Thermodynamics
63 flashcards covering Thermodynamics for the MCAT Chem / Phys / Psych / Soc section.
Thermodynamics is the study of energy and how it transforms in systems, focusing on concepts like heat, work, and energy transfer. At its core, it explains why certain processes happen spontaneously, such as chemical reactions in the body or the operation of engines. For example, it helps us understand how energy flows in living organisms, from metabolism to maintaining body temperature, making it essential for grasping fundamental principles in physics and chemistry.
On the MCAT, thermodynamics appears in the Chemical and Physical Foundations section, often through multiple-choice questions testing the laws of thermodynamics, enthalpy, entropy, and Gibbs free energy. Common traps include confusing reversible and irreversible processes or misapplying equations, so watch for context clues in word problems. Focus on real-world applications, like how these principles relate to biological systems, and practice calculating values like free energy changes to predict reaction spontaneity.
A concrete tip: Always double-check units when solving thermodynamic equations.
Terms (63)
- 01
First Law of Thermodynamics
The first law states that the change in internal energy of a system equals the heat added to the system minus the work done by the system, conserving energy.
- 02
Internal Energy
Internal energy is the total kinetic and potential energy of all molecules within a system, a state function that depends only on the system's current state.
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Heat
Heat is the transfer of thermal energy between systems due to a temperature difference, typically measured in joules or calories.
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Work in Thermodynamics
Work in thermodynamics is the energy transfer associated with a force acting through a distance, such as expansion or compression of a gas.
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Second Law of Thermodynamics
The second law states that the entropy of an isolated system always increases, implying that natural processes are irreversible and heat cannot flow from a colder to a hotter body unaided.
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Entropy
Entropy is a measure of the disorder or randomness in a system, related to the number of possible arrangements of its particles, and it tends to increase in spontaneous processes.
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Enthalpy
Enthalpy is the heat content of a system at constant pressure, defined as the sum of internal energy and the product of pressure and volume.
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Gibbs Free Energy
Gibbs free energy measures the maximum reversible work a system can perform at constant temperature and pressure, calculated as enthalpy minus the product of temperature and entropy.
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State Function
A state function is a property that depends only on the current state of the system, not on the path taken to reach that state, such as internal energy or enthalpy.
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Path Function
A path function is a quantity that depends on the specific process or path taken, such as heat and work, rather than just the initial and final states.
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Adiabatic Process
An adiabatic process is one in which no heat is exchanged with the surroundings, so changes in internal energy are due solely to work.
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Isothermal Process
An isothermal process occurs at constant temperature, meaning the internal energy of an ideal gas remains constant, with heat and work balancing each other.
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Isobaric Process
An isobaric process happens at constant pressure, where the system can expand or contract while exchanging heat to maintain that pressure.
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Isochoric Process
An isochoric process occurs at constant volume, so no work is done, and any heat added goes entirely into changing internal energy.
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Carnot Cycle
The Carnot cycle is an idealized reversible cycle for a heat engine, consisting of two isothermal and two adiabatic processes, representing maximum efficiency.
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Efficiency of a Heat Engine
Efficiency of a heat engine is the ratio of work output to heat input, always less than 1, and for a Carnot engine, it depends on the temperatures of the hot and cold reservoirs.
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Ideal Gas Law
The ideal gas law, PV = nRT, relates pressure, volume, number of moles, gas constant, and temperature for an ideal gas, assuming no molecular interactions.
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Kinetic Theory of Gases
The kinetic theory explains gas behavior through the motion of molecules, stating that pressure arises from collisions with container walls and average kinetic energy is proportional to temperature.
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Maxwell-Boltzmann Distribution
The Maxwell-Boltzmann distribution describes the speeds of molecules in a gas at a given temperature, showing a range of speeds with a most probable value.
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Calorimetry
Calorimetry is the measurement of heat transfer in a chemical or physical process, often using a calorimeter to determine enthalpy changes.
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Heat Capacity
Heat capacity is the amount of heat required to raise the temperature of a substance by one degree Celsius, depending on the substance's mass and composition.
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Specific Heat
Specific heat is the heat capacity per unit mass of a substance, indicating how much heat is needed to change its temperature without a phase change.
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Latent Heat
Latent heat is the energy absorbed or released during a phase change at constant temperature, such as melting or vaporization.
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Phase Diagram
A phase diagram is a graph showing the conditions of temperature and pressure under which a substance exists as a solid, liquid, or gas.
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Critical Point
The critical point is the temperature and pressure above which a gas cannot be liquefied, regardless of pressure applied.
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Triple Point
The triple point is the unique temperature and pressure at which a substance's solid, liquid, and gas phases coexist in equilibrium.
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Vapor Pressure
Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature.
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Clausius-Clapeyron Equation
The Clausius-Clapeyron equation relates the vapor pressure of a substance to temperature, used to calculate changes in boiling point with pressure.
- 29
Hess's Law
Hess's law states that the total enthalpy change for a reaction is the sum of enthalpy changes for individual steps, regardless of the pathway.
