A chemical formula represents a substance using element symbols, while a chemical equation represents a chemical change using chemical formulas. Being able to read and write equations reveals the essence of chemical changes.
Basic Knowledge
A chemical formula represents the composition of a substance using element symbols and small numbers at the lower right (subscripts). A subscript of 1 is omitted. Examples: H2O(water), CO2(carbon dioxide), NaCl(sodium chloride).
A chemical equation writes a chemical change in the form “reactants → products.” Coefficients are adjusted so that the types and numbers of atoms are equal before and after the reaction (a coefficient of 1 is omitted).
Steps for writing:
① Write the chemical formulas of the reactants and products on the left and right of the arrow (→)
② Adjust the coefficients so that the number of each atom is equal on both sides
③ Reduce the coefficients to the smallest whole-number ratio
Example: formation of water 2H2 + O2 → 2H2O
Check: left side H=4・O=2, right side H=4・O=2 ✓
Important equations:
・Combustion of carbon: C + O2 → CO2
・Combustion of magnesium: 2Mg + O2 → 2MgO
・Hydrochloric acid and iron: Fe + 2HCl → FeCl2 + H2
Chemical formula（a formula representing a substance's composition with element symbols and subscripts）
Chemical equation（an equation representing reactants → products with chemical formulas）
Coefficient（a whole number placed before a chemical formula to balance the number of atoms）
Reactant（a substance present before a chemical change. The left side of the arrow）
Product（a substance formed after a chemical change. The right side of the arrow）
Combustion（a chemical change in which a substance combines with oxygen and gives off light and heat）
Further Study
Chemical equations are the “language” of chemistry. When an equation is written accurately, it also shows the quantitative relationship (mole ratio) among the substances involved in the reaction. At the middle-school level, the most important task is checking that the coefficients are balanced.
Tips for balancing coefficients: ① Balance the atom that appears in the greatest number of substances first. ② Balance atoms contained in only one substance first. ③ Check everything at the end and reduce to the smallest whole-number ratio.
Chemical formulas requiring attention:
・Carbon dioxide CO2 (different from CO. CO is carbon monoxide)
・Both water vapor and water are H2O (only their states differ)
・Sodium chloride NaCl (an ionic crystal that does not form molecules)
・Copper oxide CuO (copper and oxygen in a 1:1 ratio)
- Chemical equation = reactants → products (the arrow points in one direction)
- Make the types and numbers of atoms equal on both sides of the equation
- A coefficient is before an atom or formula; a subscript is at the lower right after an atom
- A coefficient of 1 is not written
2H2O: two H2O molecules (4 H atoms and 2 O atoms)- Formation of water:
2H2 + O2 → 2H2O - Combustion of carbon:
C + O2 → CO2
Points to Note
① A coefficient (number of molecules) and a subscript (number of atoms) have completely different meanings. In 2H2O, 2 is the coefficient (number of molecules), while in H2, 2 is the subscript (number of H atoms). ② A subscript cannot be changed afterward (the substance itself would change). Only coefficients are used to balance the number of atoms. ③ The direction of the arrow shows the direction of the chemical change and differs in meaning from the mathematical equals sign “=”. ④ CO (carbon monoxide) and CO2 (carbon dioxide) are completely different substances.
Practice
- Write the chemical equation for the reaction of hydrogen and oxygen that produces water.
- Balance the coefficients in the following chemical equation:
__H2 + __N2 → __NH3 - Write the chemical equation for the combustion of carbon and name the reactant and product.