Understanding Chemical Equilibrium: Dynamic Balance in Reversible Reactions

1. Chemical Equilibrium 1 Some chemical reactions appear to stop after a period of time. The concentration of reactants stops decreasing, and the concentration of products stops increasing. Does that mean that nothing is happening at the molecular level?

2. Chemical Equilibrium 2 • No! • Reactants are still reacting to make products. • Products are also reacting to re-form the reactants. • The reaction is going “forward” – from left to right the way the chemical equation would be written – and “backward” – the opposite direction. • The concentrations are not changing because the forward and reverse reactions are happening at the same rate. That’s called dynamic, or chemical, equilibrium.

3. Chemical Equilibrium 3 In this illustration, we see the reaction I2 + H2 2HI . In the initial state illustration, we have only the reactants, hydrogen gas and iodine gas. In the second frame, representing the equilibrium state, we have some of the product, hydrogen iodide, but we still have some iodine gas and some hydrogen gas. Has the reaction stopped?

4. Chemical Equilibrium 4 http://image.tutorvista.com/content/chemical-equilibrium/hydrogen-iodide-formed.jpeg No! Hydrogen and iodine gases are still reacting to form hydrogen iodide, and hydrogen iodide molecules are breaking up and re-forming hydrogen and iodine gases! The forward and reverse reactions are happening at the same rate, so the system is in dynamic __________________.

5. Chemical Equilibrium 5 http://upload.wikimedia.org/wikipedia/commons/a/a9/ChemicalEquilibrium.jpg In this graph, the reactants (red line) decrease in concentration while the products (blue line) increase in concentration , but only for one second. What is happening at the end of that second?

6. Chemical Equilibrium 6 Dynamic equilibrium has been reached. Chemical equilibrium is another way to say it, but dynamic equilibrium is so much more descriptive! Dynamic means that something is changing, while equilibrium means that it remains the same. How is that appropriate? The molecules themselves are still changing, but the overall mixture of reactants and products is staying the same.

7. That symbol is used in place of the one-direction arrow to write the equation for a reaction that goes on both directions. CH4(g) + H2O(g) <--> CO(g) + 3H2(g) is an example of a reversible reaction. If it is in equilibrium, then the rate of the forward and reverse reactions is the same. This DOES NOT mean that there are equal amounts of products and reactants. Chemical Equilibrium 7

8. Chemical Equilibrium 8 • Analogy: A movie theater sits next to a taco shop. Every evening, people patronize these two establishments. • Most of the time, the taco shop is full, with 30 diners eating there. • The movie theater is also nearly full, with 250 viewers at a time. • All evening, people travel between these two establishments, eating, watching movies, and sometimes eating some more. On average, 45 people travel from the taco shop to the theater every hour. 45 people also go from the theater to the taco shop. • The rate travel in each direction is equal, while the number of people in each establishment maintains its inequality. • Dynamic equilibrium is being maintained.

9. Chemical Equilibrium 9 • LeChatelier’s ( luh shat-ell-yay’s) Principle is used to predict how a reaction that is at equilibrium and is then disturbed in some way will then shift to re-gain its equilibrium. • Analogy: If I push you to the left, you will shift your weight to the right to regain your balance. • LeChatelier’s Principle deals with changes in concentration, temperature, pressure, and volume.

10. Chemical Equilibrium 10 • Let’s say we have the following reversible reaction that is at equilibrium: • A + B ↔ 2C + D • If we add extra B, we are increasing the concentration of a reactant. The reaction will increase in the forward direction, shifting to the right, as the reactions try to regain equilibrium. • If we add extra D, the reverse reaction will increase, shifting the reaction towards the left, as the extra product gets used up in an effort to regain equilibrium.

11. Chemical Equilibrium 11 • According to LeChatelier’s Principle, for any reversible reaction at equilibrium: • Increasing the concentration of reactant will cause the reaction to shift right in order to regain equilibrium. • Increasing the concentration of a product will cause the reaction to shift left in order to regain equilibrium.

