The Effect of a Temperature Change on Equilibrium

According to Le Chatelier s principle, if the temperature of a system at equilibrium is changed, the system will shift in a direction to counter that change. So, if the temperature is increased, the reaction will shift in the direction that tends to deaease the temperature and vice versa. Recall from Chapter 6 that an exothermic reaction (negative AH) emits heat  

We can think of heat as a product in an exothermic reaction. In an endothermic reaction (positive AH), the reaction absorbs heat. 

At constant pressure, raising the temperature of an exothermic reaction—think of this as adding heat—is similar to adding more product, causing the reaction to shift left. For example, the reaction of nitrogen with hydrogen to form ammonia is exothermic  

Conversely, lowering the temperature causes the reaction to shift right, releasing heat and producing more products because the value of K has increased  

In contrast, for an endothermic reaction, raising the temperature (adding heat) causes the reaction to shift right to absorb the added heat. For example, the following reaction is endothermic  

In Section 3.9, we classify chemical reactions according to whether they absorb or emit heat energy in tiie course of the reaction. Recall that an exothermic reaction (one witii a negative emits heat. 

In contrast, an endothermic reaction (one with a positive AHfxn) absorbs heat. Endothermic reaction A -I- B -I- Heat - C + D 

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Determining the Effect of a Temperature Change on Equilibrium (14.9) Add heat... 

The effect of a temperature change on solubility equilibria such as these can be predicted by applying a simple principle. An increase in temperature always shifts the position of an equilibrium to favor an endothermic process. This means that if the solution process absorbs heat (AHsoin. > 0), an increase in temperature increases the solubility. Conversely, if the solution process is exothermic (AH < 0), an increase in temperature decreases the solubility. 

In summary, to use Le Chatelier s principle to describe the effect of a temperature change on a system at equilibrium, treat energy as a reactant (in an endothermic process) or as a product (in an exothermic process), and predict the direction of the shift as if an actual reactant or product is added or removed. Although Le Chatelier s principle cannot predict the size of the change in A, it can correctly predict the direction of the change. 

What property of a reaction allows you to predict the effect of a temperature change on the equilibrium ... 

We have seen how Le Chatelier s principle can be used to predict the effects of several types of changes on a system at equilibrium. To summarize these ideas, Table 17.2 shows how various changes affect the equilibrium position of the endothermic reaction N204( ) t" 2N02( ). The effect of a temperature change on this system is shown in Figure 17.13. 

Effect of a Temperature Change on Equilibrium Again, the effect of temperature on a reaction allows chemists to choose conditions that will favor desired reactions. Higher temperatures favor endothermic reactions, while lower temperatures favor exothermic reactions. Most reactions will occur faster at higher temperature, so the effect of temperature on the rate, not just on the equilibrium constant, must be considered. 

Le Chatelier s principle can be used to predict the effect of a temperature change on the position of an equilibrium. The key factor to he considered here is whether the forward reaction is exothermic (a negative AH value) or endothermic (a positive AH value)(see Chapter 5). Remember that, in a reversible reaction, the reverse reaction has an enthalpy change that is equal and opposite to that of the forward reaction. 

At constant temperature, a decrease in volume (iucrease in pressure) increases the concentrations of both A and D. In the expression for Q, the concentration of D is squared aud the conceutratiou of A is raised to the first power. As a result, the numerator of Q increases more than the denominator as pressure increases. Thus, Q > K, and this equilibrium shifts to the left. Couversely, an increase in volume (decrease in pressure) shifts this reaction to the right until equilibrium is reestablished, because Q < K. We can summarize the effect of pressure (volume) changes on this gas-phase system at equilibrium. 

Le Chateliers principle can be used to predict the effect of a change in temperature on the position of an equilibrium. In general, an increase in temperature causes Ihe endothermic reaction to occur. This absorbs heat and so tends to reduce the temperature of the system, partially compensating for the original temperature increase. 

Ealy, Jr., "Effect of Temperature Change on Equilibrium Cobalt Complex" Chemical Demonstrations, A Sourcebook for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988), p. 60-61. Concentrated hydrochloric acid is added to pink [Co(H20)5]2+ until blue [C0CI4]2- is formed. When heated the solution turns darker blue when cooled the solution turns pink, indicating that the reaction is endothermic. Students are asked to examine the equilibrium reaction and predict how the system will shift upon the addition of water. 

Chemical equilibrium represents a balance between forward and reverse reactions. In most cases, this balance is quite delicate. Changes in experimental conditions may disturb the balance and shift the equilibrium position so that more or less of the desired product is formed. When we say that an equilibrium position shifts to the right, for example, we mean that the net reaction is now from left to right. Variables that can be controlled experimentally are concentration, pressure, volume, and temperature. Here we will examine how each of these variables affects a reacting system at equilibrium. In addition, we will examine the effect of a catalyst on equilibrium. 

A binary liquid solution, having mole fraction x of component 1, is in equilibrium with a vapor that has mole fraction v of that component. Show that for this mixture the.effect of a change in temperature on the equilibrium pressure at fixed liquid composition is approximately... 

Law of reactions—Henry Le Chatelier (1850-1936) put forward the law of reaction, which governs the effects of pressure, temperature, concentration, and catalyst on reaction equilibrium. It states that if a change of conditions (stress) is applied to a system at equilibrium, the system will respond in the way that best reduces the stress in reaching a new state of equilibrium. ... 

LAG - A delay in the effect of a changed condition at one point in the system, on some other condition to which it is related. Also, the delay in action of the sensing element of a control, due to the time required for the sensing element to reach equilibrium with the property being controlled i.e., temperature lag, flow lag, etc. 

What is the effect on the position of a reaction system at equilibrium when an exothermic reaction is performed at a higher temperature Does the value of the equilibrium constant change in this situation ... 

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