Chemistry Final Study Notes

Enthalpies of Formation

The first law of thermodynamics states that energy cannot be created or destroyed, only transformed. Within this framework, we can define enthalpy (H), which refers to the heat content of a system under constant pressure. A significant principle tied to enthalpy is Hess’ law, which asserts that the total enthalpy change of a reaction is the sum of the enthalpy changes of its individual steps, making it essential for calculating enthalpy changes in reactions.

When we discuss enthalpies of formation (ΔHºf), we refer to the enthalpy change that occurs when one mole of a substance is synthesized from its constituent elements in their standard states, typically at a temperature of 25°C (298 K) and a pressure of 1 atmosphere (atm). The standard state of an element is defined such that its ΔHºf equals 0. For instance, common standard states include O₂ (g), H₂ (g), and N₂ (g).

Examples of Enthalpies of Formation

Formation of Water: H₂ (g) + ½ O₂ (g) → H₂O (l); ΔHºf = -285.5 kJ/mol
Formation of Carbon Dioxide: C (s) + O₂ (g) → CO₂ (g); ΔHºf = -393.5 kJ/mol
Formation of Methane: C (s) + 2H₂ (g) → CH₄ (g); ΔHºf = -74.8 kJ/mol

Calculating Enthalpy of Reaction (ΔHºrxn)

The formula for determining the enthalpy change of a reaction (ΔHºrxn) is expressed as:

ΔH^{ ext{rxn}} = ∑nΔH^{ ext{f}}(products) - ∑mΔH^{ ext{f}}(reactants)

In this equation, Σ (the Greek letter Sigma) represents "the sum of," where n and m are the stoichiometric coefficients from the balanced chemical equation.

Application Examples

To calculate the enthalpy change for the reaction: 2H₂O₂ (l) → 2H₂O (l) + O₂ (g):
Utilize the enthalpies of formation:

ΔHºf [H₂O₂ (l)] = -187.8 kJ/mol
ΔHºf [H₂O (l)] = -285.8 kJ/mol

Using the formula, solve for ΔHrxn:

ΔHrxn= (2 x -285.8) - (2 x -187.8) = -196 kJ/mol

For the combustion of methane: CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l):

ΔHºf [CH₄ (g)] = -74.8 kJ/mol
ΔHºf [CO₂ (g)] = -393.5 kJ/mol
ΔHºf [H₂O (l)] = -285.8 kJ/mol

ΔHrxn= [-393.5 + (2 x -285.8)] - (-74.8) = -890.3 kJ/mol (exothermic reaction)

For the reaction: CaCO₃ (s) → CaO (s) + CO₂ (g), with ΔHrxn = 178.1 kJ:

Balance the reaction.
Use ΔHºf [CaO] = -635.5 kJ/mol and ΔHºf [CO₂] = -393.5 kJ/mol to calculate ΔHºf [CaCO₃]:

178.1 = -635.5 + -393.5 - ΔHºf [CaCO₃]

-635.5 + -393.5 = -1029.

Therefore, ΔHºf [CaCO₃] = 178.1 + 1029 = 1207.1 kJ/mol.

Common Misconceptions and Clarifications

It's a common mistake to assume that all elements have a formation enthalpy of zero; only elements in their standard states do.
There is often confusion between enthalpy of formation and enthalpy of reaction.
It is crucial to remember to multiply enthalpy values by the coefficients in balanced equations.

True or False: The formation enthalpy of O₂ (g) is -298 kJ/mol. False, as O₂ is in standard state and hence ΔHºf = 0 kJ/mol.

Summary of Key Concepts

Overall Definition: Enthalpy of formation refers to the energy change when one mole of a compound forms from elements in their standard states.
Essential Formula: ΔHºrxn = ∑nΔHºf(products) − ∑mΔHºf(reactants)
Practical Applications: These concepts are vital in understanding fuels, climate impact, and food energy.

Chapter 24: Organic Chemistry

Organic molecules predominantly consist of the elements carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P). While carbon is always present in these compounds, not all carbon-containing compounds are classified as organic. Key exceptions include:

CO₂: Carbon dioxide
CO: Carbon monoxide
Carbonates: CO₃ (ending with)
Cyanides: CN (ending with)
Carbides: C₂ (ending with)

Organic vs Inorganic Compounds

Examples of Organic Compounds

C₂H₄
C₂H₆O
C₂H₄O₂
C₆H₆
C₆H₁₄

Examples of Inorganic Compounds

H₂
Cl₂
H₂O
NH₃
HCN
CO
NaHCO₃ (cyanide)

Each carbon atom possesses four valence electrons, enabling it to form four covalent bonds:

Tetrahedral: All four bonds are single bonds (sp³ hybridization).
Trigonal: One double bond exists (sp² hybridization).
Linear: A triple bond exists (sp hybridization).

Solubility and Acid-Base Properties of Organic Compounds

Organic compounds demonstrate solubility primarily in non-polar substances, while hydrocarbons form the simplest class of organic compounds, comprising solely hydrogen and carbon atoms. The defining characteristic of hydrocarbons is the presence of stable carbon bonds.

Classification of Hydrocarbons vs Non-Hydrocarbons

Hydrocarbons: C₂H₄, C₆H₁₀, CH₄, C₄H₁₀
Non-Hydrocarbons: CH₂Cl₂, C₆H₁₂O₆

Types of Hydrocarbons

Alkanes: C_nH_2n+2, single bonds, saturated
Alkenes: C_nH_2n, double bonds, unsaturated
Alkynes: C_nH_2n-2, triple bonds, unsaturated
Aromatic Hydrocarbons: Ring-shaped, unsaturated
Cycloalkanes: C_nH_2n, single bonds, saturated

Physical Properties of Hydrocarbons

Generally insoluble in water
Dissolve well in nonpolar solvents
Variations in melting points depending on molecular structure

Saturation vs Unsaturation

Hydrocarbons that consist solely of single bonds (alkanes) are described as saturated.
Conversely, alkenes, alkynes, and aromatic hydrocarbons are classified as unsaturated due to the presence of fewer hydrogen atoms relative to the number of carbon atoms.