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Standard Enthalpy of Formation
Standard enthalpy of formation is the enthalpy change when one mole of a compound forms from its elements in their standard states.
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Bond Enthalpy
Bond enthalpy is the energy required to break one mole of a particular bond in a gaseous molecule, used to estimate reaction enthalpies.
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Spontaneity of Reactions
Spontaneity refers to whether a reaction can occur without external input, determined by the sign of the Gibbs free energy change.
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Free Energy Change
Free energy change is the difference in Gibbs free energy between products and reactants, negative for spontaneous reactions at constant T and P.
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Equilibrium Constant and Gibbs Free Energy
The equilibrium constant relates to Gibbs free energy by ΔG° = -RT ln K, where a positive K indicates a spontaneous forward reaction.
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Temperature Dependence of Equilibrium
The van't Hoff equation describes how the equilibrium constant changes with temperature, based on the enthalpy change of the reaction.
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Entropy Change in Reactions
Entropy change in a reaction is the difference in entropy between products and reactants, often positive for reactions increasing disorder.
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Reversible Process
A reversible process is one that can be reversed by an infinitesimal change in conditions, with no net change in the surroundings.
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Irreversible Process
An irreversible process occurs with a finite change in conditions and increases the entropy of the universe, unlike reversible ones.
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Thermodynamic Equilibrium
Thermodynamic equilibrium is a state where a system has no net changes in properties over time, with balanced macroscopic forces.
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Exothermic Reaction
An exothermic reaction releases heat to the surroundings, resulting in a negative enthalpy change for the system.
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Endothermic Reaction
An endothermic reaction absorbs heat from the surroundings, resulting in a positive enthalpy change for the system.
- 42
PV Diagram
A PV diagram plots pressure versus volume for a thermodynamic process, used to visualize work done by or on a gas.
- 43
Work Done in Gas Expansion
Work done in gas expansion is calculated as the integral of P dV, representing the area under the curve on a PV diagram.
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Heat Transfer by Conduction
Heat transfer by conduction occurs through direct molecular contact in solids, governed by the material's thermal conductivity.
- 45
Heat Transfer by Convection
Heat transfer by convection involves the movement of fluids, carrying heat via currents in liquids or gases.
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Heat Transfer by Radiation
Heat transfer by radiation occurs through electromagnetic waves, independent of a medium, as in blackbody radiation.
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Thermal Expansion
Thermal expansion is the increase in size of a material due to temperature rise, quantified by the coefficient of thermal expansion.
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Coefficient of Thermal Expansion
The coefficient of thermal expansion measures how much a material's length changes per unit length per degree of temperature increase.
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Joule-Thomson Effect
The Joule-Thomson effect is the temperature change of a gas during adiabatic expansion through a throttle, used in refrigeration.
- 50
Adiabatic Lapse Rate
The adiabatic lapse rate is the rate at which air temperature decreases with altitude due to expansion, about 10°C per kilometer in dry air.
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Common Trap: Enthalpy vs. Internal Energy
A common error is confusing enthalpy (which includes PV work) with internal energy (just the system's energy), especially at constant pressure versus constant volume.
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Standard Conditions in Thermodynamics
Standard conditions refer to 298 K and 1 atm, used as a reference for thermodynamic measurements like enthalpy and free energy.
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Third Law of Thermodynamics
The third law states that the entropy of a perfect crystal approaches zero as temperature approaches absolute zero, setting a baseline for entropy.
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Helmholtz Free Energy
Helmholtz free energy is the energy available for work at constant temperature and volume, calculated as internal energy minus TS.
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Strategy for Solving Thermodynamic Problems
When solving problems, first identify the system and process type, then apply conservation laws and state functions to track energy changes.
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Example: Work in Isothermal Expansion
For an ideal gas expanding isothermally, work is calculated as nRT ln(V2/V1), representing the energy transferred to the surroundings.
For 1 mole of gas at 300 K expanding from 1 L to 2 L, work equals 8.314 J/mol·K × 300 K × ln(2) ≈ 1,727 J.
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Entropy of Mixing
Entropy of mixing is the increase in entropy when substances mix, due to the greater number of possible arrangements.
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Chemical Potential
Chemical potential is the partial molar free energy, driving the direction of chemical reactions and phase changes.
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Fugacity
Fugacity is a corrected measure of the effective pressure of a real gas, used in place of partial pressure for non-ideal behavior.
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Nernst Equation in Thermodynamics
The Nernst equation relates Gibbs free energy to electrochemical cell potential, showing how it varies with concentrations.
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Le Chatelier's Principle
Le Chatelier's principle predicts how a system at equilibrium responds to changes in temperature, pressure, or concentration to restore balance.
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Example: Enthalpy Calculation via Hess's Law
Hess's law allows adding reaction enthalpies to find the overall enthalpy, such as for a net reaction from multiple steps.
To find CO2 formation enthalpy, add C + O2 → CO and CO + 1/2 O2 → CO2 enthalpies.
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Thermodynamic Standard State
The thermodynamic standard state is the most stable form of a substance at 1 bar and specified temperature, used for tabulating data.