Intermolecular Forces and Crystal Structures: Key Concepts for Chemistry

What is the difference between Intramolecular and Intermolecular forces?

Intramolecular forces hold atoms together within a molecule; Intermolecular forces are attractive forces between separate molecules.

Which are generally stronger: Intramolecular forces or Intermolecular forces?

Intramolecular forces (chemical bonds) are generally stronger.

What are the three main types of van der Waals forces?

1. Dipole-dipole forces 2. Hydrogen bonding 3. London Dispersion Forces (induced dipole-induced dipole)

Which intermolecular force exists between all molecules and atoms, regardless of polarity?

London Dispersion Forces (LDF).

What property determines the strength of London Dispersion Forces?

Polarizability, which generally increases with molar mass and electron cloud size.

How does molecular shape affect London Dispersion Forces?

Molecules with linear/elongated shapes have a greater surface area for contact, leading to stronger LDFs than spherical/compact molecules.

What specific atoms must Hydrogen be bonded to for Hydrogen Bonding to occur?

Nitrogen (N), Oxygen (O), or Fluorine (F).

Why does Ice have a lower density than liquid water?

Hydrogen bonding creates an open, hexagonal lattice structure in the solid that pushes molecules further apart than in the liquid state.

What is an Ion-Dipole interaction?

The attraction between an ion and the partial charge of a polar molecule.

What is Surface Tension?

The energy required to increase the surface area of a liquid by a unit amount.

How does Temperature affect the Viscosity of a liquid?

Viscosity decreases as temperature increases.

What is Capillary Action?

The spontaneous rising of a liquid in a narrow tube against gravity.

In Capillary Action, what are Cohesive Forces?

Forces between the liquid molecules themselves.

In Capillary Action, what are Adhesive Forces?

Forces between the liquid molecules and the surface of the container.

If Adhesive forces are stronger than Cohesive forces, what is the shape of the meniscus?

Concave (U-shaped).

If Cohesive forces are stronger than Adhesive forces, what is the shape of the meniscus?

Convex (inverted U).

What is the relationship between Vapor Pressure and Intermolecular Forces?

They are inversely related: Stronger IMFs lead to lower vapor pressure.

What is the Normal Boiling Point of a liquid?

The temperature at which the liquid's vapor pressure equals 1 atmosphere.

What is the Clausius-Clapeyron Equation used for?

To calculate the enthalpy of vaporization or vapor pressure at a specific temperature.

What are the three phase changes that are Endothermic?

1. Melting (Fusion) 2. Vaporization 3. Sublimation

What are the three phase changes that are Exothermic?

1. Freezing 2. Condensation 3. Deposition

What is the phase change from Solid to Gas called?

Sublimation.

What is the phase change from Gas to Solid called?

Deposition.

In a Heating Curve, what is plotted on the x-axis and y-axis?

x-axis: Heat added (or Time); y-axis: Temperature.

On the slanted lines of a heating curve, what happens to the Temperature?

It increases.

On the slanted lines of a heating curve, what happens to the Average Kinetic Energy of the particles?

It increases.

On the flat lines of a heating curve, what happens to the Temperature?

It remains constant.

On the flat lines of a heating curve, what happens to the Average Kinetic Energy?

It remains constant.

On the flat lines of a heating curve, what happens to the Potential Energy?

It increases as intermolecular forces are overcome.

Which equation is used to calculate heat during a temperature change?

q = m × C_s × ΔT.

Which equation is used to calculate heat during a phase change?

q = n × ΔH (or q = m × ΔH).

Why is the horizontal line for Vaporization typically longer than the line for Melting on a heating curve?

Because the Enthalpy of Vaporization is usually much larger than the Enthalpy of Fusion.

What is a Supercooling phenomenon in a cooling curve?

When a liquid is cooled below its freezing point without forming a solid.

What do the axes represent on a Phase Diagram?

y-axis: Pressure; x-axis: Temperature.

In a Phase Diagram, what does the Triple Point represent?

The specific temperature and pressure where solid, liquid, and gas phases coexist in equilibrium.

