General Chemistry Study Notes: Chemical Reactions and Gases

Chemical Reactions

Classifying Chemical Reactions

Chemical reactions can be classified based on the nature of the reactants and products, as well as the changes that occur during the reaction. Understanding these classifications helps predict reaction outcomes and balance chemical equations.

• Strong Electrolyte: A substance that completely dissociates into ions in solution, conducting electricity efficiently. Example: HCl in water.

• Weak Electrolyte: A substance that partially dissociates into ions in solution, conducting electricity poorly. Example: CH3COOH (acetic acid).

• Non-electrolyte: A substance that does not produce ions in solution and does not conduct electricity. Example: sucrose (table sugar).

Example: The reaction:

$\mathrm{CaH_2SO_4 (aq) + 2 HCl (aq) \rightarrow CaCl_2 (aq) + 2 H_2O (l) + SO_2 (g)}$

Here, CaH2SO4 and HCl are strong electrolytes, while SO2 is a gas and H2O is a non-electrolyte.

Net Ionic Equations

Net ionic equations show only the species that actually participate in the reaction, omitting spectator ions.

• Steps to write a net ionic equation:

  1. Write the balanced molecular equation.

  2. Identify all strong electrolytes and write them as ions.

  3. Cancel out spectator ions.

  4. Write the net ionic equation with only the reacting species.

Gases

Gas Laws and Properties

Gases are described by several physical laws that relate pressure, volume, temperature, and amount of gas. These laws are fundamental in predicting the behavior of gases under different conditions.

• Ideal Gas Law: Relates pressure, volume, temperature, and moles of gas.

$PV = nRT$

• Partial Pressure: The pressure exerted by a single component in a mixture of gases.

• Dalton's Law of Partial Pressures: The total pressure of a mixture of gases is the sum of the partial pressures of each component.

$P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$

Calculating Partial Pressures

To find the partial pressure of a gas in a mixture:

• Use the ideal gas law for each component.

• Sum the partial pressures for the total pressure.

Example: If a container holds 0.2 mol CO and 0.1 mol O2 at 100.0 kPa, calculate the partial pressures using:

$P = \frac{nRT}{V}$

Root Mean Square Speed of Gas Molecules

The root mean square (rms) speed is a measure of the average speed of gas molecules, which depends on temperature and molar mass.

$u_{\text{rms}} = \sqrt{\frac{3RT}{M}}$

• R: Universal gas constant (8.314 J/mol·K)

• T: Temperature in Kelvin

• M: Molar mass in kg/mol

Example: Calculate the rms speed of H2 at 298 K:

$u_{\text{rms}} = \sqrt{\frac{3 \times 8.314 \times 298}{0.002}}$

Effusion and Diffusion

Effusion is the process by which gas molecules escape through a small hole, while diffusion is the mixing of gases.

• Graham's Law of Effusion: The rate of effusion is inversely proportional to the square root of the molar mass.

$\text{Rate}_1 / \text{Rate}_2 = \sqrt{M_2 / M_1}$

True/False Statements about Gases

• All of the H2 gas molecules in a 1.0 L container at 25°C move slower than all the He gas molecules at 25°C. False

• At the same temperature, lighter molecules move faster than heavier molecules. True

• The pressure of a gas sample filled with 2.0 moles of H2 at 2.0 L is greater than a sample of 1.0 mole at 2.0 L. True

• Replacing half the gas with another gas of equal moles and volume will not change the total pressure. True

Lab Techniques and Procedures

Calculating Moles in Solution

In laboratory settings, it is important to calculate the amount of substance present in a solution using molarity and volume.

• Molarity (M): Moles of solute per liter of solution.

$\text{Molarity} = \frac{\text{moles of solute}}{\text{volume of solution (L)}}$

• Example: To find moles of HCl in 50.0 mL of 1.0 M HCl:

$0.050 \text{ L} \times 1.0 \text{ mol/L} = 0.050 \text{ mol}$

Tables

Partial Pressures Table

The following table summarizes partial pressures of gases in a mixture:

Species	Partial Pressure (atm)
CO	15
O2	93
N2	78

Partial Pressure Calculation Example

Species	Partial Pressure (atm)
NO	2.5
N2	1921
O2	0

Lab Calculation Table

Solution	Moles of HCl	Moles of CaCO3
22.0 mL 1.0 M HCl	0.022	0
50.0 mL 1.0 M HCl	0.050	0
0.05 mol CaCO3	0	0.05

Additional info: Some calculations and values were inferred from handwritten notes and standard chemistry procedures.