CHP 5

CHM 130 Chapter 5: Chemical Reactions, Stoichiometry, Energy & Equilibrium

Chemical Equations & Conservation of Mass

  1. Law of Conservation of Mass: The law states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products. This principle indicates that matter cannot be created or destroyed during a chemical reaction. Therefore, each atom present in the reactants will be found in the products, albeit in different molecular arrangements.

  2. Difference between Coefficient and Subscript:

    • A coefficient in a chemical reaction indicates the number of molecules or moles of a substance involved in the reaction. It is placed before a chemical formula (e.g., in the reaction $2 H2 + O2
      ightarrow 2 H_2O$, the coefficient '2' indicates that there are two molecules of water produced).

    • A subscript is part of the chemical formula that indicates the number of atoms of an element in a molecule (e.g., in $H_2O$, the subscript '2' indicates that there are two hydrogen atoms in each water molecule).

  3. Example of a Chemical Reaction: The reaction can be illustrated through the following example:

    • Reaction: $2 Al + 3 Cl2 ightarrow 2 AlCl3$
      This reaction shows that two aluminum atoms react with three diatomic chlorine molecules to form two formula units of aluminum chloride.

Types of Reactions

  • Single Replacement Reaction: An example is:

    • $Zn + CuCl2 ightarrow ZnCl2 + Cu$
      Here, zinc displaces copper in copper(II) chloride, resulting in zinc chloride and free copper.

  • Double Replacement Reaction: For instance:

    • $NaNO3 + HCl ightarrow NaCl + HNO3$
      In this reaction, sodium nitrate reacts with hydrochloric acid, resulting in sodium chloride and nitric acid.

Oxidation–Reduction Reaction

  1. Definitions:

    • Oxidation: Defined as the loss of electrons from a substance during a chemical reaction. This process may involve an increase in oxidation state.

    • Reduction: Defined as the gain of electrons by a substance, which results in a decrease in oxidation state.

  2. Electrons in Redox Reactions: In a redox reaction, if magnesium (Mg) loses electrons, it is oxidized. Consequently, the other substance must be reduced by gaining the electrons lost by magnesium.

The Mole & Avogadro’s Number

  1. Molecules in a Mole: One mole of any substance contains approximately $6.022 imes 10^{23}$ molecules, known as Avogadro's number.

  2. Calculating Molar Mass of $C3H8O_2$: The molar mass is calculated using the atomic masses:

    • Carbon (C) = 12.01 g/mol, Hydrogen (H) = 1.008 g/mol, Oxygen (O) = 16.00 g/mol.

    • Therefore, $C3H8O_2 = 3(12.01) + 8(1.008) + 2(16.00) = 36.03 + 8.064 + 32.00 = 76.094 ext{ g/mol}$.

  3. Moles in 22.0 g of CO2: Given that the molar mass of CO2 is 44.01 g/mol:

    • Number of moles = racextmassextmolarmass=rac22.0extg44.01extg/mol<br>ightarrow0.500extmolesrac{ ext{mass}}{ ext{molar mass}} = rac{22.0 ext{ g}}{44.01 ext{ g/mol}} <br>ightarrow 0.500 ext{ moles}.

Mole Ratios

  1. Using Balanced Equation: From the balanced reaction $2 H2 + O2
    ightarrow 2 H_2O$:

    • If 5.0 moles of $H_2$ are used, the number of moles of water formed is calculated using the mole ratio:

    • 5.0extmolesH<em>2imesrac2extmolesH</em>2O2extmolesH<em>2=5.0extmolesH</em>2O5.0 ext{ moles } H<em>2 imes rac{2 ext{ moles } H</em>2O}{2 ext{ moles } H<em>2} = 5.0 ext{ moles } H</em>2O.

  2. Oxygen Needed for 6.0 moles of Hydrogen: Using the same balanced equation, the moles of $O_2$ required can be calculated:

    • 6.0extmolesH<em>2imesrac1extmoleO</em>22extmolesH<em>2=3.0extmolesO</em>26.0 ext{ moles } H<em>2 imes rac{1 ext{ mole } O</em>2}{2 ext{ moles } H<em>2} = 3.0 ext{ moles } O</em>2.

Energy Changes (ΔH)

  1. Endothermic vs. Exothermic Reactions: If ΔH = +125 kJ for a reaction, the reaction is classified as endothermic. This implies that the reaction absorbs heat from its surroundings, leading to a positive change in enthalpy (ΔH).

  2. Energy Diagram: On the energy diagram for a chemical reaction:

    • Vertical Axis: Represents energy of the reactants and products.

    • Activation Energy: Refers to the minimum energy required to initiate a chemical reaction. This energy barrier must be overcome for reactants to be converted into products.

Reaction Rates

  1. Impact of Temperature on Reaction Rate: Increasing the temperature affects the reaction rate by increasing the kinetic energy of the reactants. As the temperature rises, particles move faster and collide more often and with greater energy, which enhances the rate of reaction.

  2. Role of a Catalyst: A catalyst lowers the activation energy of a reaction, thereby increasing the rate of reaction without being consumed itself in the process.

Equilibrium & Le Chatelier’s Principle

For the reaction:

  • $N2 + 3 H2
    ightleftharpoons 2 NH_3$

  1. Effect of Adding H2: If additional hydrogen (H2) is added to the system, the equilibrium shifts to the right, favoring the production of ammonia (NH3) until a new equilibrium is established.

  2. Effect of Removing NH3: If ammonia (NH3) is removed, the equilibrium will shift to the left to compensate, increasing the concentration of nitrogen (N2) and hydrogen (H2).

  3. Rates at Equilibrium: At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, indicating a state where the concentrations of reactants and products remain constant over time.