The reversible reaction Hb+4O2⇄Hb(O2)4exists in blood between hemoglobin (Hb) and oxyhemoglobin Hb(O₂)₄. According to the law of mass action, doubling the O₂ concentration would:

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11ecba2d_1202_7c0e_a3bf_c39de42639e8_MD0008_00 The general reaction in which chemical species A and B react to form products C and D is written as 11ecba2d_1202_7c0f_a3bf_0b0fb0336f63_MD0008_00 aA+bB⇄cC+dD where a, b, c, and d are the coefficients of the balanced reaction. The law of mass action states that a reaction quotient Q can be expressed as a ratio of the molar concentrations of the products over the reactants, each raised to the power of its respective balanced reaction coefficient: 11ecba2d_1202_7c10_a3bf_fff38be4c9c2_MD0008_00 The law of mass action further states that the concentrations of the chemical species will change until equilibrium is reached, such that Q and the equilibrium constant K_eq are equal. Changes to the concentration of chemical species with a higher reaction coefficient will have the greatest impact to increase or decrease the value of Q. For the reaction between hemoglobin and oxygen, Q is expressed as 11ecba2d_1202_7c11_a3bf_17727a913a1c_MD0008_00 If [O₂] is initially x mol/L and is then doubled to 2x mol/L, the change will be raised to the power of 4 ([2x]⁴ =16x⁴). Because [O₂] is in the denominator of the Q expression, this will decrease the value of the reaction quotient by a factor of 16 relative to the initial value of Q. (Choice A) The reaction quotient would decrease by half if the hemoglobin concentration were doubled instead of the O₂ concentration. (Choice C) Doubling the oxyhemoglobin concentration instead of the O₂ concentration would increase the reaction quotient by a factor of 2. (Choice D) O₂ has a coefficient of 4 in the balanced reaction equation. In the reaction quotient, [O₂] is in the denominator and the concentration of O₂ is exponentially raised to the power of 4 (not just multiplied by 4). Educational objective: The law of mass action states that a reaction quotient Q can be expressed as a ratio of the molar concentrations of the products over the reactants, each raised to the power of its respective balanced reaction coefficient. Reactions tend to proceed toward equilibrium by shifting species concentrations until Q equals the equilibrium constant K_eq.

The Reversible Reaction Hb+4O2⇄Hb(O2)4exists in Blood Between Hemoglobin (Hb)