12. Chemical Equilibrium 12 • LeChatelier’s Principle is also used for changes in temperature: • An exothermic reaction gives off energy in the form of heat. Heat is, essentially, a product. • If that exothermic reaction is at equilibrium, then the reverse reaction is absorbing that energy at the same rate that the energy is being given off in the forward direction, making no net temperature change. • N2(g) + 3H2(g) <--> 2NH3 + 91.8 kJ • This reaction is at equilibrium at a certain temperature.

13. 13N2(g) + 3H2(g) <--> 2NH3 + 91.8 kJ • If we add extra energy by increasing the temperature of this system, the reaction will shift in the direction that uses up that excess energy. • It shifts in the endothermic direction. • It shifts to the left.

14. 14N2(g) + 3H2(g) <--> 2NH3 + 91.8 kJ • If the above system of reactions is at equilibrium and we decrease its temperature, how will the rates of forward and reverse reactions adjust to regain equilibrium? • The reaction will shift in the way that gives off energy to make up for the loss of heat. • The reaction will shift to the right.

15. Chemical Equilibrium 15 • BaCl2 + NaCO3 + energy ↔ NaCl + Ba(CO3)2 • If the above endothermic reaction is reversible and at equilibrium, what effect will an increase in temperature have? • An increase in temperature always causes a reaction to shift in the direction that will use up the excess energy. • In this case, the reaction will shift to the left.

16. Chemical Equilibrium 16 • According to LeChatelier’s Principle, for any reaction that is at equilibrium: • An increase in temperature will cause the reaction to shift in the direction that consumes the energy in order to regain equilibrium. • A decrease in temperature will cause the reaction to shift in the direction in which energy is given off in order to regain equilibrium.

17. Volume and pressure of gases are, as you may remember from studying properties of gases, inversely related. We can increase the pressure of a gas by decreasing the volume. We can also decrease the pressure by increasing the volume. Chemical Equilibrium 17 What does this have to do with LeChatelier’s Principle?

18. Chemical Equilibrium 18 • Chemical reactions at equilibrium will also shift in response to changes in their pressure, which are usually caused by changes in volume. Only gases are subject to this part of LeChatelier’s Principle. • C2H6(g) + Cl2(g) <---> C2H5Cl(s) + HCl(g) • The coefficients for the substances in the above reaction are 1, 1, 1, and 1. Notice which substances are gases. • There are two moles of gases on the left side and only one mole of gas on the right side of the equation.

19. 19C2H6(g) + Cl2(g) <---> C2H5Cl(s) + HCl(g) • Let’s say that equation is at equilibrium. It is at equilibrium at a certain pressure. • If we increase the pressure by decreasing the volume, the system will shift to reduce the pressure again. • The left side of the equation has two moles of gas, while the right has just one mole of gas. • Which side will give the lower pressure at a given volume?

20. 20C2H6(g) + Cl2(g) <---> C2H5Cl(s) + HCl(g) • If we increase the pressure by decreasing the volume, the system will shift to the right, decreasing the amount of gas, reducing the pressure to regain equilibrium. • If, on the other hand, we increase the volume, decreasing the pressure, which way will the system shift? • It will shift toward the side with a greater volume of gas, the left, to regain equilibrium.

21. Chemical Equilibrium 21 • H2(g) + N2(g) ↔ H3N(g) represents the reaction illustrated above. Write the equation in balanced form on your study guide. • In which picture above is the pressure lowest? • Frame b shows the pressure being greatly increased by the volume being decreased. Notice what happens in frame c. Which way has the reaction shifted? Why?

22. Chemical Equilibrium 22 In frame c, the reaction has shifted to the right, because the right side of the reaction has only one mole of gas, leading to lower pressure.

23. Chemical Equilibrium 23 • According to LeChatelier’s Principle, a reversible chemical reaction at equilibrium will react to a disturbance in its pressure in order to regain its equilibrium: • If the pressure is increased, the reaction will shift to the side that has fewer moles of gas in order to regain its equilibrium • If pressure is decreased, the reaction will shift toward the side with more moles of gas in en effort to regain its equilibrium. • If the two sides of the equation have equal numbers of moles of gas, pressure changes will have no effect on the system.