Unit 10: Energy in Chemical Reactions Cypress Creek High School Chemistry 1K Chapter 10

Enthalpy Change Problem Use standard heat of formation to calculate ΔHrxn for the combustion of methane: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) Find the standard heat of formation values from the chart or information provided. Use the formula and multiply each term by the coefficient of the substance in the balanced chemical equation: ΔHrxn = ΣHf(products) - ΣHf(reactants) ΔHrxn = [(-394 kJ) + (2)(-286 kJ)] – [(-75 kJ) + (2)(0 kJ)] ΔHrxn = [-966 kJ] – [-75 kJ] = -891 kJ 3. Identify the reaction: exothermic reaction! Molecule Standard Heat of Formation ΔHf0 CH4 (g) -75 kJ/mole C2H2 (g) 227 kJ/mole C2H6 (g) -85 kJ/mole C6H12O6 (s) -1268 kJ/mole O2 (g) 0 kJ/mole H2 (g) CO2 (g) -394 kJ/mole H2O (l) -286 kJ/mole

Enthalpy Change from Standard Heat of Formation Use standard heat of formation to calculate ΔHrxn for the combustion of methane CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) We can summarize Hess’s Law into the following equation: ΔHrxn = ΣΔHf(products) - ΣΔHf(reactants) The symbol Σ means “to take the sum of the terms.”

Hess’s Law Example Calculate DH for the overall reaction: C2H4(g) + H2(g) → C2H6(g), from the following: C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) DH = -1411. kJ C2H6(g) + 3½O2(g) → 2CO2(g) + 3H2O(l) DH = -1560. kJ H2(g) + ½O2(g) → H2O(l) DH = -285.8 kJ What do you do? Decide how you need to manipulate the intermediate reactions to algebraically cancel out unnecessary chemicals and leave the overall reaction.

Hess’s Law Practice Calculate DH for the overall reaction: C2H4(g) + H2(g) → C2H6(g), from the following: C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) DH = -1411. kJ C2H6(g) + 3½O2(g) → 2CO2(g) + 3H2O(l) DH = -1560. kJ H2(g) + ½O2(g) → H2O(l) DH = -285.8 kJ What do you do? There is no H2O in the overall reaction, but there is C2H4. So take step 2 and flip it (product become reactant and reactants become products) 2CO2(g) + 3H2O(l) → C2H6(g) + 3½O2(g) DH = 1560. kJ This means that the DH need to have it’s sign flipped

Hess’s Law Practice Calculate DH for the overall reaction: C2H4(g) + H2(g) → C2H6(g), from the following: C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) DH = -1411. kJ 2CO2(g) + 3H2O(l) → C2H6(g) + 3½O2(g) DH = 1560. kJ H2(g) + ½O2(g) → H2O(l) DH = -285.8 kJ What do you do? Check to see if unnecessary chemicals can be cancelled out.

Hess’s Law Practice Calculate DH for the overall reaction: C2H4(g) + H2(g) → C2H6(g), from the following: C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) DH = -1411. kJ 2CO2(g) + 3H2O(l) → C2H6(g) + 3½O2(g) DH = 1560. kJ H2(g) + ½O2(g) → H2O(l) DH = -285.8 kJ What do you do? After crossing everything out that you can you are left with the overall reaction. Now add up the DHs. (-1411 + 1560 + -285.8) = -136.8

Reaction Rate Reaction Rate is the number of atoms, ions or molecules that react in a given time to form products We measure either the rate of disappearance of the reactants or the rate of appearance of one of the products In simpler terms: The average rate is the change in a given quantity during a specific period of time. 8

Reaction Rate Graph For the reaction H2 + I2 → 2HI, What is happening to the concentration of the reactants? What is happening to the concentration of the products? 9

Measuring Reaction Rates Change in Electrical Conductivity Change in Color Change in Pressure Change in Volume 10

Factors that Effect Reaction Rate Physical State of the Reactants The more freedom of movement, the more collisions As state of matter changes from solid to gas, rate increases

Factors that Effect Reaction Rate Concentration of Reactants The more particles, the more collisions As concentration increases, rate increases 12

Factors that Effect Reaction Rate Temperature of Reactants As temperature increases, particles move faster; there are more high-energy collisions 13

Factors that Effect Reaction Rate Particle Size (surface area) For solids, breaking up big pieces increases surface area, increasing rate by having more places for the molecules to interact Ionic compounds have more surface area than covalent 14

Factors that Effect Reaction Rate Pressure – Gases only! As pressure increases the concentration increases, so you will have more collisions 15

Factors that Effect Reaction Rate Catalysts – compounds added to speed up reaction but not used in the reaction. Lowers activation energy but does not change ΔH Examples: enzymes, catalytic converters (Pt, plates that convert NO → N2 + O2 and CO → CO2) 16

Catalyst 17

Catalysts Without a catalyst Enthalpy Time (min)

Catalysts Lowers activation Without a energy catalyst Enthalpy With a Time (min)

Catalysts DOES THE CATALYST CHANGE THE DH? NO! Lowers activation energy Without a catalyst Enthalpy H reactants With a catalyst H products Time (min)

Factors that Effect Reaction Rate Inhibitors Compounds added to a reaction that slow it down Examples: Lead in diesel, preservatives in food. 21

Reaction Rates Discussion Why do you add lighter fluid to charcoal in a grill? How does opening and closing air vents on a charcoal grill effect how fast the coals burn? How can an explosion occur in a grain elevator when grain is not considered flammable? 22

Reaction Rate Laws Rate Laws relate the reaction rate and reactant concentration We write the rate laws for elementary reactions from the equation. They include only the gaseous or aqueous reactants! Solids & liquids are not included because they have a constant concentration that does not increase or decrease

Rate Law A Rate Law is an expression that shows how the rate depends on concentration of reactants Example: Rate laws use reactants only k is a constant (called the rate constant) n is the order (exponent) of the reactant n is generally the coefficient of the chemical in the balanced equation. Reaction is xth order in A Reaction is yth order in B Reaction is (x +y)th order overall aA + bB cC + dD Rate = k [A]x[B]y

Rate Laws Practice Write the rate laws for the following reactions: 4A (s) + 2B2 (aq) + 4C (g) → 4ABC (g) Zn (s) + 2HCl (aq) → ZnCl2 (aq) + H2 (g) Ans: Rate = k[B2]2[C]4 Ans: Rate = k[HCl]2

Order of Reaction Order of Reaction For the reaction with the rate = k[A]2[B] Rate is 2nd order with respect to A Rate is 1st order with respect to B Rate is 3rd order overall (add exponents) How does concentration affect reaction rate? If [A] doubles what happens to the rate? If [B] triples, what happens to the rate?

Chemical Reactions Example Remember the classic science fair project - the homemade volcano? It’s done by combining vinegar and baking soda (and some red food coloring for effect). These substances undergo a chemical reaction, evident by the overflow and bubbles. This reaction produces three new substances: water, carbon dioxide, and sodium acetate (used in hot packs) Reaction of baking soda and vinegar: http://www.metacafe.com/watch/706897/vinegar_grenade/