How to read, balance, and classify a chemical reaction
TEAS 7 outline: Chemical reactions
Also covers HESI A2 Chemistry (Reaction types)
- Reactants go left of the arrow; products go right.
- Atoms are never created or destroyed, so equations must balance.
- Balance by changing coefficients, never subscripts.
- Common types: synthesis, decomposition, single and double replacement, combustion.
- Exothermic releases heat (ΔH negative); endothermic absorbs heat (ΔH positive).
Key facts
- Reactants are written on the left of the arrow and products on the right.
- Coefficients (big numbers in front) can change to balance an equation; subscripts (small numbers inside a formula) cannot, because they define the substance.
- Balanced means the same number of each kind of atom on both sides, which follows from the law of conservation of matter.
Show all 7 key factsShow fewer
- Synthesis: A + B → AB. Decomposition: AB → A + B.
- Single replacement: one element replaces another in a compound. Double replacement: two compounds swap partners.
- Combustion is a fuel reacting rapidly with oxygen, releasing heat and often light; methane burns to carbon dioxide and water.
- Exothermic reactions release heat (ΔH negative); endothermic reactions absorb heat (ΔH positive).
Quick check1 question · 2 more at the end
Which equation shows a single-replacement reaction?
Correct answer: D.
In a single-replacement reaction, one element takes the place of another element in a compound, so each side has one element and one compound. Here zinc replaces hydrogen in hydrochloric acid, forming zinc chloride and releasing hydrogen gas. Compare the patterns: synthesis ends with a single product, double replacement swaps partners between two compounds, and combustion is a fuel reacting with oxygen.
A Two elements joining into one product is a synthesis reaction (A + B → AB), not a replacement.
B Methane reacting with oxygen to give carbon dioxide and water is combustion; no element replaces another in a compound.
C Two compounds swapping partners is double replacement (here a precipitation reaction), which has two compounds on each side.
D Correct: the element zinc replaces hydrogen in HCl, leaving hydrogen as a free element, which is the single-replacement pattern A + BC → AC + B.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: In a chemical reaction, reactants (left of the arrow) rearrange into products (right of the arrow). Atoms are never created or destroyed, so a balanced equation has the same number of each atom on both sides; you balance it by changing coefficients, never subscripts. Common types are synthesis, decomposition, single replacement, double replacement, and combustion.
1 What do the parts of a chemical equation mean?A chemical equation is a short sentence about a reaction. 1 min
A chemical equation is a short sentence about a reaction. The substances you start with, the reactants, go on the left. The substances that form, the products, go on the right. An arrow between them means "react to form."
Take methane burning:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
The large numbers in front are coefficients: they tell you how many units of each substance take part. The small numbers inside a formula are subscripts: they tell you how many atoms are in one unit, so they define what the substance is. The letters in parentheses give the physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
2 Why do equations have to be balanced?In a reaction, atoms are rearranged into new combinations, but none appear or disappear. 1 min
In a reaction, atoms are rearranged into new combinations, but none appear or disappear. This is the law of conservation of matter. So a correct equation must have the same number of each kind of atom on both sides. That also means the total mass of the reactants equals the total mass of the products.
Notice that atoms have to balance, but the number of molecules does not. In the methane equation there are 3 molecules on the left and 3 on the right. But in 2H₂ + O₂ → 2H₂O there are 3 on the left and only 2 on the right, and it is still balanced.
3 How do you balance an equation?A useful habit: balance an element that appears in several substances, often oxygen, last. 1 min
Use balancing by inspection:
- Write the unbalanced equation with correct formulas: CH₄ + O₂ → CO₂ + H₂O.
- Count each atom on both sides. Carbon: 1 and 1. Hydrogen: 4 and 2. Oxygen: 2 and 3.
- Change coefficients only. Put a 2 in front of H₂O to get 4 hydrogens on the right.
- Recount oxygen: the right now has 2 (in CO₂) + 2 (in 2H₂O) = 4. Put a 2 in front of O₂.
- Check every element once more and make sure the coefficients are the smallest whole numbers that work.
A useful habit: balance an element that appears in several substances, often oxygen, last. Never "fix" an equation by changing a subscript. Changing H₂O to H₂O₂ would balance the numbers but turn water into hydrogen peroxide, a different substance.
4 What are the main types of reactions? 1 min
| Type | Pattern | Example |
|---|---|---|
| Synthesis | A + B → AB | 2Na + Cl₂ → 2NaCl |
| Decomposition | AB → A + B | 2H₂O₂ → 2H₂O + O₂ |
| Single replacement | A + BC → AC + B | Zn + 2HCl → ZnCl₂ + H₂ |
| Double replacement | AB + CD → AD + CB | 2KI + Pb(NO₃)₂ → PbI₂ + 2KNO₃ |
| Combustion | Fuel + O₂ → products + heat | CH₄ + 2O₂ → CO₂ + 2H₂O |
Quick ways to tell them apart:
- Synthesis ends with one product, and decomposition starts with one reactant.
- In single replacement, a lone element kicks another element out of a compound (zinc takes hydrogen's place).
- In double replacement, two compounds swap partners. When one product is an insoluble solid, it is called a precipitation reaction.
- In combustion, a fuel reacts rapidly with oxygen and gives off heat and often light. Methane and other fuels made of carbon and hydrogen produce carbon dioxide and water when they burn completely.
5 What do exothermic and endothermic mean?An exothermic reaction releases heat to its surroundings; combustion is the classic example, giving off heat and light. 1 min
Reactions also move energy. An exothermic reaction releases heat to its surroundings; combustion is the classic example, giving off heat and light. Its enthalpy change, ΔH, is negative because the system loses heat.
An endothermic reaction absorbs heat from its surroundings, so ΔH is positive. An instant cold pack works this way: when the salt and water inside mix, the process absorbs heat, and the pack feels cold against your skin. Your skin is losing heat to the pack, not gaining "cold."
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Balance with coefficients, not subscripts; changing H₂O to H₂O₂ turns water into a different substance.
Watch out: Atoms must balance, not molecules; 2H₂ + O₂ → 2H₂O has 3 molecules on the left and 2 on the right, and that is fine.
Watch out: A negative ΔH means heat is released (exothermic), not absorbed.
Watch out: Single replacement has one element and one compound on each side; double replacement has two compounds on each side.
Watch out: A cold pack feels cold because it absorbs heat from your skin, not because it gives off cold.
How to remember it
Memory hook: "Coefficients count, subscripts say who." Coefficients tell you how many; subscripts tell you who the substance is, so they never change. For energy: exo = exit (heat leaves), endo = enter (heat comes in).
Self-check:
- Balance H₂ + O₂ → H₂O and explain why you cannot write H₂ + O₂ → H₂O₂ instead.
- Name the reaction type for each: AB → A + B; A + BC → AC + B; AB + CD → AD + CB.
- A reaction makes its test tube feel hot. Is it exothermic or endothermic, and what is the sign of ΔH?