Reversible Reactions 1. שיווי משקל כימי תגובות הפיכות Suppose SO2 and O2 are present initially. As they collide, the forward reaction begins. 2SO2(g) + O2(g) 2SO3(g) As SO3 molecules form, they also collide in the reverse reaction that forms reactants. 2SO3(g) 2SO2(g) + O2(g) This reversible reaction is written with a double arrow. forward 2SO2(g) + O2(g) 2SO3(g) reverse

תגובות הפיכות A reversible reaction proceeds in both the forward and reverse directions. As a result there are two reaction rates: the rate of the forward reaction and the rate of the reverse reaction. When molecules begin to react, the rate of the forward reaction is faster than the rate of the reverse reaction. As reactants are consumed and products accumulate, the rate of the forward reaction decreases, whereas the rate of the reverse reaction increases.

תגובות הפיכות

שיווי משקל כימי At equilibrium, the rate of the forward reaction becomes equal to the rate of the reverse reaction. the forward and reverse reactions continue at equal rates. no further changes occur in the concentration of reactants and products. Insert bottom page 7 right column, reaction diagram. 4

דוגמא נוספת לתגובה שבשיווי משקל Given the reaction, H2(g) + I2(g) 2HI(g) at equilibrium, the forward and reverse reactions occur at the same time and are shown together using a double arrow. Forward reaction: H2(g) + I2(g) 2HI(g) Reverse reaction: 2HI(g) H2(g) + I2(g) H2(g) + I2(g) 2HI(g)

המשך הדוגמא H2(g) + I2(g) 2HI(g) Initially, the reaction flask contains only the reactants H2 and I2. The forward reaction begins to produce HI. As the reaction proceeds, there is less H2 and I2 and more HI, which increases the rate of the reverse reaction. At equilibrium, the concentrations of reactants and product are constant. The reaction continues, with the rate of the forward reaction equal to the rate of the reverse reaction.

תרגיל 1 Learning Check Write the forward and reverse reactions for the following. CH4(g) + 2H2S(g) CS2(g) + 4H2(g)

תרגיל 2 Learning Check Complete with equal, not equal, forward, reverse, changes, or does not change. 1. Reactants form products in the ________ reaction. 2. At equilibrium, the reactant concentration _______. 3. When products form reactants, it is the _______ reaction. 4. At equilibrium, the rate of the forward reaction is ______ to the rate of the reverse reaction. 5. If the forward reaction is faster than the reverse, the amount of products ________.

Equilibrium Constants 2. קבועי שיווי משקל Equilibrium Constants For the reaction, the equilibrium constant expression, Kc, gives the concentrations of the reactants and products at equilibrium, the square brackets indicate the moles/liter of each substance, and the coefficients b and a are written as superscripts that raise the moles/liter to a specific power.

השלבים בכתיבת קבוע שיווי משקל

דוגמא Write the Kc expression for the following reaction. Step 1 Write the balanced chemical equation. Step 2 Write the concentrations of the products as the numerator and the reactants as the denominator. Step 3 Write any coefficient in the equation as an exponent. 2 2

תרגיל Learning Check Which of the following is the correctly written Kc expression for the reaction shown below? A. B. C. D.

שיווי משקל הומוגני והטרוגני In a homogeneous equilibrium, all reactants and products in the reaction are in the same physical state. In a heterogeneous equilibrium, the reactants and products are in two or more physical states. The concentration of solids and liquids are constant, and therefore omitted from the equilibrium constant expression.

Learning Check תרגיל Write the equilibrium constant expression for the following reaction at equilibrium.

השלבים בחישוב קבוע שיווי משקל

דוגמא What is the Kc for the following reaction at 427 ˚C if the equilibrium concentrations are [H2] = 0.20 M, [I2] = 0.20 M and [HI] = 1.47 M? Step 1 Write the Kc expression for the equilibrium. Step 2 Substitute equilibrium (molar) concentrations and calculate Kc.

Learning Check תרגיל Calculate the Kc for the reaction, with the following equilibrium concentrations,

Reaching Chemical Equilibrium 4. שימוש בקבועי שיווי משקל Reaching Chemical Equilibrium The values of equilibrium constants can be large or small. The size of the Kc depends on whether equilibrium is reached with more products than reactants (Kc is large), or more reactants than products (Kc is small).

Kc ערך גבוהה של When a reaction has a large equilibrium constant, it means the forward reaction produced a large amount of products when equilibrium was reached. the equilibrium mixture contains mostly products.

המשך Kc is Large.

Kc ערך נמוך של When a reaction has a small equilibrium constant, it means the reverse reaction converted a large amount of products back to reactants. the equilibrium mixture contains mostly reactants.

המשך Kc is Small.

