2.3 Temperature, Pressure, and Composition Scales
2 min read•july 25, 2024
Temperature scales are essential in chemical engineering, allowing us to measure and compare heat levels. From Celsius to Kelvin, each scale serves a unique purpose, with conversion formulas bridging the gaps between them.
Pressure and composition scales are equally crucial, helping engineers quantify forces and mixture components. Understanding these scales and how to convert between them is fundamental for solving complex chemical process problems.
Temperature Scales
Temperature scales and conversions
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- Celsius (°C) measures water's freezing (0°C) and boiling (100°C) points used globally for everyday measurements
- Kelvin (K) absolute temperature scale starts at 0 K (absolute zero) employed in scientific calculations
- Fahrenheit (°F) prevalent in the United States defines water's freezing at 32°F and boiling at 212°F
- Rankine (°R) absolute temperature scale relates to Fahrenheit with 0°R at absolute zero
- Conversion formulas enable switching between scales:
Pressure and Composition Scales
Pressure scales and conversions
- measured from perfect vacuum (zero pressure) always positive used in thermodynamics
- measured relative to can be positive or negative common in industry
- represents negative gauge pressure below atmospheric pressure
- Atmospheric pressure standard value: 1 atm = 101.325 kPa = 14.7 psi
- Conversion formulas:
- Absolute pressure = Gauge pressure + Atmospheric pressure
- Vacuum pressure = Atmospheric pressure - Absolute pressure
Composition representations in chemistry
- Composition quantifies relative amounts of components in mixtures
- ratio of component mass to total mixture mass (sum = 1)
- Mole fraction ratio of component moles to total mixture moles (sum = 1)
- ratio of component volume to total mixture volume (ideal gases)
- measures mass of solute per unit volume of solution (g/L, mol/L)
Composition calculations and conversions
- Mass fraction to mole fraction:
- Divide mass fraction by molecular weight
- Normalize results to ensure sum equals 1
- Mole fraction to mass fraction:
- Multiply mole fraction by molecular weight
- Normalize results to ensure sum equals 1
- Volume fraction equals mole fraction for ideal gas mixtures (Amagat's law)
- Concentration calculations include molarity (moles solute/liter solution) and molality (moles solute/kg solvent)
- Density and specific gravity relate mass and volume fractions
Applications of temperature, pressure, and composition
- Material balances require consistent units for temperature and pressure with appropriate composition conversions
- Vapor-liquid equilibrium calculations use absolute pressure and temperature scales considering partial pressures and mole fractions
- Gas law applications (Ideal gas law: ) employ absolute temperature and pressure scales
- Heat transfer problems apply temperature conversion formulas and consider temperature-dependent properties
- Reaction kinetics utilize appropriate composition scales (concentrations) and account for temperature effects on reaction rates
Key Terms to Review (20)
Absolute Pressure: Absolute pressure is the total pressure exerted on a system, measured relative to a perfect vacuum, where the pressure is zero. This measurement provides a true indication of the pressure without any influence from atmospheric pressure, making it crucial in various scientific calculations and applications, especially in thermodynamics and fluid mechanics.
Atmospheric Pressure: Atmospheric pressure is the force exerted by the weight of air above a given point, measured in units such as Pascals (Pa) or atmospheres (atm). It plays a crucial role in various processes involving temperature, pressure, and composition scales, as it affects gas behavior, boiling points, and phase changes in substances.
Barometer: A barometer is an instrument used to measure atmospheric pressure, which is crucial for understanding weather patterns and the behavior of gases. By providing a reading of pressure changes in the atmosphere, barometers play a key role in predicting weather changes and understanding how pressure affects chemical processes. Barometers can be used to calibrate other instruments and establish conditions needed for various experiments.
Boiling Point: The boiling point is the temperature at which a liquid's vapor pressure equals the external pressure surrounding it, resulting in the transition of the liquid into vapor. This key physical property is influenced by factors such as temperature, pressure, and the composition of the liquid, making it crucial in various processes involving phase changes.
Celsius Scale: The Celsius scale is a temperature measurement system where the freezing point of water is defined as 0 degrees Celsius and the boiling point as 100 degrees Celsius at standard atmospheric pressure. This scale is widely used in scientific contexts and everyday applications for measuring temperature, making it essential for understanding thermal properties in chemical processes.
Clausius-Clapeyron Equation: The Clausius-Clapeyron equation is a fundamental relationship in thermodynamics that describes the phase transition between two states of matter, particularly the change in vapor pressure with temperature. This equation helps to establish how pressure and temperature affect the equilibrium between phases, making it essential for understanding phase changes and the associated latent heat.
