Chemical reactions

Definition of a chemical reaction

A process that exchanges one bonding arrangement between atoms for a different bonding arrangement (this can happen between atoms, molecules, or compounds)

Molecule vs. compound

Molecule: any arrangement of 2+ atoms bonded together (H₂)

Compound: any molecule with 2+ dissimilar elements (H₂O)

Ionization energy and organization of the periodic table

Ionization energy: the energy required to remove the most loosely held electron

Generally:

• elements on the left side of the periodic table (the metals) have low ionization energy and often lose electrons (become cations) during chemical reactions

• elements on the right side of the periodic table (the nonmetals) have high ionization energy and tend to gain electrons (become anions) during chemical reactions

Ionic vs. covalent bonds

Ionic bonds: complete transfer of electrons, creates ions, typically occurs between elements with a high difference in electronegativity

Covalent bonds: sharing of electrons, typically between elements with a similar number of valence electrons/low difference in electronegativity

Moles & why they’re used in chemical equations

• Mole: unit of measurement equal to ~6.022 × 10 particles of a substance

• Used to preserve ratios in chemical equations (where atomic mass would not) and is used as the coefficient

• ex: 2H₂O + O₂ > 2H₂O is read as 2 moles of hydrogen react w/ 1 mole of oxygen to create 2 moles of water

Single replacement reaction

Reaction type in which one element replaces another similar element

Double replacement reaction

Reaction type in which the positive and negative ions of two ionic compounds replace each other/switch

Decomposition reaction

Type of reaction in which a compound breaks apart into simpler substances

Synthesis reaction

Type of reaction in which substances combine to create a more complex substance

Oxidation-Reduction reactions

Type of reaction that involves the transfer of electrons, changing oxidation numbers (the number of electrons an atom gains or loses to create a chemical bond)

• Oxidation: loss of electrons, making oxidation number go up

• Reduction: gain of electrons, making oxidation number go down

Rate of reaction

The speed at which reactions occur (influenced by various factors including temperature, concentration, pressure, etc.)

Influence of temperature on rate of reaction

An increase in temperature creates an increased likelihood that molecules will run into each other (as movement increases) and therefore an increased likelihood of product formation

Endothermic reaction

Reaction where heat is a reactant (increase of temperature increases the rate of reaction)

Exothermic reaction

Reaction where heat is a product — an increase in temperature will cause an initial increase in rate of reaction, then slow or lead to a reverse reaction

Influence of concentration on chemical reactions

• Increased concentration of reactants = increased likelihood of reaction (more chances for molecules to collide)

• Increased concentration of products = slowed rate of reaction (less uncollided molecules moving around)

Influence of pressure on chemical reactions

For gaseous molecules, increased pressure leads to an increased rate of reaction (pressure drives the molecules together, increasing chance of collision)

Activation energy

• The minimum energy required to initiate a chemical reaction

• A hill/barrier — the smaller the barrier, the faster the rate of reaction

• Can be lowered with catalysts

Catalyst

• A substance that increases the rate of a chemical reaction without undergoing permanent chemical change itself

• Lower the activation energy required for a reaction, provide an alternate pathway

• Facilitate chemical bond cleavage and new bond formation

• Often used by cells (as enzymes) in biologically important reactions to decrease activation energy (as increasing temperature/pressure/concentration isn’t feasible in a living cell)

Le Chatelier’s Principle

When a chemical reaction at equilibrium is perturbed, it responds by proceeding in a direction that will restore the equilibrium