CHEM 110: Forces between Atoms, Molecules, and Ions - Properties of Liquids and Vaporization

Course Overview and Logistics

• CHEM 110 General Chemistry, University of Nebraska Lincoln, Fall 2026.

• Class Date: Friday, Aug 28th, 2026.

• Schedule Note: Labs and Recitation will meet this week.

Properties of Liquids

• Learning Objectives for Section 12.7:

  • Distinguish cohesive and adhesive forces.

  • Explain the molecular basis of surface tension.

  • Explain the molecular basis of viscosity.

  • Explain the phenomenon of capillary action in terms of the relative strength of cohesive and adhesive forces.

  • Describe the formation of concave and convex menisci.

• Surface Tension:

  • Definition: Surface tension is a measure of the resistance of a liquid to an increase in its surface area.

  • Molecular Basis: Molecules are most stable when attractive forces are maximized by having as many neighboring molecules surrounding them as possible.

  • Energy States: Surface molecules have fewer neighbors than interior molecules, giving surface molecules greater potential energy and making them less stable.

  • Interior Forces: A molecule within the interior of a liquid experiences no net pull in any direction because forces are balanced on all sides.

  • General Trend: Surface tension decreases with decreasing strength of intermolecular forces.

• Viscosity:

  • Definition: Viscosity is a liquid's resistance to flow.

  • Molecular Significance: Viscosity gives an indication of how fast molecules move past each other.

  • Temperature Effects: Viscosity decreases as temperature increases.

  • Intermolecular Force Effects: Viscosity increases as cohesive forces (intermolecular forces) increase.

  • Units of Measurement:

  • Viscosity is measured in poise ($\text{P}$) or centipoise ($\text{cP}$).

  • $1\,\text{poise} = 1\,\text{g/cm}\cdot\text{s}$.

  • Viscosity Data of Ethylene Glycol at Varying Temperatures:

  • At $25^\circ\text{C}$: $16.1\,\text{cP}$.

  • At $100^\circ\text{C}$: $1.98\,\text{cP}$.

  • Viscosity Data of Various Substances at $20^\circ\text{C}$:

  • Water: $1.00\,\text{cP}$.

  • Honey: $10\,\text{cP}$.

  • Ketchup: $50\,\text{cP}$.

  • Tar: $30,000\,\text{cP}$.

  • Glass: $10^{18} - 10^{21}\,\text{cP}$.

• Cohesive vs. Adhesive Forces:

  • Intermolecular forces affect physical properties including surface tension, viscosity, vaporization, and vapor pressure.

  • Characterization of Forces: Categorized into strong adhesive forces versus weak adhesive forces.

  • Cohesive Forces: Occur within the molecules of a single phase of matter and result in the tendency of a substance to resist separation (causing like particles to stick together).

  • Adhesive Forces: Occur between the molecules of two or more different phases of matter and result in the tendency of two substances to stick to each other.

• Capillary Action:

  • Intermolecular Mechanism: Attraction between molecules in the liquid phase (single phase) is due to cohesive forces, whereas attraction between molecules in the liquid phase and solid phase (two or more different phases) results from adhesive forces.

  • Application: Medical technicians take advantage of capillary action when taking a sample of blood.

  • Requirement: Driven by strong adhesive forces.

• Meniscus Formation:

  • Influenced directly by the relative strengths of cohesive and adhesive forces.

  • Concave Meniscus:

  • Example: Water (dyed red).

  • Mechanism: The meniscus is concave because water molecules experience strong adhesive forces and are more attracted to the glass container than to each other.

  • Convex Meniscus:

  • Example: Mercury.

  • Mechanism: Atoms of mercury experience weak adhesive forces with glass and are more attracted to each other (strong cohesive forces), causing the meniscus of mercury to be convex.

Effects of Intermolecular Forces on Vaporization

• Learning Objectives for Section 12.8:

  • Explain the molecular basis of vaporization.

  • Explain dynamic equilibrium in the context of vapor pressure.

  • Distinguish boiling point and normal boiling point.

