Overview
This lecture covers the distinction between physical and chemical changes, introduces chemical equations, explains how to write and balance these equations, and defines key terminology related to chemical reactions.
Physical vs. Chemical Changes
- Physical changes alter appearance but not the chemical composition of a substance (e.g., boiling water, crushing ice).
- Chemical changes (chemical reactions) involve the rearrangement of atoms to form new compounds.
- Common examples of chemical changes include the oxidation of iron and the burning of a candle.
Chemical Equations and States
- Chemical equations use arrows to indicate that a reaction occurs, with reactants on the left and products on the right.
- Formulas and physical states (solid = s, liquid = l, gas = g, aqueous = aq) must be included in equations.
- Most metals and ionic compounds are solids; Mercury and Bromine are liquids; hydrogen, nitrogen, oxygen, fluorine, chlorine, noble gases, and ammonia are gases at room temperature.
- The (aq) symbol denotes substances dissolved in water.
Writing and Interpreting Chemical Equations
- Diatomic elements (e.g., O₂, H₂, F₂) must be written as molecules when alone.
- Ionic compounds' formulas are determined by considering the charges and using the criss-cross method.
- Use the correct state symbols based on the substance and reaction conditions.
Balancing Chemical Equations
- The Law of Conservation of Mass requires the same number and type of atoms on both sides of the reaction.
- Stoichiometric coefficients (big numbers in front of formulas) are used to balance equations, not subscripts.
- Multiply coefficients by subscripts to calculate the total number of atoms of each element.
- Coefficients must represent the lowest possible whole-number ratio.
- For polyatomic ions appearing on both sides, treat them as units when balancing.
Example Applications
- Oxygen and hydrogen react to form water: 2H₂(g) + O₂(g) → 2H₂O(l), balanced using coefficients.
- Sodium and fluorine form sodium fluoride: 2Na(s) + F₂(g) → 2NaF(s), balanced by adding coefficients to match fluorine atoms.
- Magnesium and oxygen form magnesium oxide: 2Mg(s) + O₂(g) → 2MgO(s), balanced to ensure equal atoms on both sides.
- Reactions involving polyatomic ions require balancing each ion as a whole unit.
Key Terms & Definitions
- Physical Change — change affecting form but not chemical composition.
- Chemical Change (Reaction, rxn) — process where substances form new compounds.
- Reactant — substance present at the start of a reaction.
- Product — substance formed as a result of a reaction.
- Stoichiometric Coefficient — number placed in front of a chemical formula to balance the equation.
- Diatomic Element — element that exists as a molecule of two atoms (e.g., O₂).
- Aqueous (aq) — dissolved in water.
- Law of Conservation of Mass — total mass and atom count must be conserved in a reaction.
Action Items / Next Steps
- Practice writing and balancing chemical equations from word descriptions.
- Learn and memorize the common states of elements and compounds at room temperature.
- Review polyatomic ion formulas and charges for balancing reactions.