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