In a Phase Diagram, what does the Critical Point represent?

The temperature and pressure above which a distinct liquid and gas phase no longer exist.

What is a Supercritical Fluid?

A state of matter beyond the critical point that has the high density of a liquid and the low viscosity of a gas.

In the Phase Diagram for Water, what is unique about the slope of the solid-liquid equilibrium line?

It has a negative slope.

What does a negative slope on the solid-liquid line of a phase diagram indicate about density?

The solid phase is less dense than the liquid phase.

What is the slope of the solid-liquid line in the Phase Diagram for Carbon Dioxide (CO2)?

Positive (leans right), indicating the solid is denser than the liquid.

What does the direct transition from solid to gas at 1 atm imply about the Triple Point of Carbon Dioxide (CO2)?

The Triple Point pressure is above 1 atm (5.1 atm).

What is the Enthalpy of Fusion (ΔH_fus)?

The energy required to convert 1 mole of a solid to a liquid at its melting point.

What is the Enthalpy of Vaporization (ΔH_vap)?

The energy required to convert 1 mole of a liquid to a gas at its boiling point.

What is Chemical Kinetics?

The study of the rates of chemical reactions and the factors that affect them (e.g., concentration, temperature, catalysts).

What is the definition of Reaction Rate?

The change in concentration of a reactant or product per unit of time.

What is the general formula for reaction rate involving a change in concentration Δ[A]?

Rate = Δ[A]/Δt.

Why is a negative sign used when calculating the rate of disappearance of a reactant?

Because the concentration of reactants decreases over time (is negative), and reaction rates are always expressed as positive quantities.

What are the standard units for Reaction Rate?

M/s (Molarity per second) or mol L⁻¹ s⁻¹.

In the reaction 2N2O5 -> 4NO2 + O2, how does the rate of disappearance of N2O5 relate to the rate of formation of O2?

The rate of O2 formation is half the rate of N2O5 disappearance (based on stoichiometric coefficients 2:1).

What is the formula relating the rates of reactants and products for the general reaction aA + bB -> cC + dD?

Rate = -1/a Δ[A]/Δt = -1/b Δ[B]/Δt = 1/c Δ[C]/Δt = 1/d Δ[D].

What is the Average Rate of a reaction?

The change in concentration measured over a specific interval of time.

What is the Instantaneous Rate of a reaction?

The rate at a specific point in time, determined by the slope of the tangent line to the concentration-time curve.

What is the Initial Rate?

The instantaneous rate at time t = 0.

What is a Rate Law (or Rate Equation)?

A mathematical expression that relates the rate of a reaction to the concentration of reactants.

In the rate law Rate = k[A]^n, what does k represent?

The Rate Constant.

In the rate law Rate = k[A]^n, what does n represent?

The Reaction Order.

How is the Overall Reaction Order calculated?

By summing the exponents (orders) of all reactants in the rate law (m + n).

Can reaction orders be determined from the balanced chemical equation coefficients?

No, they can only be determined by experiment.

What is the Method of Initial Rates?

A method to determine reaction orders by varying the initial concentration of one reactant while holding others constant and observing the change in initial rate.

If doubling the concentration of a reactant has no effect on the rate, what is the reaction order for that reactant?

Zero Order.

If doubling the concentration of a reactant doubles the rate, what is the reaction order for that reactant?

First Order.

If doubling the concentration of a reactant quadruples the rate, what is the reaction order for that reactant?

Second Order.

Does the Rate Constant (k) change when reactant concentrations change?

No, k is independent of concentration.

What is the primary factor that changes the value of the Rate Constant (k)?

Temperature.

What are the units of the rate constant k for a Zero Order reaction?

M/s (or mol L⁻¹ s⁻¹).

What are the units of the rate constant k for a First Order reaction?

s⁻¹ (or 1/time).

What are the units of the rate constant k for a Second Order reaction?

M⁻¹ s⁻¹ (or L mol⁻¹ s⁻¹).

What defines the structure of a Cesium Chloride (CsCl) unit cell?

A primitive cubic lattice of Cl⁻ ions with a Cs⁺ ion in the center (or vice versa).

