Chapter 10: Thermochemistry

Thermochemistry What is Energy? • List as many types of energy you can think of: Energy • Energy is the ability to do work o It is measured in Joules (J) o Potential energy: stored energy o Kinetic energy: energy of motion Heat & Temperature Heat & temperature are different! • Temperature is the measure of the average kinetic energy of random motions of particles in a substance Measured in degrees • Heat is the measure of the total amount of energy Measured in Joules or calories Aside: 1 kilocalorie or 1000 calories = 1 Calorie (used for food) • Calorimetry: is the measurement of heat changes for physical & chemical processes • A calorimeter is an insulated container used to measure the amount of heat absorbed or released Exothermic and Endothermic •Exothermic process: •Heat exits out of the system •the system loses heat as the surroundings heat up • a negative heat change •Endothermic Process: • Heat enters or goes into the system • a process that absorbs heat as the surroundings cool down • a positive heat change Specific Heat Capacity (Cp) • It is the amount of heat (energy) needed to increase the temp of 1 g of a substance by 1 °C Q = (m)(Cp)(ΔΤ) Q = heat in Joules (J) m = mass in grams (g) Cp = specific heat (J/g•°C) ● ΔΤ = change in temp (°C) Temperature Change ●ΔΤ = Τf – Τi ΔΤ is positive if temp is increasing ΔΤ is negative if the temp is decreasing Specific Heat Problems 1. How much heat is transferred from 1000 g of water if the change in temp is 37 °C ? (Cp of water = 4.18 J/g•°C) 154,660 J 2. How much heat is released as 200 g of hot water cools from 65 °C to 35 °C ? -25,080 J How much heat is lost as 500 g of Al is cooled from 200 °C to 25 °C? (cp of Al = 0.897 J/g•°C) -78,488 J 3. Specific Heat Problems 4. 5. 6. What is the mass of ethanol if 6.2 x 104 J of heat is gained as the ethanol is heated from 25 °C to 37 °C? (cp of ethanol = 2.438 J/g•°C) 2119 g A 65 g of metal at 95 °C is placed in a calorimeter. The final temp is 28 °C. If -2.8 x 104 J of heat is released, what is the specific heat of the metal? 6.43 J/g˚C What is the final temp of 880 g of water if 3.7 x 105 J of heat are absorbed & the initial temp of water is 25 °C? ΔT = 100.6˚C Tfinal = 125.6˚C Heating Curve Heating Curve temperature boiling melting gas solid liquid time Heating Curve of Water Temperature 🡪 endothermic ΔHfus solid ΔHvap liquid exothermic Heat supplied 🡪 gas Heat of fusion (enthalpy) • • • • Is the energy needed to melt one mole ΔHfus occurs at the melting/freezing point Temperature stays constant Solid to liquid: ΔHfus is positive (heat in) • liquid to solid: ΔHfus is negative (heat out) * The heat value is the same (just the sign changes) *FUSION- think “ice” Heat of Vaporization (enthalpy) • Is the energy needed to boil one mole • ΔHvap occurs at the boiling/condensing point • Temperature stays constant • Liquid to gas: ΔHvap is positive (heat in) • Gas to liquid: ΔHvap is negative (heat out) * The heat value is the same (just the sign changes) *VAPORIZATION- think “vapor” For Phase Changes • Heat of fusion (ΔHfus): o Energy needed to melt 1 mole Q = (ΔHfus)(# of moles) • Heat of vaporization (ΔHvap): o Energy needed to boil 1 mole Q = (ΔHvap)(# of moles) To do problems: While a substance is changing state (melting, boiling, etc), Q = (ΔHfus)(# of moles) OR Q = (ΔHvap)(# of moles) MUST be used While a substance is in the solid, liquid or gas phase, Q = m C ΔT MUST be used *remember MELTING: ΔHfus is positive FREEZING: ΔHfus is negative BOILING ΔHvap is positive CONDENSING ΔHvap is negative *for many calculations both equations must be used (sometimes more than once) Heat Calculations 1. How much energy is needed to boil 850 g of mercury? (ΔHvap of Hg is 59.1 kJ/mol) 2. What amount of energy is needed to melt 48.5 g of water? (ΔHfus of H2O is 6.00 kJ/mol) 3. What is the mass of ethanol (C2H6O) if 208 kJ of heat are added when it boils? (ΔHvap of C2H6O is 38.6 kJ/mol) Heat Calculations 1. 2. 