For this question, I understand how the species are affected, but I don't know how to represent this on a graph of molar concentration versus time: Given the equilibrium equation: XY(g) + heat...

For this question, I understand how the species are affected, but I don't know how to represent this on a graph of molar concentration versus time:

Given the equilibrium equation:

XY(g) + heat ---> X(g) + Y(g)

If initially, at equilibrium, the [XY] = 3.0 M, the [X] = 5.0 M and the [Y] = 6.0 M, draw a graph showing qualitatively what happens to the concentrations of each species as the following changes are made to the system

Time I -temperature is decreased.

Time II - Some X(g) is removed from the system

Time III - Some XY(g) is added to the system

Time IV - The total pressure is increased.

Asked on by sandblom

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ncchemist | eNotes Employee

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This question deals with LeChatelier's Principle, which states that if a chemical reaction at equilibrium is changed by either temperature, pressure, or concentration of one of the chemical species, the reaction will shift in a direction to counteract the change and form a new equilibrium.  So if more reactant is added to the reaction, the equilibrium will shift to the right to consume the extra reactant added.  Similarly, if more product is added to the reaction, the equilibrium will shift to the left.  It appears you want to see how the shift is plotted on a graph of concentration versus time.  So let's go through each case and discuss the effects and the plots.

1.  Temperature decrease.  This is an endothermic reaction, meaning that it requires heat to proceed.  This is shown by the fact that heat is listed as a reactant in the equation.  If the temperature is decreased, this means that heat is removed from the reaction, and the equilibrium will shift to the left (reactants).  So if we are looking at a plot of the concentration of XY versus time, the result would be a line with a positive slope (meaning the line starts in the lower left of the graph and rises with moving along the x axis to the upper right of the graph) because amount of XY is increasing with time.  If we were plotting the concentrations of X and/or Y over time, the result would be a line with a negative slope (upper left to bottom right) because the products are decreasing over time.

2.  X is removed.  If X is removed, the equilibrium will shift to the right to compensate and form more products.  Plotting the concentration of X versus time will be a line with a positive slope.

3.  XY is added.  If XY is added to the system, the equilibrium will shift to the right to compensate and form more products.  Plotting XY concentration versus time will result in a line with a negative slope.

4.  Increasing pressure.  A change in pressure does not really affect the concentrations of any of the species but the reaction can shift to accommodate the pressure change.  The reactant consists of one volume of gas while the products consist of two volumes of gas. 
Increasing the pressure of the system will cause the side with fewer total volumes of gas to dominate, so the equilibrium will shift to the left (reactants).  Plotting XY concentration versus time will give a line with a positive slope while plotting X and/or Y concentrations versus time will give a line with a negative slope.

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