Concentration: Concentration refers to the amount of a substance present in a given volume or mass of a mixture or solution. It’s an essential concept in understanding how different variables interact in a system, affecting both the physical and chemical properties of materials involved. Concentration plays a crucial role in processes such as reaction rates, solubility, and the efficiency of separation processes, impacting calculations and designs in chemical engineering.
Dalton's Law of Partial Pressures: Dalton's Law of Partial Pressures states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of each individual gas. This concept is crucial for understanding how gases behave under varying temperature and pressure conditions, as it helps quantify the composition of gas mixtures in different environments.
Degree Celsius: Degree Celsius (°C) is a unit of temperature measurement used to express the thermal state of an object or environment. It is based on the metric system and is defined by the freezing point of water at 0 °C and the boiling point at 100 °C under standard atmospheric conditions. This scale is widely used in scientific contexts and everyday life, making it essential for understanding temperature-related properties in various processes.
Fahrenheit Scale: The Fahrenheit scale is a temperature measurement system where the freezing point of water is defined as 32 degrees and the boiling point as 212 degrees at standard atmospheric pressure. This scale was developed by Daniel Gabriel Fahrenheit in the early 18th century and is primarily used in the United States and some Caribbean nations, highlighting its significance in everyday temperature measurements and various scientific applications.
Gauge pressure: Gauge pressure is the pressure of a fluid measured relative to the atmospheric pressure. It indicates how much the pressure in a system exceeds the ambient atmospheric pressure and is critical for understanding various processes in chemical engineering, where accurate pressure measurements are essential for efficiency and safety.
Kelvin scale: The kelvin scale is a temperature measurement system that uses absolute zero as its null point, where 0 K represents the complete absence of thermal energy. It is widely used in scientific contexts, particularly in physics and chemistry, because it provides a direct correlation to thermodynamic temperature and allows for straightforward calculations involving gas laws and other thermodynamic principles.
Mass Fraction: Mass fraction is a dimensionless quantity that represents the ratio of the mass of a component in a mixture to the total mass of the mixture. This concept is vital for understanding composition scales, as it helps quantify how much of a specific substance is present relative to others within a mixture, influencing calculations related to temperature and pressure in chemical processes.
Melting Point: The melting point is the temperature at which a solid becomes a liquid, indicating a phase change that occurs at a specific temperature for a given substance. This temperature is crucial in understanding material behavior under various conditions, as it impacts how substances interact in chemical processes. Knowing the melting point helps in determining suitable applications for materials and understanding their properties in relation to temperature, pressure, and composition scales.
Pascal: A pascal is a unit of pressure defined as one newton per square meter, symbolized as Pa. It is part of the International System of Units (SI) and is commonly used in various scientific and engineering applications to measure pressure, stress, and internal pressure in fluids. Understanding pascals is crucial for comprehending the behavior of gases and liquids in chemical processes, as pressure significantly influences phase behavior and reaction kinetics.
Rankine Scale: The Rankine scale is a temperature scale that begins at absolute zero and uses the same increments as the Fahrenheit scale. This means that temperatures on the Rankine scale are measured in degrees Rankine (°R) and are used primarily in engineering applications, particularly in thermodynamics and heat transfer where absolute temperature measurements are critical.
Raoult's Law: Raoult's Law states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent present. This principle is crucial in understanding how temperature, pressure, and composition scales influence the behavior of mixtures, especially when dealing with ideal solutions. By using Raoult's Law, one can predict how the addition of a solute affects the vapor pressure of a solvent, making it essential for calculations involving phase equilibria.
Thermometer: A thermometer is a scientific instrument used to measure temperature, providing crucial data in various processes and applications. It operates based on physical properties of materials, such as expansion or contraction of liquids or the change in electrical resistance, allowing for precise temperature readings essential for chemical processes. Understanding how thermometers function and their calibration is vital for ensuring accurate temperature measurements in experimental and industrial settings.
Vacuum Pressure: Vacuum pressure is the pressure in a system that is below atmospheric pressure, often measured in units such as inches of mercury (inHg) or pascals (Pa). It reflects the difference between atmospheric pressure and the absolute pressure within a container, highlighting how much lower the pressure is compared to the surrounding environment. Understanding vacuum pressure is crucial in various applications, such as in distillation processes and in systems where reduced pressure can affect boiling points and reaction rates.
Volume Fraction: Volume fraction is a measure of the concentration of a component in a mixture, defined as the ratio of the volume of that component to the total volume of the mixture. This concept is particularly important in understanding how different substances interact within a system, influencing properties like density and phase behavior.