Chemistry Notes

Energy in Reactions

Chemistry studies how matter interacts, focusing on energy changes during processes.
These processes can be physical or chemical.

Definition: A change that is observable but doesn't alter the substance's composition.
Example: Melting ice (H2O(s) ilda H2O(l)) or vaporizing water (H2O(l) ilda H2O(g)).
The chemical composition remains the same (e.g., H2O remains H2O).
Physical Properties: Measurable characteristics, like the melting point of water.
- Melting point of ice: 0^\circ C (32°F, 273 K)
- Freezing point of liquid water: 0^\circ C
- At 0^\circ C, ice melts and water freezes, establishing an equilibrium.

Definition: A change that involves altering the composition of a substance through a chemical reaction.
Example: Burning hydrogen gas (H2) with oxygen (O2) to produce water (H_2O).
- 2H2 + O2 ilda 2H_2O
Conservation of Mass: Matter is neither created nor destroyed in a chemical reaction.
- Elements are not transmuted; atoms are merely rearranged.
- The total mass of reactants equals the total mass of products.
Balancing chemical equations ensures the conservation of mass.

Common metric units:
- 1 kilogram (kg) = 1,000 grams (g)
- 1 gram (g) = 1,000 milligrams (mg)
- 1 gram (g) = 1 \times 10^6 micrograms (\mug) (1 millionth of a gram)
Micrograms are used for very small doses (e.g., selenium in multivitamins).

Common units:
- 1 liter (L) = 1,000 milliliters (mL)
- 1 milliliter (mL) = 1 cubic centimeter (cc or cm^3)

Example: Converting ounces to kilograms.
- Given: 1 pound (lb) = 16 ounces (oz) and 1 pound (lb) = 454 grams (g).
Strategy: Ounces → Pounds → Grams → Kilograms
Calculation: 4 \text{ oz} \times \frac{1 \text{ lb}}{16 \text{ oz}} \times \frac{454 \text{ g}}{1 \text{ lb}} \times \frac{1 \text{ kg}}{1000 \text{ g}}
- Units cancel out, leaving the desired unit.
Place the desired unit in the numerator for each conversion step.

Significant figures indicate the precision of measured numbers.
In measured numbers, the last digit has uncertainty.
Example: Measuring a string with a ruler marked in half inches.
- Report the length as 2.55 inches (estimated).
- This number has three significant figures.

Nonzero digits are always significant.
- 41 has two significant figures.
Zeros at the end of a number without a decimal point are not significant.
- 410 has two significant figures.
- 1. has three significant figures.
Zeros between nonzero digits are significant.
- 401 has three significant figures.
Zeros to the left of the first nonzero digit are not significant (placeholders).
- 0.041 has two significant figures.
Zeros at the end of a number with a decimal point are significant
- 1.30 has three significant figures.

Align the numbers by their decimal points, then apply operation.
Round the answer to the fewest number of decimal places.
Example:
89.332 \text{ g} + 1.1 \text{ g} = 90.432 \text{ g} \approx 90.4 \text{ g}

Apply the operation
Round the answer to the same number of significant figures as the number with the fewest significant figures.
Example:
2.8 \times 4.5039 = 12.61092 \approx 13

Periodic table arranges elements by atomic number (Z).
- The atomic number (Z) indicates the number of protons in the nucleus.
Mass number (A) is the sum of protons and neutrons in the nucleus.

An element (X) designated with mass number as a superscript and atomic number as a subscript (_Z^AX).
Example: Oxygen-16 (_{8}^{16}O) has 8 protons and 8 neutrons.

Isotopes are variants of an element with different numbers of neutrons but same number of protons.
Example: Isotopes of hydrogen.
- Hydrogen (_1^1H): 1 proton, 0 neutrons.
- Deuterium (_1^2H or D): 1 proton, 1 neutron.
- Tritium (_1^3H): 1 proton, 2 neutrons (radioactive, used in medicine).