Comprehensive Chemistry Review Notes for DOST Scholars

NCR Alliance of DOST Scholars: Chemistry Intensive Review

Speaker and Session Information

• Instructor: Therese Josefa M. Lagumbay.
• Instructor Profile: Incoming 4th Year Medical Technology Student at Centro Escolar University - Manila.
• Credentials: DOST Scholar Awardee (Year 2023) and Student Volunteer.
• Philosophy: Believes that education is a powerful tool that should continuously be empowered and made accessible.
• Core Physics Formulas Referenced:
  • Sum of Forces: $\Sigma F = M \times a$
  • Mass-Energy Equivalence: $E = M \times c^2$
  • Kinetic Energy: $E_k = \frac{1}{2} m \times v^2$

Questions and Discussion

Question 1: Classification of Matter

• Question: Which of the following is classified as a pure substance?
  • A. Saltwater
  • B. Air
  • C. Gold
  • D. Milk
• Answer: C. GOLD.
• Detailed Explanation:
  • Matter Hierarchy:
    1. Pure Substances: These consist of only one type of particle and have fixed chemical properties. This includes Elements (e.g., Gold, represented as $Au$ with atomic number $79$ and mass $196.97$) and Compounds.
    2. Mixtures: Physical combinations of two or more substances.
       • Homogeneous: Uniform composition (e.g., Saltwater, Air which contains $N_2$, $O_2$, and $CO_2$).
       • Heterogeneous: Non-uniform composition (e.g., Milk).

Question 2: Physical vs. Chemical Properties

• Question: Which property is considered a chemical property?
  • A. Density
  • B. Color
  • C. Melting point
  • D. Flammability
• Answer: D. FLAMMABILITY.
• Explanation of Concepts:
  • Physical Property: These properties can be observed or measured without changing the composition of the substance. They are often reversible and describe the state of matter (e.g., $H_2O$ moving between liquid, solid, and gas states involves no change in chemical identity).
  • Chemical Property: These describe a substance's potential to undergo chemical changes. A chemical change is often irreversible and alters the composition, transforming the substance completely into a different substance.

Question 3 & 6: Atomic Models and Energy Levels

• Question 3: Who proposed the planetary model of the atom?
  • Answer: C. NIELS BOHR.
• Question 6: Who proposed that electrons move in specific energy levels around the nucleus?
  • Answer: C. Niels Bohr.
• Historical Context of Atomic Models:
  • John Dalton: Proposed the solid sphere model (Dalton's Atomic Model).
  • J.J. Thomson: Proposed the "Plum Pudding" model (Sphere of positive charge with embedded negatively charged electrons).
  • Ernest Rutherford: Proposed the Nuclear Model (Planetary appearance where electrons orbit a central nucleus composed of protons and neutrons).
  • Niels Bohr: Known as the "Architect of the Atom." He detailed specific electron orbits or energy levels, such as those seen in a nitrogen atom.

Question 4: Atomic Number Components

• Question: The atomic number represents the number of
  • A. Neutrons
  • B. Electrons only
  • C. Protons
  • D. Nucleons
• Answer: C. PROTONS.
• Constituents of the Atom:
  • Protons: Atoms with a specific number of protons behave the same; they determine the identity of the element.
  • Neutrons: Located in the nucleus; carry no charge.
  • Electrons: Orbit the nucleus; have negligible mass in many calculations.
  • Nucleons: A collective term for the components found in the nucleus (protons and neutrons).

Question 5: Isotopes

• Question: Isotopes of an element differ in the number of ________.
  • A. Neutrons
  • B. Electrons
  • C. Protons
  • D. Valence electrons
• Answer: A. NEUTRONS.
• Isotope Characteristics:
  • Isotopes have the same number of protons but different neutron counts.
  • Stability: Some are stable while others are unstable (radioisotopes) and emit radiation.
  • Examples: Isotopes of Hydrogen
    1. Protium: $1$ proton, $0$ neutrons.
    2. Deuterium: $1$ proton, $1$ neutron.
    3. Tritium: $1$ proton, $2$ neutrons.