  • Calculate the enthalpy of vaporization, the normal boiling point of a substance, or the temperature needed to achieve a desired vapor pressure using the Clausius-Clapeyron equation.

• Fundamentals of Vaporization:

  • Definition: Vaporization is the process of converting a substance from the liquid phase into the gaseous phase ($(l) \rightarrow (g)$).

  • Condensation: The opposite phase process of converting a gas into a liquid ($(g) \rightarrow (l)$).

  • Rate Influences:

  • The rate of vaporization increases with increasing temperature.

  • The rate of vaporization increases with increasing surface area.

  • The rate of vaporization increases with decreasing strength of intermolecular forces.

  • Behavior in Open vs. Closed Containers:

  • Open Containers: Vapor molecules generally spread out faster than they can condense. The net result is that the rate of vaporization is greater than the rate of condensation, leading to a net loss of liquid.

  • Closed Containers: Vapor is not allowed to spread out indefinitely. The net result is that, over time, the rates of vaporization and condensation become equal.

  • Energetics:

  • Vaporization requires an input of energy to overcome attractions between molecules, making it an endothermic process.

  • Condensation is an exothermic process.

  • Enthalpy of vaporization is symbolized by $\Delta H_{\text{vap}}$.

  • Operational Relationships:

  • A low rate of vaporization leads to fewer molecules above the threshold energy, resulting in lower vapor pressure.

  • Higher temperatures yield more molecules with enough energy to vaporize.

• Vapor Pressure and Dynamic Equilibrium:

  • Definition: Vapor pressure is the pressure of a gas when it is in dynamic equilibrium with its liquid.

  • Governing Factors: Vapor pressure depends on intermolecular forces of attraction between liquid particles and their temperature.

  • Volatility Principle: Weak intermolecular forces result in volatile substances with high vapor pressures.

  • Characteristics of Dynamic Equilibrium:

  • Both forward and reverse processes occur simultaneously.

  • $\text{Forward Rate} = \text{Reverse Rate}$ ($\text{evaporation} \rightleftharpoons \text{condensation}$).

  • Vapor pressure is established specifically because of a dynamic equilibrium process.

  • Le Châtelier's Principle:

  • Principle: If a system in equilibrium is disturbed, the system adjusts to the disturbance to reestablish equilibrium.

• Boiling Phenomena:

  • Boiling Point: A substance boils at the temperature at which its vapor pressure equals the external pressure.

  • Normal Boiling Point: The specific temperature at which a liquid's vapor pressure equals $1\,\text{atm}$ ($760\,\text{Torr}$).

  • Real-World Application Context: Geothermal boiling phenomena observed at Crater Hills Geyser, Yellowstone National Park.

• Vapor Pressure Dependence on Temperature:

  • Thermal Energy Effect: At higher temperatures, more molecules possess sufficient thermal energy to escape into the gas phase, causing vapor pressure to increase as temperature increases.

  • Direct vs. Inverse Temperature Relationships: Unlike vapor pressure (which increases with temperature), other physical properties such as viscosity and surface tension decrease with increasing temperature.

  • Temperature Dependence Summary:

  • The rate of vapor pressure change depends on the strength of intermolecular forces.

  • Increasing temperature increases the number of molecules able to escape a liquid at its surface.

  • As temperature increases, vapor pressure increases.

  • Small changes in temperature can produce large changes in vapor pressure.

• The Clausius-Clapeyron Equation:

  • Mathematical Relationship: The vapor pressure above a liquid varies exponentially with changes in temperature (the relationship is non-linear).

  • Variable Definitions:

  • $P_{\text{vap}}$: Vapor pressure.

  • $\Delta H_{\text{vap}}$: Enthalpy of vaporization in $\text{kJ/mol}$.

  • $R$: Gas constant equal to $8.314\,\text{J/(mol}\cdot\text{K)}$.

  • $T$: Absolute temperature in Kelvin ($\text{K}$).

  • $C$: Constant that depends on the specific identity of the substance.