What is the coordination number of the ions in a Cesium Chloride (CsCl) structure?

8.

In a Sodium Chloride (NaCl) structure, what type of lattice do the Cl⁻ ions form?

Face-Centered Cubic (FCC).

In a Sodium Chloride (NaCl) structure, where are the Na⁺ ions located?

In the octahedral holes.

What is the coordination number of the ions in a Sodium Chloride (NaCl) structure?

6.

How many net formula units of NaCl are in one unit cell?

4 formula units (4 Na⁺ and 4 Cl⁻).

In a Zinc Blende (ZnS) structure, what type of lattice do the S²⁻ ions form?

Face-Centered Cubic (FCC).

In a Zinc Blende (ZnS) structure, where are the Zn²⁺ ions located?

In one-half of the tetrahedral holes.

What is the coordination number of the ions in a Zinc Blende (ZnS) structure?

4.

What is Lattice Energy (or Lattice Enthalpy)?

The energy required to separate one mole of a solid ionic compound into its gaseous ions (endothermic).

According to Coulomb's Law, how does ionic charge affect Lattice Energy?

Higher ionic charge (Q₁ Q₂) leads to higher (more negative/exothermic formation) lattice energy.

According to Coulomb's Law, how does ionic radius (distance) affect Lattice Energy?

Smaller ionic radius (shorter distance d) leads to higher (more negative/exothermic formation) lattice energy.

What leads to higher lattice energy?

Smaller ionic radius (shorter distance) leads to higher lattice energy.

Which compound has a higher lattice energy magnitude: MgO or NaF?

MgO (due to +2/-2 charges vs +1/-1).

Which compound has a higher lattice energy magnitude: NaCl or LiF?

LiF (due to smaller ionic radii).

What is the Born-Haber Cycle used to calculate?

Lattice Energy (which cannot be measured directly).

What does the Enthalpy of Sublimation step represent in the Born-Haber cycle?

Converting the solid metal into gaseous atoms (e.g., Na(s) -> Na(g)).

What does the Bond Dissociation Energy step represent in the Born-Haber cycle?

Breaking the bond of the diatomic non-metal gas (e.g., 1/2 Cl2(g) -> Cl(g)).

What does the First Ionization Energy step represent in the Born-Haber cycle?

Removing an electron from the gaseous metal atom (e.g., Na(g) -> Na+(g) + e-).

What does the Electron Affinity step represent in the Born-Haber cycle?

Adding an electron to the gaseous non-metal atom (e.g., Cl(g) + e- -> Cl-(g)).

What are the general mechanical properties of Ionic Solids?

Hard and brittle.

Why do Ionic Solids conduct electricity when molten or dissolved, but not when solid?

In solid form, ions are fixed; in liquid/solution, ions are mobile and can carry charge.

What occupies the lattice points in a Molecular Solid?

Molecules (e.g., H2O, CO2).

What holds the particles together in a Molecular Solid?

Intermolecular forces (IMFs) like London Dispersion, Dipole-Dipole, or Hydrogen Bonding.

What are the typical melting points of Molecular Solids?

Low to moderate.

What occupies the lattice points in a Network Covalent Solid?

Atoms bonded by continuous covalent bonds.

Give three examples of Network Covalent Solids.

1. Diamond 2. Graphite 3. Quartz (SiO2).

What characterizes an Amorphous Solid?

A lack of long-range order (no repeating crystal lattice), e.g., Glass, Rubber.

What is a Substitutional Alloy?

An alloy where solute atoms replace solvent atoms in the crystal lattice (atoms must be similar in size).

What is an Interstitial Alloy?

An alloy where small solute atoms fit into the holes (interstices) between the larger metal atoms.

What is a crystal lattice?

A three-dimensional arrangement of atoms, ions, or molecules within a crystalline solid.

What is a unit cell?

The smallest collection of atoms, ions, or molecules that, when repeated, produces the crystal lattice.

What do the lattice points represent in a crystal lattice?

The locations of atoms, monoatomic ions, polyatomic ions, or molecules.