Kc סיכום ערכי ה A reaction that favors products has a large Kc. with about equal concentrations of products and reactants has a Kc close to 1. that favors reactants has a small Kc.

שוניםKc דוגמאות לערכי

תרגיל Learning Check For each Kc, indicate whether the reaction at equilibrium contains mostly reactants or products.

Kc השלבים בשימוש ערכי

Kc דוגמא- חישוב ריכוזים בשיווי משקל בשימוש At equilibrium, the reaction, has a Kc of and contains What is the equilibrium concentration of PCl5? Problem Facts

המשך הדוגמא Step 1 Write the Kc expression for the equilibrium equation. Step 2 Solve the Kc expression for the unknown concentration. Step 3 Substitute the known values into the rearranged Kc expression and calculate.

Learning Check תרגיל The Kc is 2.0 for the reaction, If the equilibrium concentrations are and [ what is the equilibrium concentration of Br2? A. 0.39 M B. 0.78 M C. 1.3 M

Le Châtelier’s Principle 4. שינויים בתנאי שיווי המשקל ועקרון לה שטליה Le Châtelier’s Principle Chemical equilibrium can be disturbed by a change in concentration, volume, or temperature. Altering any of these conditions puts the system under stress. Le Châtelier’s principle states that when a system at equilibrium is disturbed, the system will shift in the direction that will reduce that stress. there will be a change in the rate of the forward or reverse reaction to return the system to equilibrium.

Changing Concentrations א. שינויים בריכוזים Changing Concentrations For the following reaction at equilibrium, the rate of the forward reaction is increased to relieve the stress when more H2 or I2 is added, or HI is removed. the rate of the reverse reaction is increased to relieve the stress when H2 or I2 is removed or more HI is added.

Adding Reactant or Product סיכום- ההשפעה של הוספת מגיב או תוצר Adding Reactant or Product The equilibrium shifts toward products when H2(g) or I2(g) is added. reactants when HI(g) is added. Add H2 or I2 Add HI

Removing Reactant or Product המשך הסיכום- ההשפעה של הוצאת מגיב או תוצר Removing Reactant or Product The system shifts toward a reverse reaction when H2 or I2 is removed. a forward reaction when HI(g) is removed.

Concentration Changes and Equilibrium שינויי ריכוזים ושיווי משקל Concentration Changes and Equilibrium (a) The addition of H2 places stress on the equilibrium system of H2 (b) To relieve the stress, the forward reaction converts some reactants, to product, HI(g). (c) A new equilibrium is established when the rates of the forward reaction and the reverse reaction become equal.

ב. הוספת זרז Adding a catalyst lowers the activation energy of the forward reaction. increases the rate of the forward reaction. lowers the activation energy of the reverse reaction. increases the rate of the reverse reaction. decreases the time to reach equilibrium. has no effect on the concentrations at equilibrium.

תרגיל Learning Check Predict any shift in the forward or reverse reactions for each of the following changes on the reaction. 1. H2S(g) is added. 2. NH3(g) is removed. 3. A catalyst is added.

ג. שינויים בנפח Changing the volume of a gas mixture at equilibrium will change the concentrations of gases in the mixture, upsetting the equilibrium. Decreasing the mixture volume will increase the concentration of gases. Increasing the mixture volume will decrease the concentration of gases.

סיכום- ההשפעה של השינויים בנפח Decreasing the volume of the gas mixture shifts the equilibrium towards the fewer number of moles. Increasing the volume of the gas mixture shifts the equilibrium toward the larger number of moles.

Volume Decrease and Equilibrium דוגמא להשפעה של השינויים בנפח Volume Decrease and Equilibrium A decrease in the volume of the container causes the system to shift in the direction of fewer moles of gas. An increase in the volume of the container causes the system to shift in the direction of more moles of gas.

Temperature Change and Endothermic Reactions ד. שינויים בטמפרטורה ותגובות אנדותרמיות Temperature Change and Endothermic Reactions For an endothermic reaction at equilibrium, heat is a reactant. A decrease in temperature (T) removes heat, shifting the equilibrium toward the reactants. An increase in temperature adds heat, shifting the equilibrium toward the products.

Temperature Change and Exothermic Reactions ד. שינויים בטמפרטורה ותגובות אקסותרמיות Temperature Change and Exothermic Reactions For an exothermic reaction at equilibrium, an increase in temperature adds heat and the equilibrium shifts toward the reactants. a decrease in temperature (T) removes heat and the equilibrium shifts toward the products.

Effects of Changes on Equilibrium סיכום ההשפעות של השינויים על שיווי המשקל Effects of Changes on Equilibrium

תרגיל Learning Check Indicate if each change on a reaction at equilibrium shifts towards the products or reactants or does not change the equilibrium. 1. adding NO(g) 2. raising the temperature 3. increasing the volume