CP Chapter 17 Thermochemistry Thermochemistry Thermochemistry is the study of energy that occur during chemical reactions and phase changes (changes of state) The Nature of Energy Energy is the ability to do work or produce Heat is energy transferred from a sample of temperature to temperature. Represented by the letter Q or q. Kinetic vs. Potential Energy Two main types of energy kinetic and potential o Kinetic energy of o Potential energy due to position or energy in chemical bonds Chemical potential energy - the energy stored in a substance because of its composition Example: gasoline Law of Conservation of Energy Law of conservation of energy states that in any chemical reaction or physical process, energy can be from one form to another, but it is neither created nor destroyed. Temperature vs. Heat Temperature is a measure of the average kinetic energy of the molecular in a sample. The Kelvin temperature and kinetic energy are proportional. Temperature is a of energy. Heat is the total energy of molecular motion, dependent upon amount, size, and type of particles. Heat is. System and Surroundings System The part of the universe on which contains the or process you wish to study. Surroundings Everything in the universe than the system. When heat is transferred it can flow in or out of the system Endothermic vs. Exothermic An Exothermic process is one that (evolved) heat to its surroundings (feels warm) o Energy products < Energy reactants An Endothermic process is one that heat from the surroundings (feels cold) o Energy products > Energy reactants Exothermic Process Endothermic Process 1
Measuring Heat calorie - the amount of heat required to raise the temperature of one gram of pure by one degree Celsius Calorie nutritional calorie; o 1 Calorie = 1000 calories = 1 kilocalories (kcal) Joule SI unit of heat o 1 calorie = J Specific Heat = C p Specific heat of a substance is the amount of heat required to raise the temperature of one gram of that by one degree Celsius. Unit for specific heat is J/g o C Specific Heat cont... Each substance has a specific heat Water = 4.184 J/g o C Gold = 0.129 J/g o C Copper = 0.386 J/g o C The lower the specific heat the lower the amount energy is required to raise its temperature. Calculating Heat Released and Absorbed Q = m( T)C p o Q = Heat o m is mass o T is temperature change Tfinal -Tinitial o C p is specific heat at a constant pressure The unit for heat (Q) is Joules (J) or kilojoules (kj) If reaction is heat (Q) is positive If reaction is exothermic heat (Q) is Converting Energy Units Calorie/calorie/ kilocalorie calorie/joule A cereal has 155 nutritional Calories per serving. How many calories, kilocalories and Joules is this? A person on a diet consumed 1350 Calories in one day. How many calories, kilocalories and Joules is this? 2
Calculating Heat If the temperature of 56.6 g of ethanol increases from 45.0 o C to 80.0 o C, how much heat has been absorbed by the ethanol? Specific heat of ethanol = 2.44 J/g o C A 4.00 g sample of a substance was heated from 274 K to 314 K and absorbed 32 J of heat, what is the specific heat of the substance? If 98,000 J of energy are added to 6200 g of water at 14.00 o C, what will the change in temperature of the water be? Specific heat of water = 4.184 J/g o C Calorimetry and Enthalpy Calorimetry Calorimetry is the of the heat flow into or out of a system for chemical or physical processes o Based on the law of conservation of Calorimetry Example Heat by the system is equal to the heat absorbed by the surroundings Heat absorbed by the system is equal to the heat released by the surroundings An ice cube is added to a warm cup of water. o The amount of heat used to melt the ice cube is the same amount of heat lost by the warm water. Measuring Heat Heat changes that occur during chemical and physical processes can be measured by a. A calorimeter is an device for measuring the amount of heat absorbed or released during chemical or physical processes Enthalpy and Enthalpy Changes Enthalpy (H) is the content of a system at constant Thermochemistry uses the in enthalpy( H) to study heat changes At constant pressure, Q = H 3
Endothermic and Exothermic Processes All chemical and physical changes release or absorb heat Exothermic process heat is released and goes from the system to the surroundings o Feels - E products < E reactants Endothermic process heat is absorbed and goes from the surrounding to the system o Feels - E products > E reactants Heat of Reaction Heat of reaction ( H rxn) The change of enthalpy in a reaction Exothermic reaction H rxn is negative o Products have a enthalpy than the reactants (Energy released) H products < H reactants Endothermic reaction H rxn is positive o Products have a enthalpy than the reactants (Energy absorbed) H products > H reactants Thermochemical Equation A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products (1 atm, 25 o C) and the energy, expressed as a change in enthalpy, H NH 4NO + - 3(s) NH 4 (aq) + NO 3 (aq) H = 27 kj Endothermic CaO (s) + H 2O (l) Ca(OH) 2(s) H = -65.2 kj Exothermic Thermochemical Equations for Endothermic Reactions