Question 7: Compounds and Molecules

• Question: A molecule formed by atoms of different elements is called a ________.
  • A. Compound
  • B. Mixture
  • C. Ion
  • D. Metal
• Answer: A. COMPOUND.
• Definitions:
  • Compound: Formed when atoms of different elements bond chemically (e.g., $H_2O$ consists of Oxygen and Hydrogen atoms).
  • Mixture: Substances not chemically bonded.
  • Ion: An atom with an electrical charge difference; can be a Cation (positive, due to losing electrons) or an Anion (negative, due to gaining electrons).

Question 8: Molecular Polarity

• Question: Which molecule is polar?
  • A. $O_2$
  • B. $H_2O$
  • C. $N_2$
  • D. $Cl_2$
• Answer: B. $H_2O$.
• Polarity Explanation:
  • A polar molecule is characterized by having two opposite properties or charges moving toward poles.
  • In $H_2O$, oxygen gains electron density (partial negative charge, $\delta-$) while hydrogen atoms lose electron density (partial positive charges, $\delta+$).

Question 9, 10 & 11: Acid-Base Chemistry

• Question 9: Bases generally feel
  • Answer: C. SLIPPERY.
  • Explanation (Saponification): Bases feel slippery because they react with oils on the skin to produce soap through saponification (e.g., oily skin + $NaOH$).
• Question 10: What is formed when an acid reacts with a base?
  • Answer: B. SALT AND WATER.
  • Neutralization Reaction Example:
    • $\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$
    • $HCl + NaOH \rightarrow NaCl + H_2O$
• Question 11: Brønsted-Lowry Theory
  • Acid: A proton ($H^+$) donor.
  • Base: A proton ($H^+$) acceptor.

Question 12 & 13: Types of Chemical Reactions

• Question 12: What type of reaction is represented by $A + B \rightarrow AB$?
  • Answer: C. SYNTHESIS.
• Question 13: What type of reaction is represented by: $2H_2 + O_2 \rightarrow 2H_2O$?
  • Answer: D. Combination.
• Reaction Types Breakdown:
  • Synthesis (Combination): Reactants combine into one product ($A + B \rightarrow AB$). Example using Sodium and Chlorine: $Na_{ (s)} + Cl_{ (g)} \rightarrow NaCl_{ (s)}$ (Note: Sodium explodes in water; Chlorine was used as a chemical weapon; Salt seasons soup).
  • Decomposition: A reactant is broken down ($AB \rightarrow A + B$).
  • Combustion: Reacting a hydrocarbon with oxygen to produce water, carbon dioxide, heat, and light ($C_x H_y + O_2 \rightarrow H_2O + CO_2 + \text{Heat/Light}$).
  • Double Replacement: Two compounds exchange ions ($AB + CD \rightarrow AD + CB$).

Question 14: Chemical Equilibrium

• Question: Which of the following statements are correct at equilibrium?
  • Answer: C. The rate of the forward reaction is equal to the rate of the reverse reaction.
• Key Equilibrium Concept: Equilibrium is reached when the rate of the forward reaction ($mol/s$) matches the rate of the reverse reaction. Concentrations of reactants and products remain constant over time but are not necessarily equal.

Question 15: Dilution Calculation

• Question: A student is asked to make $250\,mL$ of $0.1\,M\,H_2SO_4$ from $18\,M$ concentrated acid. What volume of the concentrated acid is needed?
• Given:
  • $M_1 = 18\,M$
  • $V_1 = ?$
  • $M_2 = 0.1\,M$
  • $V_2 = 250\,mL$
• Formula: $M_1 V_1 = M_2 V_2$
• Step-by-step Calculation:
  1. $18 \times V_1 = 0.1 \times 250$
  2. $18V_1 = 25$
  3. $V_1 = \frac{25}{18}$
  4. $V_1 = 1.39\,mL$
• Answer: A. $1.39\,mL$.

Question 16: Balancing Equations

• Question: Which coefficient balances the equation $H_2 + O_2 \rightarrow H_2O$?
  • Answer: B. 2.
• Balancing Process:
  • Unbalanced: $H_2 + O_2 \rightarrow H_2O$ ($H=2, O=2$ vs. $H=2, O=1$).
  • Balanced: $2H_2 + O_2 \rightarrow 2H_2O$ ($H=4, O=2$ on both sides).

