Chem notes 6

Exothermic and Endothermic Reactions

• Exothermic Reactions:
  • Energy is released.
  • Products have less energy than reactants.
  • Reactants start at a higher energy level and end at a lower energy level.
• Endothermic Reactions:
  • Energy is absorbed.
  • Products have more energy than reactants.
  • Reactants start at a lower energy level and end at a higher energy level.

Heat

• Definition: The transfer of thermal energy due to a temperature difference.
  • Thermal energy moves from hotter objects to colder objects.
  • Example: Heat transfers from warmer water to an ice cube.
• Alternative Definition: The total energy of all molecular motion inside an object.
  • A lake can have more heat than a cup of hot coffee due to the sheer volume of water.
• Measurement:
  • Measured in joules and calories.
  • Calories: can be lowercase (c) or uppercase (C).
    • Uppercase C calorie is a food calorie.
• Temperature: Measures the average heat.

Temperature Scales

• Fahrenheit:
  • Water freezes at 32°F and boils at 212°F.
  • Considered illogical due to its arbitrary divisions.
• Celsius:
  • Water freezes at 0°C and boils at 100°C.
  • Divides the range between freezing and boiling into 100 equal units.
• Kelvin:
  • Freezing point of water: 273.15 K.
  • Boiling point of water: 373.15 K.
  • Same increment size as Celsius, offset by 273.15.
  • T(K) = T(°C) + 273.15
  • 0 K is absolute zero, the lowest possible temperature where all molecular motion stops.

Temperature Conversion Formulas

• Fahrenheit to Celsius: (Not recommended to memorize; use online converters instead.)
• Celsius to Kelvin:
  • K = °C + 273

Absolute Zero

• Is 0 Kelvin.
• Theoretically, it's the point at which all particle motion stops.
• Has not been reached in a laboratory setting.

Energy Units

• Joule: Abbreviated as J.
• Calorie:
  • Lowercase c calorie: 1 cal = 4.184 J
  • Uppercase C calorie (food calorie): 1 C = 1000 c
  • Also equivalent to 1 kilocalorie.
• Kilowatt-hour:
  • 1 kWh = 3.60 \times 10^6 J

Example Problem: Converting Calories to Joules

• Problem: How many joules are in a 60 Calorie serving?
• Conversion Steps:
  • Convert Calories (C) to calories (c):
    • 60 C \times \frac{1000 c}{1 C} = 60,000 c
  • Convert calories (c) to joules (J):
    • 60,000 c \times \frac{4.184 J}{1 c} = 251,040 J
• Significant Figures:
  • Round the final answer to three significant figures: 2.51 \times 10^5 J

Specific Heat Capacity

• Definition: The amount of heat (in joules) required to change the temperature of one gram of a substance by 1 degree Celsius.
• Units: Joules per gram degree Celsius (J/g°C).
• It measures how much heat a substance absorbs before its temperature changes.
• Example:
  • Copper: specific heat capacity of 0.385 J/g°C
  • Iron: specific heat capacity of 0.449 J/g°C
  • Copper heats up faster than iron because it has a lower specific heat capacity.

Specific Heat Capacity of Water

• Water has a high specific heat capacity: 4.184 J/g°C
• The specific heat capacity of Water is high because:
  • Water molecules are small, so more can fit into a given space.
  • Water molecules form hydrogen bonds, requiring more energy to break these bonds and increase molecular motion.

Impact of Water's High Specific Heat Capacity

• Coastal regions, like Hawaii and Santa Barbara, experience smaller temperature variations because water absorbs heat.

Heat Calculation Equation

• q = mc\Delta T
  • q = heat (joules or calories)
  • m = mass (grams)
  • c = specific heat capacity (J/g°C)
  • \Delta T = change in temperature (final temperature - initial temperature)

Example Problem: Heat Absorption

• Problem: How much heat must be absorbed by 125 grams of ethanol to change its temperature from 21.5°C to 34.8°C?
• Given:
  • m = 125 g
  • \Delta T = 34.8°C - 21.5°C = 13.3°C
  • c = 2.42 J/g°C
• Calculation:
  • q = (125 g) \times (2.42 J/g°C) \times (13.3°C) = 4024.55 J
• Final Answer: 4.02 \times 10^3 J

Example Problem: Finding Mass

• Problem: The temperature of a lead fishing weight rises from 26°C to 38°C as it absorbs 11.3 joules of heat. What is the mass of the fishing weight in grams?
• Given:
  • \Delta T = 38°C - 26°C = 12°C
  • q = 11.3 J
  • c = 0.128 J/g°C
• Rearranged Equation:
  • m = \frac{q}{(c \Delta T)}
• Calculation:
  • m = \frac{11.3 J}{(0.128 J/g°C \times 12°C)} = 7.34 g
• Final Answer: 7.3 g

Elements to Memorize

• Memorize the symbols and names of specific elements (from Chapter 4).

Quiz

• A quiz will be given after Exam 1.
• Memorize the symbols and names of the elements.

Atoms and Elements

• Atom: The smallest unit of an element.
• Element: A type of atom, defined by the number of protons.
• Changing the number of protons changes the element.
• Allotropes: Different forms of the same element (e.g., carbon as charcoal, graphite, and diamond).

Atomic Models

• Democritus:
  • Proposed the idea of the atom (smallest indivisible unit).
  • Lacked experimental evidence.
• John Dalton:
  • Atomic model: small, indivisible sphere.
  • Ideas accepted due to more experimental evidence.
  • Dalton's Atomic Theory (Handout Notes):
    1. Matter is made of atoms that are indivisible and indestructible (partially disproven).
    2. All atoms of an element are identical (disproven by isotopes).
    3. Atoms of different elements have different weights and chemical properties.
    4. Atoms of different elements combine in simple whole-number ratios to form compounds (e.g., H2O).
    5. Atoms cannot be created or destroyed; during decomposition, atoms remain unchanged.
• J.J. Thomson:
  • Plum Pudding Model: Atoms contain negatively charged particles (electrons) in a positive background.
  • Experiment: Cathode ray tube.
    • Cathode Ray Tube: Vacuum tube with two electrodes (cathode and anode).
    • Experiment: Applying a magnetic field to a cathode ray demonstrated that the beam consisted of negatively charged particles (electrons).
• Ernest Rutherford:
  • Discovered that most of the atom is empty space with a positive center called the nucleus.
  • Discovered the proton
  • Experiment: Gold foil experiment.
    • Shot alpha particles (positive) through gold foil.
    • Observed that most particles passed through, but some were deflected, indicating a positive center.
    • Contradicted Thomson's model.