Chemical Reaction Problems- Exercises with Answers
Chemical Reaction Problems: Exercises with Answers
Chemical reaction problems show up everywhere in chemistry—from homework to exams to real applications. If you're struggling with them, the solution isn't motivational quotes. It's practice, and knowing the specific moves that actually work.
This guide gives you both. Real problems, real answers, and the reasoning behind each one. No filler.
Why Chemical Reaction Problems Trip People Up
Most students fail chemical reaction problems for the same reasons:
- They don't balance equations properly before solving
- They confuse reaction types and apply the wrong formulas
- They skip units or forget to convert them
- They memorize steps without understanding why those steps work
The fix is straightforward: learn the patterns, then drill them until they're automatic.
The Four Main Types of Chemical Reactions
Before diving into problems, you need to know which reaction type you're dealing with. Each has its own pattern.
1. Combination (Synthesis) Reactions
Two or more substances combine to form a single product.
General form: A + B → AB
Example: 2H₂ + O₂ → 2H₂O
2. Decomposition Reactions
A single compound breaks down into simpler substances.
General form: AB → A + B
Example: 2H₂O → 2H₂ + O₂
3. Single Replacement (Single Displacement) Reactions
One element replaces another in a compound.
General form: A + BC → AC + B
Example: Zn + 2HCl → ZnCl₂ + H₂
4. Double Replacement (Double Displacement) Reactions
The positive ions of two compounds switch places.
General form: AB + CD → AD + CB
Example: AgNO₃ + NaCl → AgCl + NaNO₃
Comparison of Reaction Types
| Reaction Type | General Pattern | Key Indicator |
|---|---|---|
| Combination | A + B → AB | Two reactants, one product |
| Decomposition | AB → A + B | One reactant, two or more products |
| Single Replacement | A + BC → AC + B | One element + one compound |
| Double Replacement | AB + CD → AD + CB | Two compounds swap ions |
How to Balance Chemical Equations
Balancing is step one for almost every reaction problem. Get this wrong, and everything else falls apart.
The Process
- Write the unbalanced equation using chemical formulas
- Count atoms of each element on both sides
- Add coefficients (whole numbers) in front of formulas to make atom counts equal
- Check your work — recount every element
Rule: Never change subscripts. Only add coefficients.
Balancing Example
Unbalanced: Fe + O₂ → Fe₂O₃
Count atoms: Fe (1 left, 2 right), O (2 left, 3 right)
Balance iron first: 2Fe + O₂ → Fe₂O₃
Now balance oxygen: 2Fe + 3/2 O₂ → Fe₂O₃
Multiply everything by 2: 4Fe + 3O₂ → 2Fe₂O₃
Check: 4 Fe on each side ✓, 6 O on each side ✓
Practice Problems with Answers
Problem 1: Combination Reaction
Predict the product and balance:
Na + Cl₂ → ?
Answer: 2Na + Cl₂ → 2NaCl
Explanation: Sodium (Na) combines with chlorine gas (Cl₂) to form sodium chloride (NaCl). Balance by adding coefficient 2 in front of Na and NaCl.
Problem 2: Decomposition Reaction
Predict the products and balance:
HgO → ?
Answer: 2HgO → 2Hg + O₂
Explanation: Mercury(II) oxide decomposes into mercury metal and oxygen gas. The oxygen atoms pair up as O₂.
Problem 3: Single Replacement
Predict the products and balance:
Mg + HCl → ?
Answer: Mg + 2HCl → MgCl₂ + H₂
Explanation: Magnesium replaces hydrogen in hydrochloric acid. Magnesium is more reactive, so the reaction proceeds. Balance by adding coefficient 2 to HCl.
Problem 4: Double Replacement
Predict the products and balance:
Pb(NO₃)₂ + KI → ?
Answer: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
Explanation: Lead nitrate reacts with potassium iodide. Lead and potassium swap partners. Lead iodide forms a yellow precipitate. Balance by adding coefficient 2 to KI and KNO₃.
Problem 5: Stoichiometry Problem
How many grams of water form when 4g of hydrogen reacts completely with oxygen?
2H₂ + O₂ → 2H₂O
Answer: 36 grams of H₂O
Step-by-step:
- Molar mass H₂ = 2 g/mol. Moles H₂ = 4g ÷ 2 g/mol = 2 mol
- From equation: 2 mol H₂ produces 2 mol H₂O (1:1 ratio)
- Moles H₂O = 2 mol
- Molar mass H₂O = 18 g/mol
- Mass H₂O = 2 mol × 18 g/mol = 36 g
Problem 6: Limiting Reagent Problem
10g of iron reacts with 8g of sulfur. Which is limiting? What mass of FeS forms?
Fe + S → FeS
Answer: Iron is limiting. 13.7 g of FeS forms.
Step-by-step:
- Moles Fe = 10g ÷ 55.85 g/mol = 0.179 mol
- Moles S = 8g ÷ 32.07 g/mol = 0.249 mol
- 1:1 ratio needed. Fe runs out first → Fe is limiting
- Moles FeS = 0.179 mol
- Molar mass FeS = 87.91 g/mol
- Mass FeS = 0.179 × 87.91 = 13.7 g
Common Mistakes That Will Cost You Points
- Forgetting to balance before calculating — always check balance first
- Ignoring state symbols — (s), (l), (g), (aq) matter in some problems
- Using wrong molar mass — double-check your periodic table values
- Dropping coefficients when calculating moles — the coefficient matters
- Assuming equal masses = equal moles — they almost never are
Quick Reference: What to Do First
When you see any chemical reaction problem:
- Identify the reaction type
- Write the correct formulas for all compounds
- Balance the equation
- Convert given information to moles if needed
- Use mole ratios from the balanced equation
- Convert back to the requested units
That's it. No magic, no extra steps. Just follow the checklist.
The only way to get faster is to do more problems. This guide has the patterns. Use them.