🫴Physical Science Unit 6 – Chemical Reactions and Equations

Key Concepts and Definitions

• Chemical reaction process in which one or more substances (reactants) are converted into one or more different substances (products)
• Reactants starting materials that undergo change during a chemical reaction
• Products substances formed as a result of a chemical reaction
  • Can have different physical and chemical properties compared to the reactants
• Chemical equation symbolic representation of a chemical reaction using chemical formulas
  • Reactants are written on the left side of the arrow and products on the right side
• Stoichiometry quantitative study of the amounts of reactants and products involved in a chemical reaction
• Mole unit of measurement in chemistry that represents a specific number of particles (6.022 × 10^23)
• Balanced equation chemical equation in which the number of atoms of each element is equal on both sides of the equation
• Reaction rate measure of how fast a chemical reaction proceeds, often expressed as the change in concentration of a reactant or product per unit time

Types of Chemical Reactions

• Synthesis reaction two or more reactants combine to form a single product (A + B → AB)
• Decomposition reaction a single compound breaks down into two or more simpler substances (AB → A + B)
• Single displacement reaction a single element replaces another element in a compound (A + BC → AC + B)
• Double displacement reaction two compounds exchange parts, forming two new compounds (AB + CD → AD + CB)
  • Also known as metathesis reactions
• Combustion reaction a substance reacts with oxygen, releasing heat and often light (Fuel + O2 → CO2 + H2O)
• Acid-base reaction reaction between an acid and a base, producing a salt and water (HCl + NaOH → NaCl + H2O)
• Redox reaction reaction involving the transfer of electrons between species, resulting in changes in oxidation states
  • Oxidation loss of electrons and increase in oxidation state
  • Reduction gain of electrons and decrease in oxidation state

Balancing Chemical Equations

• Law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction
  • Total mass of the reactants must equal the total mass of the products
• Balancing equations process of adjusting coefficients to ensure the number of atoms of each element is equal on both sides
• Steps to balance an equation:
  1. Identify the reactants and products
  2. Write the unbalanced equation
  3. Count the number of atoms of each element on both sides
  4. Adjust coefficients to balance the equation (never change subscripts)
  5. Verify that the equation is balanced
• Coefficients numbers placed in front of chemical formulas to balance the equation
  • Represent the relative amounts of reactants and products
• Subscripts part of the chemical formula and cannot be changed when balancing equations
  • Indicate the number of atoms of each element within a compound

Stoichiometry and Calculations

• Molar mass mass of one mole of a substance, expressed in grams per mole (g/mol)
  • Calculated by adding the atomic masses of all atoms in a compound
• Mole ratios relationships between the amounts of reactants and products in a balanced chemical equation
  • Used to convert between moles of different substances in a reaction
• Limiting reactant reactant that is completely consumed first, limiting the amount of product formed
• Excess reactant reactant that remains after the limiting reactant is consumed
• Theoretical yield maximum amount of product that can be obtained based on the balanced equation and the amount of limiting reactant
• Actual yield amount of product actually obtained in a real reaction
  • Often less than the theoretical yield due to various factors (side reactions, incomplete reactions, etc.)
• Percent yield ratio of the actual yield to the theoretical yield, expressed as a percentage
  • $\text{Percent Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%$

Reaction Rates and Kinetics

• Reaction rate measure of how fast a chemical reaction proceeds
  • Expressed as the change in concentration of a reactant or product per unit time
• Factors affecting reaction rates:
  • Temperature higher temperatures generally increase reaction rates by providing more kinetic energy for collisions
  • Concentration higher concentrations of reactants lead to more frequent collisions and faster reaction rates
  • Surface area larger surface areas of solid reactants expose more particles to collisions, increasing reaction rates
  • Catalysts substances that speed up reactions without being consumed, lowering the activation energy
• Collision theory explains how reactions occur when reactant particles collide with sufficient energy (activation energy) and proper orientation
• Activated complex unstable intermediate formed during a reaction when reactant particles collide with enough energy
  • Represents the highest energy state along the reaction pathway
• Reaction mechanism series of elementary steps that describe how a reaction proceeds from reactants to products
  • Includes the formation and breakdown of intermediate species
• Rate-determining step slowest step in a reaction mechanism, which determines the overall reaction rate

Factors Affecting Chemical Reactions

• Temperature higher temperatures increase the average kinetic energy of particles, leading to more frequent and energetic collisions
  • Arrhenius equation relates reaction rate to temperature: $k = A e^{-E_a/RT}$
• Concentration higher concentrations of reactants result in more particles per unit volume, increasing the likelihood of collisions
  • Reaction rates are often proportional to the concentrations of reactants
• Pressure for gaseous reactions, higher pressures lead to more collisions between reactant particles, increasing reaction rates
• Surface area for heterogeneous reactions involving solids, larger surface areas expose more reactant particles to collisions
  • Grinding or crushing solids can increase reaction rates
• Catalysts substances that lower the activation energy of a reaction without being consumed
  • Provide an alternative reaction pathway with a lower energy barrier
  • Can be homogeneous (in the same phase as reactants) or heterogeneous (in a different phase)
• Inhibitors substances that slow down or prevent chemical reactions by interfering with the reaction mechanism
  • Can be used to control unwanted reactions or preserve products

Real-World Applications

• Combustion reactions:
  • Burning of fossil fuels (coal, oil, natural gas) for energy production
  • Internal combustion engines in vehicles
• Chemical synthesis:
  • Production of ammonia (Haber-Bosch process) for fertilizers and other applications
  • Manufacture of pharmaceuticals, plastics, and other synthetic materials
• Environmental chemistry:
  • Formation and decomposition of ozone in the atmosphere
  • Acid rain caused by the reaction of air pollutants with water in the atmosphere
• Biochemistry:
  • Enzymatic reactions in living organisms
  • Metabolic pathways (glycolysis, citric acid cycle, etc.)
• Food chemistry:
  • Maillard reaction between amino acids and sugars, responsible for browning and flavor development in cooked foods
  • Fermentation reactions in the production of beer, wine, and other alcoholic beverages

Common Challenges and Tips

• Balancing equations:
  • Start with the most complex compound or the element that appears in only one compound on each side
  • Adjust coefficients systematically, focusing on one element at a time
  • Remember that subscripts cannot be changed when balancing
• Stoichiometry:
  • Clearly identify the given information and the desired quantity
  • Use dimensional analysis to set up problems, canceling units to ensure the correct answer
  • Double-check calculations and unit conversions
• Reaction mechanisms:
  • Break down the overall reaction into elementary steps
  • Identify intermediates and catalysts involved in the mechanism
  • Determine the rate-determining step based on the slowest elementary step
• Conceptual understanding:
  • Relate chemical reactions to everyday experiences and real-world examples
  • Use analogies and visual aids to help grasp abstract concepts
  • Practice problem-solving and critical thinking skills through worked examples and practice problems