Question 17: Molar Mass Calculation

• Question: What is the molar mass of $H_2O$?
• Atomic Masses:
  • $H = 1\,g/mol \times 2 = 2\,g/mol$
  • $O = 16\,g/mol \times 1 = 16\,g/mol$
• Calculation: $16 + 2 = 18\,g/mol$.
• Answer: C. $18\,g/mol$.

Question 18: Empirical Formula Calculation

• Question: A $3.00\,g$ sample of a hydrocarbon undergoes complete combustion producing $8.80\,g$ of $CO_2$ and $3.60\,g$ of $H_2O$. What is the empirical formula?
• Calculation Steps:
  1. Moles of C from $CO_2$: $\frac{8.80\,g}{44.0\,g/mol} = 0.20\,mol\,C$
  2. Moles of H from $H_2O$: $\frac{3.60\,g}{18.0\,g/mol} = 0.20\,mol\,H_2O \times 2 = 0.40\,mol\,H$
  3. Simplest Whole-Number Ratio:
     • $C = \frac{0.20}{0.20} = 1$
     • $H = \frac{0.40}{0.20} = 2$
• Formula: $CH_2$.
• Answer: D. $CH_2$.

Question 19: Gas Laws and Molar Mass

• Question: A $0.2\,g$ sample of an unknown gas occupies $0.082\,L$ at $1\,atm$ and $273\,K$. What is its molar mass?
• Given: $m = 0.2\,g$, $V = 0.082\,L$, $P = 1\,atm$, $T = 273\,K$. Gas constant $R = 0.0821\,L\cdot atm/mol\cdot K$.
• Step 1: Solve for Moles (n) using Ideal Gas Law ($PV = nRT$):
  • $n = \frac{PV}{RT} = \frac{1 \times 0.082}{0.0821 \times 273}$
  • $n \approx \frac{0.082}{22.4133} \approx 0.00366\,mol$
• Step 2: Calculate Molar Mass (MM):
  • $MM = \frac{m}{n} = \frac{0.2\,g}{0.00366\,mol} = 54.6\,g/mol$
• Answer: B. $54.6\,g/mol$.

Question 20: Thermodynamics

• Question: An exothermic reaction:
  • Answer: B. Releases heat.
• Terminology:
  • Exothermic: Releases heat; reaction is hotter than surroundings. Potential energy of products is lower than reactants (energy released).
  • Endothermic: Absorbs heat; reaction is cooler than surroundings. Potential energy of products is higher than reactants (energy absorbed from activation energy).

Question 21, 23 & 25: Quantum Mechanics and Bonding

• Question 21: Orbitals
  • Orbitals describe the probability of finding an electron. Orbital shapes include $s$, $p$, $d$, and $f$.
• Question 22: Level of Theory
  • Quantum theory explains matter/energy at the atomic and subatomic level.
• Question 23: Heisenberg Uncertainty Principle
  • Proposed by Werner Heisenberg. It states that you cannot simultaneously measure both the exact position and the exact momentum (mass $\times$ velocity) of a microscopic particle.
• Question 24: Energy Levels
  • The maximum number of electrons in the second energy level is 8. Levels are classified: I ($K$), II ($L=8$), III ($M=18$), IV ($N=32$).
• Question 25: Bond Types
  • Covalent Bond: Involves the sharing of available valence electrons (Nonmetal + Nonmetal/Metalloid).
  • Ionic Bond: Involves the transfer of electrons (Metal + Nonmetal).
  • Metallic Bond: Electrostatic attraction between metal ions and a "sea" of electrons.
  • Hydrogen Bond: Electrostatic attraction between a molecule containing hydrogen and one containing a lone pair.

Tips and Final Encouragement

• Study Tips: Focus on the foundation of each topic; find clue words in questions; stay calm and collected.
• Words of Encouragement: "You believe, a man can change his destiny? – I think a man does what he can, until his destiny is revealed." (The Last Samurai).
• Closing Message: God bless to all future DOST Scholars!