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Chemistry Exam Notes

Types of Reactions

  • The student should study the types of reactions to understand each one.
  • Single Replacement:
    • A single element and a compound react.
    • Example: Fe + NaBr3 \rightarrow FeBr3 + Na
    • After the reaction, there will be a different single element.
    • In the example, iron (Fe) forms a positive ion, so it will be the single element on the product side.
    • Sodium (Na) and Bromide (Br) form the new compound, Sodium Bromide (NaBr).
    • Sodium has a +1 charge, and Bromide has a -1 charge, so they combine directly.
  • Balancing Equations:
    • Balance chemical equations using coefficients.
    • Balance with coefficients: Adjust the numbers in front of the compounds to ensure the number of atoms of each element is equal on both sides of the equation.
    • The example equation is unbalanced: Fe + NaBr3 \rightarrow FeBr3 + Na
    • Original count (reactants): 1 Fe, 3 Na, 3 Br
    • To balance, start with the element that is not balanced (e.g., Na).
    • Place a coefficient of 3 in front of NaBr: Fe + 3NaBr3 \rightarrow FeBr3 + 3Na
    • Now, balance the remaining elements by inspection.

Double Replacement Reactions

  • Double Replacement:
    • Two compounds react, and the positive ions switch places.
    • Example: NaOH + H2SO4 \rightarrow Na2SO4 + H_2O
    • Sodium (Na) combines with Sulfate (SO4), and Hydrogen (H) combines with Hydroxide (OH).
    • Sodium has a +1 charge, and Sulfate has a -2 charge, so they combine as Na2SO4.
    • Hydrogen and Hydroxide combine to form water (H_2O or HOH).
  • The general process is:
    • Identify the positive and negative ions in the reactants.
    • Swap the positive ions and combine them with the negative ions from the other reactant.
    • Make sure the resulting compounds are neutral by balancing the charges.
    • The number of atoms in the products is calculated based on the charges (oxidation numbers) of the new molecule.

Balancing Double Replacement Reactions

  • Example: NaOH + H2SO4 \rightarrow Na2SO4 + H_2O
    • Reactant count: 1 Na, 1 OH, 2 H, 1 SO_4
    • Product count: 2 Na, 1 SO_4, multiple H and O in water.
    • Place a 2 in front of NaOH to balance Sodium: 2NaOH + H2SO4 \rightarrow Na2SO4 + H_2O
    • Place a 2 in front of H2O to balance Hydrogen and Hydroxide: 2NaOH + H2SO4 \rightarrow Na2SO4 + 2H2O

Key Principles for Predicting Products

  • Positive Ion:
    • The positive ion from one reactant combines with the negative ion from the other reactant.
  • Charge Balance:
    • Ensure the compounds formed are neutral by properly balancing the charges.
  • Subscripts:
    • Subscripts on the reactant side do NOT determine the subscripts on the product side.
  • Balancing:
    • Balancing is based on making sure the compounds are neutral.
    • Balancing ensures there's no net charge in the compound.
  • Underlying Knowledge:
    • This unit builds upon the previous unit.
    • A solid understanding of the previous unit is essential for predicting products.
    • Example: PbSO4 + AgNO3 \rightarrow Pb(NO3)2 + Ag2SO4
      • Lead (Pb) has a +2 charge because Sulfate (SO_4) has a -2 charge.
      • Silver (Ag) has a +1 charge, and Nitrate (NO_3) has a -1 charge.
      • Ensure charges remain consistent on both sides of the reaction.

Oxidation Numbers

  • Example: MnO2 + Sn(OH)4 \rightarrow Mn(OH)4 + SnO2
    • Manganese (Mn) has a varying oxidation number, so it must be figured out.
    • Oxygen has a -2 charge, thus 2 oxygen atoms make it 4 minuses, which means Mn is +4 to balance it.
    • Sn(OH)₄: Hydroxide (OH) has a -1 charge, and there are four of them, so 10 is +4 to balance it.
    • Mn(OH)₄: The 4 hydroxides give it 4 minuses so Mn is +4 to balance it.
    • The Oxidation Is Potential:
      • +1,+2, skip the DIP, +3, +/4 (plus or minus), -3, -2, Oxygen, -1, -7
      • The oxidation number does not change when it goes to the product side.

Balancing Combustion Reactions

  • Combustion Reactions:
    • Involve a hydrocarbon (a compound containing carbon and hydrogen) or a compound containing carbon, hydrogen, and oxygen reacting with oxygen.
    • Products are always carbon dioxide and water.
    • Example: C2H4O2 + O2 \rightarrow CO2 + H2O
  • Balancing Steps:
    • Write CHO on both sides to keep track of atoms.
    • Balance carbon first, then hydrogen, then oxygen.
    • Adjust coefficients to ensure the number of atoms of each element is equal on both sides.
    • Ensure total number of oxygen atoms matches on both sides.
      • If you have C5H{12}O2 + O2
        • You will get CO2 and H2O EVERY SINGLE TIME.
        • Match your beginning accounts to look like the step that you want to take and copy the carbons you have from the reaction side.
        • Then to balance your equation look at your changing hydrogen and your oxygens. You do not look there until you change your hydrogen.

Double Replacement Examples

  • Identify positive ions: In a compound, the positive ion includes the hydrogen, potassium, and/or silver. The negative ions include the rest i.e. chlorine.
    • Mn(OH)₄ + SnO₂ is balanced with the same +4 charge because they balance already.
    • For compound plus compounds, you must have double replacement.
      • You would swap the partners from there.
    • With MnO₄ + ZnCl₂ you will have MnCl in order to correctly balance it.
      • The one balanced is: Zn + MnO₄
        • To do this you must know what the charges on MnO₄ are.
          • The easy way to remember that charge is that with one potassium, potassium has one positive charge. Which means that Mn is actually one minus.

Nuclear Chemistry

  • Definition:
    • The study of changes to the nucleus of an atom.
  • Radioactivity:
    • Emission of radiation.
  • Radioactive or Nuclear Decay:
    • The process by which an unstable nucleus loses energy and matter by emitting radiation.
    • An unstable nucleus occurs when there are different numbers of neutrons and protons.
  • Alpha Particle:
    • Symbol: H e
    • Mass number: 4
    • Atomic number: 2
  • Beta Particle:
    • It has a zero and a negative one because all of that is an electron. It is the loss of an electron by an unstable nucleus.
    • Notation: e
    • Mass number: 0
    • Atomic number: -1

Nuclear Equations

  • Balancing:
    • Ensure the mass numbers and atomic numbers are equal on both sides of the equation.
  • Example (Beta Emission):
    • Protactinium-234 (Pa-234) undergoes beta decay.
    • Equation: Be (e) + 234 / 92 x
    • The symbol for beta is e with a zero and a negative one from the equation
      • Zero plus what equals two thirty four equals zero, giving it the 234 / -1 plus what give me 91
  • Example (Alpha Emission):
    • Antimony-116 (Sb-116) emits an alpha particle.
    • Equation: Sb-116 → He + In
    • Find Antimoy on the periodic table, that gives you 116 / 52 Sb / He + In
      • Four plus what gives me 116? That is 112. From that you have two plus what gave me 51? 49.
    • From that you would look for number 49 in a new element.
  • Key Points:
    • All reactions will involve emission, not absorption.
    • Focus on Alpha and Beta particles only.
    • Match and balance the sides using mass and atomic numbers.