Chemical Reactions: Writing and Balancing

• Study notes for chemical reactions include writing reactions, classifying reactions, and chemical equilibrium.

Writing Reactions

• Physical changes differ from chemical changes.
• Collision theory is important.
• Signs such as color change, precipitate formation, gas production, temperature change, or light emission can indicate a chemical reaction.
• The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction.
• In chemical reactions, coefficients indicate the number of molecules, while subscripts indicate the number of atoms within a molecule.
• Chemical reactions can be labeled and interpreted using correct notation.
• Chemical reactions can be balanced to adhere to the Law of Conservation of Mass.
• Balanced chemical reactions can be written from a written description of a reaction.
• Mg3(PO4)2 contains 3 Mg atoms, 2 P atoms, and 8 O atoms.
• 3CO contains 3 C atoms and 3 O atoms.
• 2Ca(OH)2 contains 2 Ca atoms, 4 O atoms, and 4 H atoms.

Balancing Reactions

• Balanced reactions include:
  • 2NF3 → N2 + 3F2
  • 2AlCl3 + 3K2SO4 → 6KCl + Al2(SO4)3
  • 2C6H10 + 17O2 → 12CO2 + 10H2O
  • Pb(OH)2 + 2HCl → 2H2O + PbCl2
• The balanced chemical equation for the reaction of solid aluminum with oxygen gas to create solid aluminum oxide is: 4Al(s) + 3O2(g) → 2Al2O3(s).
• Chemical reaction: process involving rearrangement of atoms and molecules to form new substances.
• Reactants: substances initially involved in a chemical reaction.
• Products: substances that are formed as a result of a chemical reaction.
• Aqueous: a solution in which the solvent is water.

Classifying Reactions

• Reactions can be classified into different types based on their characteristics.
• Reactions from Concept 1:
  • NF3 → N2 + F2: Decomposition
  • AlCl3 + K2SO4 → KCl + Al2(SO4)3: Double Displacement
  • C6H10 + O2 → CO2 + H2O: Combustion
  • Pb(OH)2 + HCl → H2O + PbCl2: Double Displacement
  • Solid aluminum reacts with oxygen gas to create solid aluminum oxide: Synthesis
• Neutralization reaction: a reaction between an acid and a base, resulting in the formation of a salt and water.
• Reactions can be classified based on their equation or description.
• The activity series can be used to determine if a chemical reaction will occur.
• Products resulting from a chemical reaction can be predicted when given the reactants.

Writing, Balancing, and Classifying Reactions

• Solid calcium and liquid water react to form aqueous calcium hydroxide and gaseous hydrogen: Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g); Single Displacement.
• Copper (II) nitrate and magnesium react to form magnesium nitrate and copper: Cu(NO3)2 + Mg → Mg(NO3)2 + Cu; Single Displacement.
• Propane gas (C3H8) is burned: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g); Combustion.

Predicting Products and Balancing Equations

• Combustion: C6H12O6 + 6O2 → 6CO2 + 6H2O
• Synthesis: 2K + Cl2 → 2KCl
• Decomposition: Mg(OH)2 → MgO + H2O
• Single: 2LiF + Mg → No reaction (Mg is not more reactive than Li)
• Double: 2Na3PO4 + 3CaCl2 → 6NaCl + Ca3(PO4)2
• Oxide: a compound containing oxygen and another element.
• Electrolysis: the process in which electric current is passed through a substance to effect a chemical change.
• Hydrocarbon: a compound containing only carbon and hydrogen atoms.
• Precipitate: an insoluble solid that emerges from a liquid solution.
• Neutralization: a chemical reaction in which an acid and a base react quantitatively with each other.
• Acid: a molecule or ion capable of donating a proton or forming a covalent bond with an electron pair.
• Base: a molecule or ion able to accept a hydrogen ion from an acid.
• Salt: a chemical compound formed from the reaction of an acid with a base.
• Activity series: a list of elements ordered by their relative reactivity.

Chemical Equilibrium

• Chemical equilibrium is a state where the rate of forward and reverse reactions are equal, resulting in no net change in reactant and product concentrations.
• Le Chatelier's principle describes how a system at equilibrium responds to changes in conditions.
• Changes in concentration, temperature, and pressure can affect a chemical reaction in equilibrium.
• The response to a specific change in condition on a chemical reaction in equilibrium can be predicted.
• Diagrams and graphs that represent reactions in chemical equilibrium can be interpreted.

Equilibrium Example

• For the reaction N2O4(g) + heat ⇌ 2NO2(g):
  • The reaction is endothermic because heat is absorbed (heat is a reactant).
  • If NO2 is added, the reaction will shift to the left to consume the added NO2 and re-establish equilibrium.
  • If N2O4 is removed, the reaction will shift to the left to produce more N2O4 and re-establish equilibrium.
  • If NO2 is removed, the reaction will shift to the right to produce more NO2 and re-establish equilibrium.
  • If heat is added, the reaction will shift to the right, favoring the production of NO2.
  • If the temperature is decreased, the reaction will shift to the left, favoring the production of N2O4 and releasing heat.
  • If pressure is increased, the reaction will shift to the left, favoring the side with fewer moles of gas (N2O4).
  • If pressure is decreased, the reaction will shift to the right, favoring the side with more moles of gas (NO2).
  • If volume is decreased, pressure increases, so the reaction will shift to the left, favoring the side with fewer moles of gas (N2O4).
  • If volume is increased, pressure decreases, so the reaction will shift to the right, favoring the side with more moles of gas (NO2).
• Chemical equilibrium: the state in which both reactants and products are present in concentrations which have no further tendency to change with time.
• Le Chatelier's Principle: states that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.
• Exothermic: a reaction or process that releases heat.
• Endothermic: a reaction or process that absorbs heat.