General Chemistry CHM110: Climate Change, Atomic Mass, and Mole Concepts

Climate Change

Carbon Cycle

The carbon cycle describes the movement of carbon among Earth's atmosphere, biosphere, oceans, and geosphere. It is a dynamic system involving both natural and human-induced processes.

• Photosynthesis: Plants absorb CO2 and convert it to glucose and oxygen. $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

• Respiration: Organisms break down glucose, releasing CO2 and water. $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

• Industry: Human activities, such as cement production (decomposition of CaCO3), release CO2. $CaCO_3 \rightarrow CaO + CO_2$

• Major Carbon Reservoirs: Atmosphere, plants, soils, oceans, and Earth's crust.

Example: The pie chart in the notes shows the sources of global CO2 emissions, with electricity and heat production being the largest contributor.

Atomic Mass and Isotopes

Isotopes and Atomic Mass

Isotopes are atoms of the same element (same atomic number, Z) but with different numbers of neutrons. Most elements have two or more naturally occurring isotopes.

• Atomic Mass: The atomic mass listed in the periodic table is the weighted average of all naturally occurring isotopes of an element.

• Example: Carbon has three isotopes: $^{12}C$, $^{13}C$, and $^{14}C$. The average atomic mass is calculated as: $\text{Avg. atomic mass of C} = 11.868 + 0.143 + 0.0001 = 12.011$

Table: Calculation of Average Atomic Mass (Carbon)

Mass #	Relative %	Contribution to Relative Atomic Mass
12	98.90%	12 x 98.90% = 11.868
13	1.10%	13 x 1.10% = 0.143
14	~0.001%	14 x 0.001% = 0.0001

Calculating Average Atomic Mass

• Multiply the mass of each isotope by its fractional abundance, then sum the results.

• Example (Silicon):

  • $^{28}Si$: 27.9769 amu, 92.21%

  • $^{29}Si$: 28.9766 amu, 4.70%

  • $^{30}Si$: 29.9738 amu, 3.09%

  $\text{Avg. atomic mass of Si} = (27.9769 \times 0.9221) + (28.9766 \times 0.0470) + (29.9738 \times 0.0309) = 28.0855$

Table: Calculation of Average Atomic Mass (Silicon)

Isotope	Mass (amu)	Abundance (%)
$^{28}Si$	27.9769	92.21
$^{29}Si$	28.9766	4.70
$^{30}Si$	29.9738	3.09

Molecules and Moles

The Mole Concept

The mole (mol) is the SI unit for the amount of substance. One mole contains Avogadro's number ($6.022 \times 10^{23}$) of entities (atoms, molecules, or ions).

• Definition: 1 mole = $6.022 \times 10^{23}$ particles

• Molar Mass: The mass in grams of 1 mole of a substance, numerically equal to its atomic or molecular mass in amu.

• Example: 1 mol C atoms = 12.01 g; 1 mol H2O molecules = 18.0 g

Calculating Mass, Moles, and Number of Particles

• Number of moles: $\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

• Number of particles: $\text{number of particles} = \text{moles} \times 6.022 \times 10^{23}$

• Example: How many atoms in 4.76 mol Mg? $4.76 \text{ mol} \times 6.022 \times 10^{23} = 2.87 \times 10^{24}$ atoms

Sample Calculations

• Mass to Moles: $\text{moles} = \frac{48.7 \text{ g}}{39 \text{ g/mol}} = 1.25$ mol (for K)

• Moles to Atoms: $0.4408 \text{ mol} \times 6.022 \times 10^{23} = 2.65 \times 10^{23}$ atoms (for C)

• Atoms to Mass: $3.80 \times 10^{23}$ atoms of Kr = $0.6313$ mol; $0.6313 \text{ mol} \times 83.80 \text{ g/mol} = 52.9$ g

Greenhouse Effect

Overview

The greenhouse effect is a natural process where certain atmospheric gases absorb and emit infrared radiation, warming Earth's atmosphere.

• About 53% of solar energy reaches Earth's surface; the rest is absorbed or reflected by the atmosphere.

• Greenhouse gases (GHGs) absorb about 80% of Earth's outgoing infrared radiation.

• Enhanced greenhouse effect (due to increased GHGs) leads to global warming.

Molecular Shapes: VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory

VSEPR theory predicts the shape of molecules based on the repulsion between electron groups around a central atom.

• Electron Groups: Single, double, triple bonds, and lone pairs each count as one group.

• Common Geometries:

  • 2 groups: Linear (180°)

  • 3 groups: Trigonal planar (120°)

  • 4 groups: Tetrahedral (109.5°)

• Example: CO2 is linear; H2O is bent due to two lone pairs on O.

Table: Electron Group Geometry and Molecular Shape

Electron Groups	Bonding Domains	Nonbonding Domains	Molecular Geometry	Bond Angle
2	2	0	Linear	180°
3	3	0	Trigonal planar	120°
4	4	0	Tetrahedral	109.5°
4	3	1	Trigonal pyramidal	~107°
4	2	2	Bent	~104.5°

How Greenhouse Gases Absorb Infrared Radiation

Molecular Vibrations and IR Absorption

Greenhouse gases absorb IR radiation by undergoing vibrational transitions. Only molecules with three or more atoms (or with a dipole moment change during vibration) can absorb IR.

• Examples: CO2, H2O, CH4 are greenhouse gases; N2 and O2 are not.

• Vibrational Modes: Symmetric stretch (no IR absorption), asymmetric stretch, and bending (IR active).

• Electronegativity: Differences in electronegativity create partial charges, enabling IR absorption.

How Can We Learn from Our Past

Ice Core Data

Ice cores provide historical records of atmospheric gas concentrations and temperature. Air bubbles trapped in ice layers preserve samples of ancient atmospheres.

• Analysis of isotope ratios and gas concentrations reveals past climate changes.

• Correlation between CO2 levels and global temperature is observed in ice core data.

Impacts of Climate Change

• Disappearance of ice

• Sea level rise

• More extreme weather events

• Loss of biodiversity

• Harm to human populations

Ionic Compounds

Formation and Properties

Ionic compounds are formed from the electrostatic attraction between positively charged cations (usually metals) and negatively charged anions (usually nonmetals).

• Cations: Atoms that lose electrons (e.g., Na+)

• Anions: Atoms that gain electrons (e.g., Cl-)

• Monatomic ions: Ions formed from single atoms

• Polyatomic ions: Ions formed from two or more covalently bonded atoms (e.g., SO42-)

• Properties: Ionic compounds are brittle, have high melting points, and conduct electricity when molten or dissolved in water.

Naming Ionic Compounds

• Name the cation first, then the anion.

• For transition metals with variable charges, indicate the charge with Roman numerals (e.g., Fe(III) chloride).

• For monatomic anions, use the stem of the element name plus "-ide" (e.g., chloride, oxide).

• The sum of the charges in the formula must be zero.

Table: Common Polyatomic Ions

Formula	Name
NH4+	Ammonium
PO43-	Phosphate
SO42-	Sulfate
NO3-	Nitrate
ClO4-	Perchlorate
MnO4-	Permanaganate

Example: The formula for aluminum sulfate is Al2(SO4)3.

Additional info: Some context and explanations have been expanded for clarity and completeness.