Chemistry Exam Notes

Equilibrium

• Kc Calculation:

  • Kc expression example: For the reaction 2H2O, H2 and O2, Kc = \frac{[H2]^2[O2]}{[H_2O]^2}

• Effect of Temperature on K Value:

  • If increasing the temperature decreases the concentration of the product, the reaction shifts towards the reactants to reduce the temperature.
  • This indicates the forward reaction is exothermic and the reverse reaction is endothermic.
  • Increasing temperature favors the endothermic reaction.
  • Exothermic Reaction: The value of K will decrease as the reaction becomes more reactant-favored.
  • To determine this, you need to look at the relative concentrations of reactants and products, not absolute values.

Ozone Depletion

• CFCs and Ozone:

  • CFCs (chlorofluorocarbons) were used in propellants and refrigerator sealants.
  • When released, CFCs break down and form chlorine radicals.

• Ozone Layer:

  • The ozone layer (O3) deflects dangerous UV radiation from the sun, acting as a protective layer.
  • Depletion leads to a hole in the ozone layer, allowing harmful UV radiation to reach the surface, increasing the risk of skin diseases and cancer.

• Reaction Mechanism of Ozone Depletion:

  • Chlorine radical reacts with ozone (O3): Cl• + O3 → ClO• + O2
  • The resulting CLO radical reacts with another ozone molecule: ClO• + O3 → Cl• + 2O2
  • Overall Equation: 2O3 → 3O2
  • This is a free radical substitution reaction.

• Steps of CFC Breakdown:

  • CFC breaks down under UV radiation: CFCl → CF + Cl•
  • Chlorine breaks up again under UV radiation to make two chlorine radicals.

• Catalytic Effect:

  • Chlorine radicals act as catalysts, lowering the activation energy and increasing the rate of successful collisions, which leads to the depletion of O3.

Rates of Reaction

• Concentration and Rate:

  • A substance with a steeper slope on a graph indicates a higher rate of product production or reactant consumption.

• Factors Affecting Rate (Black Tea Example):

  • Surface Area: Breaking sugar into smaller pieces increases surface area, speeding up dissolution.
  • Temperature: Warming the tea allows more sugar to dissolve because warmer solutions can dissolve more solute.
  • Concentration: Increasing the amount of sugar.

• Collision Theory:

  • Concentration affects the frequency of successful collisions.

Variables in Experiments

• Independent Variable:

  • The variable you change in an experiment.

• Dependent Variable:

  • The variable you measure in response to changes in the independent variable.

• Control Variables:

  • Factors that must be kept constant to ensure a fair test (e.g., catalyst concentration, surface area, type of substance, container).

• Coffee and Cream Experiment:

  • Independent Variable: Volume of cream added.
  • Dependent Variable: Temperature change.
  • Control Variables: Initial volume of coffee, starting temperature (96 degrees), and the container used.

• Reliability:

  • Repeating the experiment and calculating the average increases reliability.

Reaction Mechanisms

• Definition:

  • Reaction mechanisms explain how reactions occur, detailing the stages involved.

• Curly Arrows:

  • Represent the movement of electrons during a reaction.
  • Reactions occur due to the movement of electrons.

• Example: Addition of Bromine to a Carbon-Carbon Double Bond

  • Step 1: Carbon-carbon double bond (high electron density) approaches bromine molecule (nonpolar).
  • Step 2: The electron density pushes electrons in the bromine molecule, creating partial charges (\delta+ and \delta−).
  • Step 3: Electrons from the double bond attack the partially positive bromine, and the bond between the two bromine atoms breaks, with electrons moving to the other bromine atom.
  • Step 4: Formation of a carbocation intermediate.
    • Bromine attaches to one carbon, creating a single bond.
    • The other carbon has a positive charge.
    • The other bromine becomes negatively charged due to gaining electrons.
  • Step 5: The negatively charged bromine attacks the carbocation, forming the final product.

• Carbocation Intermediate:

  • A structure with a positive charge on a carbon atom.
  • Unstable due to the charge.

• Electrophilic Addition:

  • A reaction where an electrophile (electron-loving species) is added to a molecule.

• Multiple Products:

  • Some reactions can yield multiple products.

• Example: Reaction with HBr
  * HBr is polar (\delta+ and \delta−).
  * Two possible products due to different carbocation intermediates.
  * Major and Minor Products: One product is usually favored over the other.
  * Alkyl Groups: Influence the stability of the carbocation intermediate.
  * More alkyl groups around the carbon with a positive charge lead to a more stable intermediate.

• Substitution Reactions:
  * One atom or group is replaced by another.

• Addition Reactions:
  * Atoms or groups are added to a molecule.

• Electrophilic vs. Nucleophilic:
  * Electrophilic: Electron-loving.
  * Nucleophilic: Nucleus-loving.

Significance of Reaction Mechanisms

• Understanding Reaction Pathways:

  • Explains why certain reactions occur and the sequence of events.

• CFCs and Ozone Depletion:

  • Explains how one chlorine radical can deplete thousands of ozone molecules due to its continuous regeneration.

• Product Formation:

  • Helps understand why some reactions produce multiple products while others produce only one.

• Isomers:

  • Compounds with the same molecular formula but different structural arrangements.