Chapter 5 - Energy and Change

Chapter 5.1: The energy of Physical, Chemical, and Nuclear Processes

Thermodynamics: a study of energy and energy transfer
Thermochemistry: the study of energy involved in chemical reactions
Studying Energy Changes: * Law of conservation of energy: the total energy of the universe is constant, can’t be destroyed or created ∆Universe = 0 * System: part of universe being studied * Surroundings: everything else in that universe * ∆Ssystem = −∆Ssurroundings
Heat and Temperature * Heat, Q: transfer of kinetic energy in joules (J) * Temperature, T: a measure of the average kinetic energy of the particles that make up a substance or system in Celsius degrees ( ̊C) or kelvins (K) * The temperature in Kelvin degrees = Temperature in Celsius degrees + 273.15
Enthalpy and Enthalpy Change: * Enthalpy, H: total internal energy of a substance at constant pressure * Enthalpy change, ∆H: relative enthalpy of the reactants and products in the system
Enthalpy Changes in Chemical Reaction: * Breaking a bond is a process that requires energy. Creating a bond is a process that releases energy. * Endothermic reaction: net absorption of energy (+) * Exothermic reaction: net release of energy (-)
Representing Enthalpy Changes: * Enthalpy of reaction, ∆Hrxn reaction: enthalpy change of a chemical reaction * Standard enthalpy of reaction, ∆H ̊rxn: enthalpy change of a chemical reaction that occurs at SATP * Standard Ambient Temperature and Pressure: 25 ̊C and 100 kPa * Enthalpy of a reaction is also called heat of reaction * Visualizing Exothermic and Endothermic reactions: * Thermochemical equation: a balanced chemical equation that indicates the amount of heat that is absorbed or released by the reaction it represents (in kJ) * You can also show enthalpy of reaction as a separate expression with ∆H ̊ * Also can be represented with an enthalpy diagram which represents reactants and products and the enthalpy of the system * Enthalpy decreases as energy are released in an exothermic reaction * Enthalpy increases as energy are absorbed in an endothermic reaction
Stoichiometry and Thermochemical Equations: * Enthalpy of reaction is linearly dependent on the number of products * If the amount of products doubles, enthalpy changes
Heat Changes and Physical Changes: * Enthalpy of vaporization, ∆Hvap: the enthalpy change for the phase change from liquid to gas * Enthalpy of condensation, ∆Hcond: the enthalpy change for the phase change of a substance from gas to liquid * Enthalpy of melting, ∆Hmelt: the enthalpy change for the phase change of a substance from solid to liquid * Enthalpy of freezing, ∆Hfre: the enthalpy change for the phase change of a substance from liquid to solid * ∆Hvap = −∆Hcond * ∆Hmelt = −∆Hfre * Enthalpy of a solution: the enthalpy change when a solute dissolves in a solvent
Energy and Nuclear Reactions: * In nuclear reactions, a significant amount of the mass of the reactants is actually converted into energy * C2 = 9.0 × 1016 m2/s2 and E = mc2E is energy in kg • m2/s2 (J)is the mass in kgc2 is the square of the speed of light * Mass defect: difference in mass between a nucleus and its nucleons * Nuclear binding energy: energy associated with the strong force that holds a nucleus together * Using the E = mc2 can be used to find this * Higher binding energy means more stable nucleus, most stable is at mass number 60 * Nuclear fission: A heavy nucleus undergoing split into lighter nuclei which releases energy * Nuclear fusion: two smaller nuclei fusing to form a larger nucleus

Chapter 5.2: Determining Enthalpy of Reaction by Experiment

Specific Heat Capacity (C): amount of energy needed to raise temperature of one gram of substance 1 celsius or 1 kelvin * In units of J/g •˚C
Heat capacity (C): heat of sample, object, or system to its change in temperature * In units of kJ/˚C
Q = m • c • ∆T * Q = heat (J) * m = mass (g) * c = specific heat capacity (J/g •˚C) * ∆T = Tf (final temperature) − Ti (initial temperature)(˚C or K)
Calorimeter: measure enthalpy changes for chemical and physical reactions
Qreaction = − Qinsulated system
Enthalpy changes represent the heat change between products and reactants at a constant temperatureShould be open to atmosphere
Coffee-cup calorimeter: calorimeter is composed of two nested polystyrene cups * Placed in 250 mL for stability * Constant-pressure calorimeter: open to atmosphere

Chapter 5.3: Hess’s Law of Heat Summation

Hess’s law of heat summation: states that the enthalpy change of a physical or chemical process depends only on the beginning conditions (reactants) and the end conditions (products). * Enthalpy change is independent of the pathway of the process and the number of intermediate steps in the process * Allows algebraically combining chemical reactions and be represented by a enthalpy diagram * To manipulate an equation, you can: * Reverse equation so products become reactants * Multiply coefficients by integer or fraction
Formation reaction: substance is formed from elements in their standard states
Standard molar enthalpy of formation, ∆H ̊ f : enthalpy change of a formation reaction in their standard states * standard molar enthalpy of formation is the amount of energy absorbed or released when one mole of a compound is formed directly from its elements in their standard states
The enthalpy of formation of an element in its standard state is zero
The reactants do not actually break down into their elements and then react to form products

Chapter 5.4: Energy sources

Energy efficiency: ratio of useful energy produced to energy used in its production, expressed as a percent * [Useful energy produced] / [ Energy used] x 100 % * Useful energy: work done * Energy used: ideal energy output * Specify how fuel is used up * Ex. natural gas is around 37% efficiency
Environmental focus on: * Non-renewable energy: coal, oil, or natural gas can never be reused

Renewable: solar energy can give a constant source of energy