Due to their structural differences, unsaturated hydrocarbons exhibit greater reactivity than their saturated counterparts.

Methods for Writing Formulas

Structural Formulas: Illustrate how atoms are interconnected.
Condensed Structural Formulas: Group atoms together to simplify representation (example: CH₃).
Isomers: Molecules can share the same molecular formula—for instance, C₆H₁₄—yet differ in atom arrangement.

Alkanes

Structure and Reactivity

Carbons in alkanes are characterized by sp³ hybridization resulting in a tetrahedral configuration.
In their linear form, all carbon atoms link in a continuous chain.
Due to carbon's tetravalency, a carbon can bond with 3 or 4 other carbon atoms, yielding branched alkanes.

Compounds with analogous molecular formulas but varying atomic connectivity qualify as structural isomers.

Nomenclature

Compound names include:

Prefix: Denotes attached groups (branches).
Base: Specifies the longest carbon chain.
Suffix: Indicates the compound type (alkane, alkene, alkyne, etc.).

Isomers and Their Properties

Isomers with identical molecular formulas exhibit various properties due to their distinct structural arrangements. When naming compounds, adhere to these steps:

Identify the longest continuous chain of carbon atoms (base).
Number the chain starting from the end closest to the first substituent.
Name the substituents; branches based on alkanes are termed alkyl groups.
Insert the name with numbers corresponding to the carbon atoms to which the substituents are affixed.
List multiple substituents alphabetically.

Cycloalkanes

Cycloalkanes represent alkanes forming rings or cycles, feasible with at least three carbon atoms.
Four-carbon rings are relatively unstable, whereas rings of five or more carbons accommodate proper bond angles.
Nomenclature includes the prefix cyclo- preceding the alkane name.

Reactivity of Alkanes

Alkanes demonstrate low reactivity due to their nonpolar characteristic and the stable C—C and C—H σ bonds. They typically don't engage in reactions with acids, bases, or oxidizers; nonetheless, combustion remains a notable reaction.

Alkenes

Alkenes feature at least one double bond, characterized by σ (sigma) and π (pi) bonds. They cannot freely rotate around the double bond, rendering them unsaturated.

Nomenclature of Alkenes

Identify the longest chain including the double bond.
Name the alkene with an ending of -ene.
Assign the lowest possible number to the double bond, and include isomer details as necessary.

Geometrical Isomers

Geometrical isomers arise when the attachment groups around carbon bonds differ, identified as cis (same side) or trans (opposite sides). These configurations are exclusive to alkenes because the double bond prevents rotation, influencing relative positions of the attached groups.

Alkynes

Alkynes contain at least one triple bond comprising one σ bond and two π bonds, rendering them unsaturated.

Nomenclature of Alkynes

Identify the longest chain including the triple bond.
Name the alkyne with an ending of -yne.
Assign the lowest possible number to the carbon atoms in the triple bond.

Additions in Reactions

Addition reactions occur with alkenes and alkynes, where atoms are added to the double or triple bond, leading to saturation:

Through hydrogenation, alkenes convert to alkanes by breaking double bonds and forming single bonds.
Alkynes can be converted to alkenes by similar processes, thus gaining saturation.
Halogenation entails introducing halogen atoms, often displacing hydrogen atoms or adding across multiple bonds.

Aromatic Hydrocarbons

Aromatic hydrocarbons are distinguished by six-membered rings comprising both localized and delocalized electrons, typically recognized as benzene. They exhibit significantly reduced reactivity compared to alkenes and alkynes due to resonance stability.

Functional Groups

The chemical behavior of organic molecules is significantly dictated by the functional groups within them. In this context, R symbolizes the alkyl component (C, H) of the organic molecule; for instance, R-OH denotes an alcohol.

Alcohols

Alcohols contain one or more hydroxyl groups (-OH) and are named based on the parent hydrocarbon by replacing the suffix with -ol, while a number identifies the carbon atom to which the -OH group attaches.

Properties and Applications of Alcohols

Alcohols are polar, resulting in solubility in water and increased boiling points.
Methanol: used as a fuel additive.
Ethanol: present in alcoholic beverages.
Ethylene glycol: utilized in antifreeze formulations.
Glycerol: acts as a cosmetic skin softener and food moisturizer.
Phenol: used in plastics and dyes; applied as a topical anesthetic in throat sprays.
Cholesterol: a critical biomolecule in cell membranes, which can precipitate to form gallstones or obstruct blood vessels.

Scientific Method

The scientific method entails a systematic approach to exploring chemical phenomena:

Identify a Problem: Formulate questions derived from observations.
State a Hypothesis: Propose a potential solution informed by thorough research.
Test the Hypothesis: Design and execute controlled experiments focusing on one variable at a time.
Form a Conclusion: Organize and analyze the data obtained.
Publish Results: Share findings within the scientific community.

Key Concepts

Dependent Variable: Represented on the y-axis; outcome of the experiment.
Independent Variable: Depicted on the x-axis; element being altered in the experiment.
Controlled Variables: Aspects that remain constant throughout the experiment.

Comprehensive Overview of Enthalpy of Formation and Organic Chemistry

Comprehensive Overview of Enthalpy of Formation and Organic Chemistry