CHEM 111 Fall 2024 Practice Problems (PDF)

CHEM 111 Fall 2024 Dr. Darrell Collison PRACTICE: Calculate the amount of energy (kJ) needed to convert 237 g of ice initially at 0.0°C to water at 80.0oC. Given DHfusion, ice = 6.01 kJ/mol, cH2O = 4.184 J/goC. 1  HESS‘S LAW...

CHEM 111 Fall 2024 Dr. Darrell Collison PRACTICE: Calculate the amount of energy (kJ) needed to convert 237 g of ice initially at 0.0°C to water at 80.0oC. Given DHfusion, ice = 6.01 kJ/mol, cH2O = 4.184 J/goC. 1  HESS‘S LAW If a reaction can be written as the sum of two or more reactions, then DHo for the overall (net) reaction is the sum of the DHo values of the contributing reactions. Consider the oxidation of carbon (graphite) to carbon dioxide: C (s) + O2 (g) à CO2 (g) DHo = ?? § It may be more convenient to measure the reaction enthalpies for steps that indirectly sum to the reaction of interest: C (s) + ½ O2 (g) à CO (g) DHo = -110.5 kJ/mol CO (g) + ½ O2 (g) à CO2 (g) DHo = -283.0 kJ/mol § These can be summed to find the overall reaction enthalpy for the reaction of interest. 2  HESS’S LAW C (s) + ½ O2 (g) à CO (g) DHo = -110.5 kJ/mol CO (g) + ½ O2 (g) à CO2 (g) DHo = -283.0 kJ/mol C (s) + O2 (g) à CO2 (g) DHo = -348.5 kJ/mol direct route indirect route 3  PRACTICE: HESS’S LAW Calculate the reaction enthalpy for the formation of natural gas (methane, CH4) from coal and hydrogen gas. Is this an exothermic or endothermic process? C (s) + 2H2 (g) à CH4 (g) DHo = ?? Given the following known reactions: 1. C (s) + O2 (g) à CO2 (g) DHo = -393.5 kJ/mol 2. H2 (g) + ½ O2 (g) à H2O (l) DHo = -285.8 kJ/mol 3. CH4 (g) + 2 O2 (g) à CO2 (g) + 2H2O (l) DHo = -890.3 kJ/mol 4 PRACTICE: Determine DrH° for the following reaction, 2 NH3(g) + 5/2 O2(g) à 2 NO(g) + 3 H2O(g) given the thermochemical equations below. N2(g) + O2(g) à 2 NO(g) DrH° = +180.8 kJ/mol-rxn N2(g) + 3 H2(g) à 2 NH3(g) DrH° = –91.8 kJ/mol-rxn 2 H2(g) + O2(g) à 2 H2O(g) DrH° = –483.6 kJ/mol-rxn 5  POLL QUESTION: Calculate the enthalpy of formation of CCl4 (g) using the thermochemical equations below: C (s) + 2Cl2 (g) à CCl4 (g) CCl4(g) + 4 HCl(g) → CH4(g) + 4 Cl2(g) ∆H° = +397.0 kJ ½ H2(g) + ½ Cl2(g) → HCl(g) ∆H° = −92.31 kJ C(s) + 2 H2(g) → CH4(g) ∆H° = −74.81 kJ A. −379.5 kJ/mol B. −102.6 kJ/mol C. +167.1 kJ/mol D. +227.9 kJ/mol 6 7 CHEM 111 Fall 2023 Dr. Darrell Collison § Enthalpy of formation, DHf § The enthalpy change when 1 mol of compound is formed from its constituent elements. § Example: ½ O2(g) + H2(g) à H2O(g) DHf of H2O(g)= -241.83 kJ/mol § Standard state (o) = Most stable form of a substance at STP § STP for thermodynamics = 1 atm and 25 oC (298.15 K). § Standard enthalpy of formation, DHof § The enthalpy change when 1 mol of compound is formed from its constituent elements, all in their standard states. ½ O2(g) + H2(g) à H2O(l) DHof of H2O(l) = -285.83 kJ/mol 1 § Standard enthalpy of formation of the most stable form of an element is zero (DHof = 0) Substance DHof (kJ/mol) C(s, graphite) 0 O(g) 247.5 O2(g) 0 N2(g) 0 H2(g) 0 2 § Compounds have non-zero DHof values, many of which are tabulated: 3 PRACTICE: Write the standard enthalpy of formation equation for sodium bicarbonate. 4 § Standard enthalpies of formation can be used to calculate standard enthalpies of reaction: ΔH°rxn = ∑ n⋅ ΔH °f (products) − ∑ m⋅ ΔH °f (reactants) Note: n & m are stoichiometric coefficients. € § Represents formation of products and breaking apart of reactants 5 PRACTICE: Calculate DH∘rxn (in kJ/mol) for the combustion of ethanol. Given: DH∘f CO2 = -393.5, DH∘f ethanol = -277.7, DH∘f H2O = -241.8 (all kJ/mol) 6 1. Breaking chemical bonds requires energy. 2. Forming bonds releases energy. 3. Enthalpy is a state function. Bond enthalpies can be used to estimate overall change in enthalpy. CH 4 (g) + Cl2 (g) " "→ CH 3Cl(g) + HCl(g) ~ ΔH rxn from bond energies € 7 § Bond enthalpy depends on: § What elements are in a bond (C-H vs. C-C) § What type of bond the atoms share (C-C vs. C=C) § The polarity of the rest of molecule (O-H bond in carboxylic acid vs. O-H bond in water)– can change bond strength value § Bond enthalpy values given are averages over a wide variety of molecules for each bond type between elements § We will do this further in discussing Lewis Structures 8 CHEM 111 Fall 2024 Dr. Darrell Collison § Electrons are the ultimate determinant of the properties & reactivity of atoms. § Understanding how electrons behave and what influences them is critical to understanding chemistry. 10 Late 1800s-early 1900s saw an increased understanding of atomic structure. JJ Thomson “Plum Pudding Bohr Model Model” 1913 1904 11  Radiant energy, known as electromagnetic radiation, moves through space as perpendicular oscillating electric and magnetic waves 12 § Wavelength (l): the distance from crest to crest or trough to trough. (typical unit m to nm) § Frequency (n): the number of crests of a wave passing a stationary point of reference per unit of time (typical unit Hz = s-1) § Amplitude (A): the height of a crest or depth of a trough 13 Wavelength Region Usual l range