CHEM EXAM 10TH GRADE
I. Periodic Table and Bonding Fundamentals
A. Atomic Structure and Stability
1. Valence Electrons: These are the electrons in an atom's outer shell. The groups (vertical columns) on the periodic table indicate how many valence electrons an atom possesses.
2. Why Atoms Bond: Atoms bond together primarily to gain a very stable valence shell, usually meaning 8 valence electrons (the octet rule). A full valence shell is stable because it has a very low potential energy.
3. Major Bonding Types: Atoms bond by either sharing electrons (covalent bonds) or by transferring electrons (ionic bonds).
B. Ionic Bonding (Electron Transfer)
• Formation: Ionic bonds occur when electrons are transferred (or "stolen") from a metal to a nonmetal.
◦ Metals lose electrons to form a Cation (a positively charged ion).
◦ Nonmetals gain electrons to form an Anion (a negatively charged ion).
• Structure: Anions and cations attract each other in large groups called a LATTICE.
• Characteristics of Ionic Substances:
1. Held together by the attraction of oppositely-charged ions.
2. Are brittle.
3. Have high melting and boiling points.
4. Are good conductors when dissolved in water or molten.
C. Covalent Bonding (Electron Sharing)
• Definition: A covalent bond is a shared pair of electrons between two non-metal atoms. Nonmetals cannot give away electrons, so they achieve a full octet by sharing valence electrons.
• Dative Covalent Compounds: A dative (or coordinate) covalent bond differs from a regular covalent bond because both the bonding electrons come from one atom.
• Diatomic Molecules: Many elements exist in nature as molecules, which are neutral groups of atoms joined by covalent bonds. You must know the 7 elements that exist as diatomic molecules.
II. Drawing Lewis Structures
Drawing Lewis structures is crucial for understanding how bonds are made and broken when calculating enthalpy changes.
Element | Valence Electrons | Preferred Number of Bonds | Rationale |
Carbon (C) | 4 | Four bonds | Needs four more electrons to get to eight. |
Nitrogen (N) | 5 | Three bonds | Needs three more electrons to satisfy the octet rule. |
Oxygen (O) | 6 | Two bonds | Needs two more electrons to get to eight; often forms double bonds. |
Fluorine (F)/Halogens | 7 | Single bond | Needs only one more electron to get to eight. |
Hydrogen (H) | 1 | Single bond | Needs only one more electron to satisfy the duplet rule. |
Steps to Draw Lewis Structures:
1. Write the element symbols.
2. Figure out how many bonds to draw (based on electrons needed to fill outer shells).
3. Draw bonds as lines (where 1 line = 1 pair of shared electrons).
4. Add remaining valence electrons as dots (lone pairs) around each atom.
5. Check for full outer shells (8 electrons for most atoms, 2 for H).
III. Stoichiometry and Conservation Laws
• Law of Conservation of Mass: This fundamental law states that mass is neither created nor destroyed during a chemical reaction or physical transformation.
• Balancing Equations: You must be able to balance chemical equations using whole number coefficients to ensure the Law of Conservation of Mass is followed (equal number of atoms of each element on both sides).
(Based on our previous conversation, here are the balanced examples):
• 3 Li + 1 AlCl₃ → 3 LiCl + 1 Al
• 2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O
• 1 C₂H₄O₂ + 2 O₂ → 2 CO₂ + 2 H₂O
IV. Energy, Heat, and Temperature
• Energy: The ability to do work or cause a change, measured in units of joules (J).
• Heat (Q): Defined as the transfer of energy between objects of different temperature. Heat spontaneously flows from objects of higher temperature to objects of lower temperature until thermal equilibrium is reached.
• Temperature (T): The average kinetic energy of the particles in a substance. If the temperature increases, the average kinetic energy of the particles is increasing.
• Enthalpy (H): The heat content of a system.
• Enthalpy Change (ΔH): The change in heat energy during a chemical reaction that is transferred between the system and surroundings.
V. Exothermic vs. Endothermic Reactions
Energy transfer always occurs between the system (the chemical reaction or process) and the surroundings (everything else).
Feature | Exothermic Reaction | Endothermic Reaction |
Energy Flow | System releases energy to the surroundings. | System absorbs energy from the surroundings. |
Temperature | Surroundings feel warm/hot (Temperature increases). | Surroundings feel cool/cold (Temperature decreases). |
Enthalpy (ΔH) | Negative (ΔH<0). | Positive (ΔH>0). |
Relative Energy | Products are LOWER in chemical potential energy than reactants. | Products are HIGHER in chemical potential energy than reactants. |
Examples | Combustion, Freezing, Condensation, Chemical hand warmer, Respiration. | Photosynthesis, Melting, Boiling, Baking bread, Chemical cold pack, Electrolysis. |
Justification using Bonds
All chemical reactions involve breaking existing bonds and forming new bonds.
• Bond Breaking needs energy (always an Endothermic process).
• Bond Making releases energy (always an Exothermic process).
• A reaction is Exothermic if more energy is released making new bonds (MAKE) than was required to break the old bonds (BREAK). (Energy Change = BREAK – MAKE, where MAKE > BREAK results in a negative value).
• A reaction is Endothermic if the energy absorbed while breaking bonds (BREAK) is greater than the energy released when forming new bonds (MAKE). (Energy Change = BREAK – MAKE, where BREAK > MAKE results in a positive value).
VI. Calculating Enthalpy Changes
A. Calculating Enthalpy using Bond Energy
Bond Enthalpy (H) is the energy required to break one mole of chemical bonds in the gaseous state. Bond enthalpy values are always positive because bond breaking is endothermic.
The formula for calculating the enthalpy change (ΔH) using bond enthalpy is: ΔH=SUM of E(bonds broken)−SUM of E(bonds formed) This formula is often summarized as: Energy Change = BREAK – MAKE.
B. Calculating Enthalpy using Calorimetry
Calorimetry is a technique used to measure the heat transfer during a process, often using a simple coffee-cup calorimeter.
1. Calculate Thermal Energy (Q) (The heat absorbed or released by the surroundings, usually water/solution): Q=mcΔT
◦ Q is thermal energy (Joules, J).
◦ m is the mass of the substance changing temperature (grams, g).
◦ c is the specific heat capacity (e.g., 4.184 J g−1 K−1 for water).
◦ ΔT is the change in temperature.
2. Calculate Enthalpy Change per Mole (ΔH) (The change of the system/reaction): ΔH=−nQ
◦ ΔH is the change in enthalpy (kJ mol−1).
◦ n is the number of moles of an identified reactant or product involved in the reaction.
◦ Crucial Note: ΔH is the opposite sign of Q because Q measures the energy change of the surroundings, while ΔH measures the energy change of the system (the reaction).
VII. Modeling Energy Changes
A. Energy Profile Diagrams
Energy profile diagrams represent the chemical potential energy throughout a reaction.
• Activation Energy (Ea): The minimum amount of energy required by reactant particles to form product particles, represented by the height of the "hump".
• For an Exothermic reaction, the products plot lower than the reactants (giving negative ΔH).
• For an Endothermic reaction, the products plot higher than the reactants (giving positive ΔH).
B. LOL Diagrams (Energy Bar Charts)
LOL diagrams model energy transfer using three types of energy:
• Eth: Thermal Energy (related to temperature).
• Eph: Phase Energy (related to state: solid, liquid, gas).
• Ech: Chemical Energy (stored in bonds).
• In an Exothermic reaction shown by an LOL diagram, the Chemical Energy (Ech) of the chemicals in the initial state is higher than the final state. The difference in Ech is immediately transferred OUT of the system as heat (Q).