June 2004-20 Which statement about a system at equilibrium is true?

(1) The forward reaction rate is less than the reverse reaction rate.
(2) The forward reaction rate is greater than the reverse reaction rate.
(3) The forward reaction rate is equal to the reverse reaction rate.
(4) The forward reaction rate stops and the reverse reaction rate continues.

Aug 2006-19 A chemical reaction is at equilibrium. Compared to the rate of the forward reaction, the rate of the reverse reaction is
(1) faster and more reactant is produced
(2) faster and more product is produced
(3) the same and the reaction has stopped
(4) the same and the reaction continues in both directions

Aug 2002-13 Which statement correctly describes a chemical reaction at equilibrium?
(1) The concentrations of the products and reactants are equal.
(2) The concentrations of the products and reactants are constant.
(3) The rate of the forward reaction is less than the rate of the reverse reaction.
(4) The rate of the forward reaction is greater than the rate of the reverse reaction.

Jan 2004-17 Given the reaction at equilibrium:
H_2(g) + Br_2(g) --> 2 HBr_(g)
The rate of the forward reaction is
(1) greater than the rate of the reverse reaction
(2) less than the rate of the reverse reaction
(3) equal to the rate of the reverse reaction
(4) independent of the rate of the reverse reaction

June 2005-18 Which statement must be true about a chemical system at equilibrium?
(1) The forward and reverse reactions stop.
(2) The concentration of reactants and products are equal.
(3) The rate of the forward reaction is equal to the rate of the reverse reaction.
(4) The number of moles of reactants is equal to the number of moles of product.

Jan 2010-30 Which quantities must be equal for a chemical reaction at equilibrium?
(1) the activation energies of the forward and reverse reactions
(2) the rates of the forward and reverse reactions
(3) the concentrations of the reactants and products
(4) the potential energies of the reactants and products

June 2010-20 Given the equation representing a phase change at equilibrium:
H₂O_(s) <==>H₂O_(l)
Which statement describes this equilibrium?
(1) The H₂O_(s) melts faster than the H₂O_(l) freezes.
(2) The H₂O_(l) freezes faster than the H₂O_(s) melts.
(3) The mass of H₂O_(s) must equal the mass of H₂O_(l).
(4) The mass of H₂O_(l) and the mass of H₂O_(s) each remain constant.

June 2007-

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