Chemical equations Knowledge-Based Study

Study notes

1. What is a Chemical Equation?

   A chemical equation is a symbolic representation of a chemical reaction. It uses chemical formulas to show the identities and relative amounts of substances involved in a physical or chemical change.

   • Reactants: The starting substances, written on the left side of the equation.
   • Products: The new substances formed, written on the right side of the equation.
   • Reaction arrow ($\rightarrow$): Separates reactants from products and indicates the direction of change, read as "yields" or "produces".

   State Symbols

   State symbols placed in parentheses after a formula indicate the physical state of each substance:

   • (s): Solid
   • (l): Liquid
   • (g): Gas
   • (aq): Aqueous solution (dissolved in water)

   Conservation of Matter

   According to the Law of Conservation of Mass, matter cannot be created or destroyed in an ordinary chemical reaction. Consequently, the total number of atoms of each element must remain the same before and after the reaction takes place.

   Coefficients vs. Subscripts

   Understanding the distinction between subscripts and coefficients is essential when working with chemical equations:

   • Subscripts: Part of the chemical formula. They denote the fixed ratio of atoms in a compound (e.g., in $\text{H}_2\text{O}$, the subscript 2 indicates two hydrogen atoms). Never change subscripts when balancing an equation, as doing so alters the chemical identity of the substance.
   • Coefficients: Whole numbers placed in front of chemical formulas (e.g., $2\text{H}_2\text{O}$). A coefficient multiplies all atoms within that chemical formula. Adjusting coefficients changes the amount of substance, not its identity.

   Balancing by Inspection

   To balance an unbalanced chemical equation, follow a systematic procedure:

   1. Write the unbalanced equation: Ensure all formulas of reactants and products are chemically correct.
   2. Count the atoms: List the number of atoms of each element on both sides of the reaction arrow.
   3. Balance elements one at a time: Use coefficients to equalize atom counts. A helpful strategy is to balance elements appearing in only one reactant and one product first, leaving elemental substances such as $\text{O}_2$ or $\text{H}_2$ for last.
   4. Treat polyatomic ions as single units: If a polyatomic ion (such as $\text{SO}_4^{2-}$ or $\text{NO}_3^-$) appears unchanged on both sides of the equation, balance it as an intact group rather than counting individual atoms.
   5. Check your result: Verify that the coefficient values represent the lowest whole-number ratio and that the atom count matches for every element.

   Interpreting Equations Quantitatively

   Balanced chemical equations communicate quantitative relationships on both the microscopic and macroscopic scales.

   Consider the synthesis of water: $$2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)$$

   • Molecular level: 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to produce 2 molecules of liquid water.
   • Molar level: 2 moles of $\text{H}_2$ react with 1 mole of $\text{O}_2$ to produce 2 moles of $\text{H}_2\text{O}$.

   The coefficients define the mole ratios between reactants and products, forming the foundational bridge for all stoichiometric calculations, including mass-to-mass and limiting reactant problems.