Properties of Liquids

• Cohesive and Adhesive Forces:

  • Cohesive forces arise from the intermolecular attractions between like molecules, while adhesive forces occur between different types of molecules. Understanding these forces is crucial for explaining phenomena like surface tension and capillary action.

• Surface Tension:

  • It’s the force that occurs at the interface between a liquid and a gas. Surface tension is enhanced by strong intermolecular forces, leading to a curved surface at the liquid's boundary. This effect allows small objects, even if denser than water, to float on the surface.

• Viscosity:

  • The measurement of a liquid's resistance to flow, viscosity varies significantly among substances and is affected by temperature and intermolecular forces. For example, as temperature increases, thermal energy overcomes intermolecular forces, thereby decreasing viscosity.

• Capillary Action:

  • This is the ability of a liquid to flow in narrow spaces without the assistance of external forces and is primarily driven by the balance of cohesive and adhesive forces. A common example is water climbing up a thin tube—a result of strong adhesive forces between water and the tube material.

Effects of Intermolecular Forces on Vaporization

• Vaporization:

  • It is a crucial property where molecules gain enough energy to transition from liquid to gas. The speed of vaporization is affected by the temperature, with higher temperatures providing molecules with more kinetic energy, resulting in a faster rate of vaporization.

• Dynamic Equilibrium of Vapor Pressure:

  • In a closed system, vaporization and condensation occur at the same rate, resulting in dynamic equilibrium. The vapor pressure is determined by temperature and intermolecular forces; substances with weak intermolecular forces typically exhibit higher vapor pressures.

• Clausius-Clapeyron Equation:

  • This equation relates the vapor pressure of a substance to its temperature and enthalpy of vaporization:
    $P_{ ext{vap}} = C e^{-\frac{\Delta H_{ ext{vap}}}{R T}}$, where $C$ is a constant for each substance, $\Delta H_{ ext{vap}}$ is the heat required for vaporization, $R$ is the gas constant, and $T$ is the absolute temperature. This equation underscores the exponential relationship between vapor pressure and temperature, as vapor pressure increases significantly with even small increases in temperature.

• Cohesive and Adhesive Forces: Cohesive forces are when like things stick together, like how water droplets stick to each other. Adhesive forces are when different things stick together, like how water sticks to the glass.

• Surface Tension: Imagine a tiny trampoline on top of water. The water is trying to hold itself together really tightly, making it hard for things to sink. Some little bugs can walk on it!

• Viscosity: This is how thick or runny a liquid is. Honey is thick and doesn’t flow easily, while water is thin and flows quickly.

• Capillary Action: Picture a straw. When you put it in a drink, the drink goes up the straw because the drink sticks to the straw. That’s because of adhesive forces!

• Cohesive and Adhesive Forces: Cohesive forces are when like things stick together, like how water droplets stick to each other. Think of how two magnets stick together!

• Adhesive Forces: These happen between different things. For example, water sticking to the glass is like how glue makes paper stick.

• Surface Tension: Imagine a tiny trampoline on top of water. The water holds itself together tightly, making it hard for things to sink. That’s why some little bugs can walk on the water!

• Viscosity: This is about how thick or runny a liquid is. Honey is thick and doesn’t want to flow easily, while water is thin and can run fast. It’s like how syrup is sticky and takes a long time to pour out!

• Capillary Action: Think about a straw. When you put it into a drink, the drink goes up the straw because the drink sticks to the straw, just like how water climbs up in a tiny tube. It’s a fun magic trick that happens because of sticky forces!

• Vaporization: This is when liquid turns into gas, like when you boil water and it steams. It's like how a puddle dries up when the sun shines!

• Dynamic Equilibrium of Vapor Pressure: This means that in a closed jar, water can make little gases above it that float around. When they fit just right, we say they are balanced, like a seesaw that doesn’t tip!

• Clausius-Clapeyron Equation: This is a fancy way of saying that when it gets hotter, water wants to turn into steam even faster! It’s like when you heat your soup and it starts to bubble rapidly.

• Boiling Point: This is the temperature when the water gets so hot that it starts to boil and bubbles up really fast, like when you make pasta!