3. How much energy is needed to boil 850 g of mercury? (ΔHvap of Hg is 59.1 kJ/mol) 4.24 mol Hg 250.6 kJ Hg What amount of energy is needed to melt 48.5 g of water? (ΔHfus of H2O is 6.00 kJ/mol) 2.69 mol 16.2 kJ What is the mass of ethanol (C2H6O) if 208 kJ of heat are added when it boils? (ΔHvap of C2H6O is 38.6 kJ/mol) 5.39 mol 248 g Transfers of Heat • Heat of reaction (ΔHrxn) (also known as enthalpy): heat energy absorbed or released during a reaction • Heat of formation (ΔHf°): is the heat energy absorbed or released, in the formation of 1 mole of a compound from its elements • Heat of solution (ΔHsol): heat energy absorbed or released when a substance dissolves Energy Diagrams • Activation energy (Ea): energy needed to start a reaction • Activated complex: the unstable arrangement of atoms that exists momentarily at the peak of the activation energy It is the intermediate between the reactants and the products Potential Energy Diagrams Energy Diagram Energy Diagrams Draw an energy diagram for an exothermic reaction Exothermic Reaction Endothermic Reaction For EXOthermic reactions: o o o o Reactants are less stable than products (higher energy state) Products are more stable than reactants (lower energy state) ΔHrxn = products - reactants ΔHrxn is negative •Draw a reaction pathway for an endothermic reaction. •For ENDOthermic reactions: o Reactants are more stable (lower energy state) o Products are less stable (higher energy state) o ΔHrxn = products - reactants o ΔHrxn is positive A+B Exothermic Reaction C+D Endothermic Reaction The activation energy (Ea) is the minimum amount of energy required to initiate a chemical reaction. 13.4 Energy Diagrams • Catalysts speed up reactions by lowering the activation energy. How would the diagram change if a catalyst were added? • Show how a catalyst would affect an endothermic diagram __________________________________________ Enthalpy • Enthalpy (H) is the total energy content of a system • Enthalpy change (ΔH) is the heat energy absorbed or released in a rxn. Review-Heat of formation • Heat of formation (ΔHf), is the heat energy absorbed or released in the formation of 1 mole of a compound from its elements • Elements DO NOT have heats of formation (they use a value of ZERO) Enthalpy Changes • Hess’s law: the total enthalpy change in a reaction ΔHrxn = sum of ΔHf, products - sum of ΔHf, reactants ΔHrxn = ∑ΔHf products - ∑ΔHf reactants Enthalpy Changes • Determine ΔHrxn for the decomposition of H2O2 • 1.Make sure equation is balanced, you need to account for how many moles of each reactant and product H2O2(l) → H2O(l) + O2(g) 2H2O2(l) → 2H2O(l) + O2(g) 2.Look up ΔHf for reactants & products on chart (Remember elements are ZERO) 3. Solve for ΔHrxn ΔHrxn = ∑ΔHf products - ∑ΔHf reactants Enthalpy Changes ΔHf: 2H2O2(l) → 2H2O(l) + O2(g) -187.6 -285.8 0 [2(-187.6)] [2(-285.8) + 0] [-375.2] [-571.6] ΔHrxn = [-571.6] – [-375.2] ΔHrxn = -196.41 kJ/mol Exothermic Note: ΔHf of O2 is 0 kJ/mol because it is an element Enthalpy Problems What is ΔHrxn for the following reactions? • Ca(OH)2(s) + CO2(g) → H2O(g) + CaCO3(s) -986.6 -393.5 -241.8 -1207.0 -1380.1 -1448.8 -1448.8 - -1380.1 = -68.7 kJ/mol EXO • • 2KClO3(s) → 2KCl(s) + 3O2(g) -391.4 -435.9 0 -782.8 -871.8 -871.8 - -782.8 = -89 kJ/mol EXO • Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g) -824.2 -110.5 0 -393.5 -331.5 -1155.7 -1180.5 -1180.5 - -1155.7 = -24.8 kJ/mol EXO Entropy • Entropy (S) is the measure of disorder of a system o High entropy: more disorder o Low entropy: more order • Ex. Clean room vs. messy room • Usually it’s easier to go from low entropy to high entropy Entropy ΔS Low Entropy High Entropy Entropy and Temperature • Does a substance at a higher temperature have higher entropy or lower entropy?? Substance at a higher temperature has more entropy. • Does a substance in its solid state have more or less entropy than in its gaseous state? Solid state of matter: Less entropy Review of terms: if applicable Term Heat Temperature Calorimetry exothermic endothermic Specific heat Heat of fusion Heat of vaporization Heat of reaction Heat of formation Heat of solution Activation energy Catalyst Enthalpy Hess’ Law entropy Definition Symbol unit Entropy and Temperature • Higher temperature = more entropy (more disorder) Solid vs Gas Exothermic reaction: enthalpy y activated complex Ea E reactants PE ΔHrxn products reaction progress