A State of Dynamic Balance

COOKES Chemical equilibrium is described by an equilibrium constant expression that relates the concentrations of reactants and products. 

Real-World Reading Link Imagine a tug-of-war between two teams. Because the rope between them is not moving, it might seem that neither team is pulling. In fact, both teams are pulling, but the forces exerted by the two teams are equal and opposite, so they are in complete balance. 

Consider the reaction for the formation of ammonia from nitrogen and hydrogen that you read about in Chapter 15. 

The equation for the production of ammonia has a negative standard free energy, AG°. Recall that a negative sign for AG° indicates that the reaction is spontaneous under standard conditions, defined as 298 K and 1 atm, but spontaneous reactions are not always fast. When carried out under standard conditions, this ammonia-forming reaction is much too slow. To produce ammonia at a rate that is practical, the reaction must be carried out at a much higher temperature and pressure. 

Explain how the graph shows that the concentrations of the reactants and products become constant. 

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We have learned much about equilibrium. It is characterized by constancy of macroscopic properties but with molecular processes continuing in a state of dynamic balance. At equilibrium we can conclude that every reaction that takes place does so at the same reaction rate as its reverse reaction. 

A state of dynamic balance in which the rates of forward and reverse reactions are equal there is no net change in concentrations of reactants or products while a system is at equilibrium. 

The rate and extent of these processes can change over time. A mass balance usually reflects one of two assumptions or their converse the system is at equilibrium or the system is at steady state. Equilibrium is a state of dynamic balance—such as that which occurs when forward and reverse reactions are equal— where there is no impetus for change within the system. Steady state simply means that the condition being evaluated (which might be concentration in a certain phase or the flow rate, for example) is essentially unchanging over a specified time period. 

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