Endothermic : H is positive; heat is on the side of the equation (Heat absorbed) NH 4NO + - 3(s) NH 4 (aq) + NO 3 (aq) H = 27 kj NH 4NO 3(s) + Heat + - NH 4 (aq) + NO 3 (aq) NH 4NO + - 3(s) + 27kJ NH 4 (aq) + NO 3 (aq) Thermochemical Equations for Exothermic Reactions Exothermic : H is negative; heat is on the side of the equation (Heat released) CaO (s) + H 2O (l) Ca(OH) 2(s) H = -65.2 kj CaO (s) + H 2O (l) Ca(OH) 2(s) + Heat CaO (s) + H 2O (l) Ca(OH) 2(s) + 65.2 kj Heat of Combustion Heat of combustion is the enthalpy change for the complete burning of one mole of the substance CH 4(g) + 2O 2(g) CO 2(g) + 2H 2O(l) H = -891kJ CH 4(g) + 2O 2(g) CO 2(g) + 2H 2O(l) + Heat CH 4(g) + 2O 2(g) CO 2(g) + 2H 2O(l) + 891kJ A piece of metal with a mass of 4.68 g absorbs 2556 J of heat when its temperature increases by 182 o C. What is the specific heat of metal? How much heat is released by the combustion of 250.0 g of octane, C 8H 18? Octane ΔH comb = -5471 kj/mol 4
Changes of State (Phase Changes) Phase Change A phase change is going from one of matter to another (Physical change) o Gas to liquid o Liquid to solid Changes of state either or (evolve/liberate) energy Phase Changes cont.. Endothermic phase changes (Absorb energy) o Melting (solid to liquid) o Evaporation/ (liquid to gas) Exothermic phase changes (Release energy) o (gas to liquid) o Freezing (liquid to solid) Temperature and Phase Change During a phase change, as energy is added or removed, there is temperature change. Latent Heat Since there is no change, we cannot use Q = m( T)C p to calculate enthalpy change. Instead we use latent (hidden) heat, which is the quantity of heat released or absorbed during a phase change. Units are usually Joule/gram or kilojoule/mole Changes of State Latent Heat of Vaporization is the heat required to one mole of a liquid o Going from liquid to gas o For water, H vap = 40.7 kj/mol Latent Heat of Condensation is the heat required to one mole of a gas o Going from gas to liquid o For water, H cond = -40.7 kj/mol Thermochemical Equations for Vaporization and Condensation The reverse process of vaporization is condensation o The process has an opposite sign o H vap = - H cond o H 2O (l) H 2O (g) H vap = 40.7 kj/mol o H 2O (g) H 2O (l) H cond = -40.7 kj/mol Changes of State Latent Heat of Fusion is the heat required to one mole of a solid substance. o Going from solid to liquid o For water, H fus = 6.01 kj/mol (Endothermic) Latent Heat of Solidification is the heat required to one mole of a liquid substance. o Going from liquid to solid o For water, H solid = -6.01 kj/mol (Exothermic) Thermochemical Equations for Fusion and Solidification The reverse process of fusion is solidification. o The reverse process has an sign o Hfus = - Hsolid o H 2O (s) H 2O (l) H fus = 6.01 kj/mol o H 2O (l) H 2O (s) H solid = -6.01 kj/mol 5
Heating Curve Explanation of Heating Curve When there is a temperature change use Q=m( T)C p to calculate the amount of energy used. When the heating curve is flat (no temperature change), there is a phase change. Use the latent heat to calculate the amount of energy used. Calculate the heat required to melt 45.6 g of water at its melting point. Latent heat of fusion ( Hfus) = 6.01 kj/mol Calculate the heat evolved to condense 75.4 g of water at its boiling point Latent heat of vaporization ( Hvap) = 40.7 kj/mol What mass of ammonia (NH 3) must be vaporizes that absorbs 345 kj of heat? Latent heat of vaporization ( Hvap) = 23.3 kj/mol 6
Hess s Law - Calculating Enthalpy Change (Heat of Reaction) Hess s Law states that if you can two or more equations to produce a equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the change for the reaction. General principles for combining thermochemical equations. 1. If a reaction is reversed, the sign of H is reversed 2. If the coefficients are multiplied, then the H is multiplied by the same factor. Calculate the enthalpy change for: 2S (s) + 3O 2(g) 2SO 3(g) H =? S (s) + O 2(g) SO 2(g) H = -297 kj 2SO 3(g) 2SO 2(g) + O 2(g) H = 198 kj 2NO (g) + O 2(g) 2NO 2(g) H =? N 2(g) + O 2(g) - 2NO (g) H = 180.6 kj N 2(g) + 2O 2(g) 2NO 2(g) H = 66.4 kj 2C (s) + O 2(g) 2CO (g) H =? C (s) + O 2(g) CO 2(g) H = 393.5 kj 2CO + O 2 2CO 2 H = -566kJ 7
C (s) + 2H 2(g) CH 4(g) H =? C (s) + O 2(g) CO 2(g) H =-393.5 kj 2H 2(g) + O 2(g) 2H 2O (l) H =-571.6 kj CH 4(g) + 2O 2(g) CO 2(g) + 2H 2O (l) H = -890.8 kj Standard Heat of Formation Standard heat of formation is defined as the in that accompanies the of one mole of the compound at its state from its elements in their standard state. Standard state is Every free element in its standard state is assigned a H f of. Free elements include elements and any molecule (H 2O 2N 2Cl 2Br 2I 2F 2) Hrxn = Use the standard enthalpies of formation to calculate Hrxn CH 4(g) +2O 2(g) CO 2(g) + 2H 2O (l) H f(co 2(g)) = -394kJ H f(h 2O (l)) = -286 kj H f(ch 4(g)) = -75kJ H f(o 2(g)) = 0 kj 8
Use Table C-13 to calculate Hrxn for the following reactions 2HF (g) H 2(g) + F 2(g) 2H 2S (g) + 3O 2(g) 2H 2O (l) + 2SO 2(g) 4Fe(s) + 3O 2(g) 2Fe 2O 3 (s) 9
2NO (g) + O 2(g) 2NO 2(g) 2CO (g) + O 2(g) 2CO 2(g) 10