Chemical Quantities and Reactions: Study Guide

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CHAPTER 7: CHEMICAL QUANTITIES & REACTIONS

The Mole and Avogadro’s Number

The concept of the mole is fundamental in chemistry for quantifying substances. One mole is defined as exactly 6.02 × 1023 particles (atoms, molecules, or formula units), known as Avogadro’s Number. The mole allows chemists to relate mass, number of particles, and volume in chemical calculations.

Mole: A counting unit, similar to a dozen, but for atoms/molecules.
Avogadro’s Number: particles per mole.
Application: Used to convert between mass, number of particles, and moles.

1-mole quantities of S, Fe, NaCl, K2Cr2O7, C12H22O11

Moles per Element in a Compound

Each compound contains a specific number of moles of each element, determined by its chemical formula. For example, in NaCl, 1 mole contains 1 mole of Na and 1 mole of Cl. In more complex compounds, the mole ratios reflect the subscripts in the formula.

Example: Aspirin (C9H8O4) contains 9 moles of C, 8 moles of H, and 4 moles of O per mole of aspirin.
Formula Subscripts: Indicate the number of moles of each element per mole of compound.

Aspirin molecule and atom counts

Molar Mass

The molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in a chemical formula.

Calculation: Add atomic masses from the periodic table for each element in the formula.
Example: Molar mass of NaCl = mass of Na + mass of Cl.
Use: Converts between grams and moles.

Chemical Equations and Balancing

Chemical equations represent reactions, showing reactants and products. Balancing equations ensures the law of conservation of mass is obeyed: the number of atoms of each element must be equal on both sides.

Reactants: Substances consumed in the reaction.
Products: Substances formed in the reaction.
Coefficients: Numbers placed before formulas to balance atoms.
Balancing Steps: Adjust coefficients, not subscripts; balance polyatomic ions as units.

Chemical equation structure How to balance a chemical equation Hints to balancing chemical equations

Types of Chemical Reactions

Chemical reactions are classified by the changes occurring in reactants and products. The main types are combination, decomposition, single replacement, double replacement, and combustion.

Combination: Two or more reactants form one product.
Decomposition: One reactant splits into two or more products.
Single Replacement: One element replaces another in a compound.
Double Replacement: Two compounds exchange elements to form new compounds.
Combustion: Fuel reacts with oxygen to produce CO2, H2O, and energy.

Combination reaction diagram Combination reaction example: Fe + S Decomposition reaction diagram Decomposition example: HgO Single replacement reaction diagram Single replacement example: Zn + HCl Single replacement example: Cu + Zn Double replacement reaction diagram Double replacement example: Na2SO4 + BaCl2 Combustion reaction diagram Combustion example: C + O2 Combustion example: CH4 + O2

Oxidation and Reduction (Redox Reactions)

Redox reactions involve the transfer of electrons. Oxidation is the loss of electrons, while reduction is the gain of electrons. The mnemonic "OIL RIG" helps remember: Oxidation Is Losing, Reduction Is Gaining (electrons).

Oxidation: Element loses electrons, increases oxidation state.
Reduction: Element gains electrons, decreases oxidation state.
Example: Mg metal oxidized to Mg2+; O2 reduced to O2−.

Oxidation and reduction diagram Oxidation vs reduction examples Oxidation-reduction example: Mg + O2

Summary of Reaction Types

The following table summarizes the main types of reactions and provides examples for each.

Reaction Type	Example
Combination	Ca(s) + Cl2(g) → CaCl2(s)
Decomposition	FeS2(s) → 2Fe(s) + 3S(s)
Single Replacement	Cu(s) + 2AgNO3(aq) → 2Ag(s) + Cu(NO3)2(aq)
Double Replacement	BaCl2(aq) + K2SO4(aq) → BaSO4(s) + 2KCl(aq)
Combustion	CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + energy

Reaction types summary table

Mole-Mole Factors in Reactions

Balanced chemical equations provide mole-mole factors, which are ratios used to convert between moles of different substances in a reaction.

Example: In 2H2 + O2 → 2H2O, the ratio is 2 mol H2 : 1 mol O2 : 2 mol H2O.
Use: Essential for stoichiometric calculations.

Mass Calculations in Reactions (Stoichiometry)

Stoichiometry involves calculating the mass of reactants and products using balanced equations, molar masses, and mole-mole factors.

Steps:
1. Convert grams to moles using molar mass.
2. Use mole-mole factor from balanced equation.
3. Convert moles to grams using molar mass.
Example: Calculate mass of CO2 produced from a given mass of C2H2.

Mass-mole-particle conversion flowchart Stoichiometry conversion factors Stoichiometry calculation steps Stoichiometry calculation example Balanced equation for C2H2 combustion

Exothermic vs Endothermic Reactions

Chemical reactions can either release energy (exothermic) or absorb energy (endothermic). The energy profile of a reaction shows the activation energy and the overall energy change.

Exothermic: Energy released; products have lower energy than reactants.
Endothermic: Energy absorbed; products have higher energy than reactants.
Activation Energy: Minimum energy required to initiate a reaction.

Exothermic vs endothermic reaction energy profiles

Reaction Rate (Kinetics)

The rate of a reaction describes how quickly reactants are converted to products. Factors affecting rate include concentration, temperature, and presence of a catalyst.

Catalyst: Lowers activation energy, increasing reaction rate.
Energy Profile: Shows difference in activation energy with and without catalyst.

Reaction rate energy profile