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Chemistry - Equilibrium

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    Study Guide Chemistry – Equilibrium 1. Quiz: Two Reaction Directions 1. Question Equilibrium is the stable situation resulting from Answer Choices • the forward reaction only • the reverse reaction only • two offsetting reactions Correct Answer two offsetting reactions Why This Is Correct Chemical equilibrium occurs when two opposing reactions — the forward reaction and the reverse reaction — are happening at the same time. These reactions offset each other so that there is no overall change in the amounts of reactants and products, even though reactions are still occurring. 2. Question When writing equilibrium reactions, a __________ is used. Answer Choices • single arrow pointing right • single arrow pointing left • double arrow

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    Study Guide Correct Answer double arrow Why This Is Correct A double arrow ( ⇌ ) shows that the reaction can proceed in both directions — forward and reverse. This symbol indicates that the system can reach equilibrium, unlike a single arrow which shows a one - direction (irreversible) reaction. 3. Question At 1.0 atmosphere of pressure and 0°C, water can exist as Answer Choices • only a solid • only a liquid • both a liquid and a solid Correct Answer both a liquid and a solid Why This Is Correct At 0°C and 1 atm pressure , water is at its melting/freezing point . At this temperature, ice and liquid water can exist together in equilibrium, meaning melting and freezing occur at the same rate. 4. Question Equilibrium positions will change depending on the

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    Study Guide Answer Choices • temperature • time of the day • use of a catalyst Correct Answer temperature Why This Is Correct Changing the temperature affects the energy of particles and shifts the equilibrium position according to Le Châtelier’s principle. A catalyst only speeds up how fast equilibrium is reached and does not change the equilibrium position, and time of day has no effect. 5. Question At equilibrium and at a fixed temperature, the forward reaction will occur at Answer Choices • the same rate as the reverse reaction • a rate twice that of the reverse reaction • a rate that is half that of the reverse reaction Correct Answer the same rate as the reverse reaction Why This Is Correct At equilibrium, the rates of the forward and reverse reactions are equal. This balance keeps the concentrations of reactants and products constant, even though reactions continue to occur.

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    Study Guide 2. Equilibrium Concentrations 2.1 What Does Chemical Equilibrium Mean? When a chemical reaction reaches equilibrium , it does not stop. Instead, the forward and reverse reactions happen at the same rate . Because of this balance, the concentrations (or pressures) of reactants and products stay constant over time . At equilibrium, these concentrations are related by a special number called the equilibrium constant , written as K . 2.2 Writing an Equilibrium Expression Let’s start with an example: The equilibrium expression for this reaction is: Important Rules to Remember: • Substances on the right side (products) go on the top of the expression. • Substances on the left side (reactants) go on the bottom . • The coefficients in the balanced equation become exponents in the expression. Because NO ₂ has a coefficient of 2, its concentration is squared.

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    Study Guide 2.3 The General Form of an Equilibrium Expression For a general reaction: The equilibrium expression is: Here: • Brackets [ ] mean concentration • Lowercase letters are the stoichiometric coefficients from the balanced equation Example: Formation of Ammonia Consider this important industrial reaction: The equilibrium expression is: This tells us how ammonia, nitrogen, and hydrogen are related at equilibrium. Example: Dissociation of Water Vapor Another example is the dissociation of water vapor: The equilibrium expression is: Each reaction has its own value of K , depending on the reaction and the temperature.

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    Study Guide 2.4 Equilibrium Constants and Units The value of K depends on: • The reaction itself • The temperature • How concentration is measured For: • Solutions → concentration is usually in molarity (Kc) • Gases → concentration can be expressed as pressure (Kp) 2.5 Calculating K from Experimental Data (Ammonia Example) For the reaction: The equilibrium constant is: Substituting the values: This large value of K means the reaction strongly favors ammonia formation .

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    Study Guide 2.6 Using K to Predict Reaction Direction You can compare current pressures or concentrations with the value of K: • If the calculated ratio is greater than K , the reaction shifts left (ammonia decomposes). • If the ratio is less than K , the reaction shifts right (more ammonia forms). The system will always adjust until equilibrium is reached. 2.7 Pure Solids and Liquids in Equilibrium Expressions Pure solids and liquids do not appear in equilibrium expressions because their concentrations stay constant. Example: The equilibrium expression is: Water (H ₂ O) is a liquid, so it is not included . Another Example with Solids The equilibrium expression is: The solids (LiBr and LiH) are required for the reaction but are not part of the K expression .

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    Study Guide Practice Applications Halogen Reaction at 100°C Given initial amounts in a 1.0 L container, equilibrium calculations can predict which direction the reaction will move. Sulfur Trioxide Formation at 700°C Given: • P ₛ ₒ ₂ = 0.2 atm • Pₒ ₂ = 0.5 atm You can calculate the pressure of SO ₃ at equilibrium. Summary • Equilibrium means no net change , not no reaction. • The equilibrium constant K relates reactants and products. • Coefficients become exponents . • Pure solids and liquids are not included . • The value of K helps predict reaction direction .

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    Study Guide 3. Quiz: Equilibrium Concentrations 1. Question The equilibrium constant is always written as a ratio of Answer Choices • products over reactants • reactants over products • products times reactants Correct Answer products over reactants Why This Is Correct By definition, an equilibrium constant (K) is written as the concentration of products divided by the concentration of reactants, each raised to the power of their coefficients from the balanced equation. 2. Question When writing an equilibrium expression, the coefficients from the balanced equation become the Answer Choices • factors of the concentration terms • exponents of the concentration terms • divisor of the concentration terms Correct Answer exponents of the concentration terms Why This Is Correct The coefficients in a balanced chemical equation indicate how many moles participate and therefore become the exponents in the equilibrium expression.

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    Study Guide 3. Question In a (K c ) expression, the concentrations of the species are expressed in terms of Answer Choices • molality • mole fraction • molarity Correct Answer molarity Why This Is Correct (K c ) uses molar concentrations (mol/L) of reactants and products at equilibrium. 4. Question In a (K p ) expression, the concentrations of the gaseous species are expressed in terms of Answer Choices • pressure • molality • molarity Correct Answer pressure Why This Is Correct (K p ) is written using the partial pressures of gaseous reactants and products.

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