Atoms in Chemical Reactions: Mastery Check · Chapter 13

Learning target: I can count atoms in a given equation, sort a reaction by how its atoms regroup, predict and explain balance readings in open and closed systems, and compute the efficiency of a chemical process from mass data.

Retrieval and mastery checkAbout 35 minIndividual MS-PS1-5 · assessesMS-ETS1-3 · practice independent practicewritingvisual modeling

Directions

Work alone without notes. Part A has twelve short items; answer every one, and show your calculation on any numeric item. Part B is one synthesis paragraph. The equations on this quiz are correct as written; your job is to count, not to change them. Points are shown for each item; the quiz is worth 30 points.

You will need: This worksheet, Pencil

Part A: Retrieval items (24 points)

1.In the equation 4Fe + 3O₂ → 2Fe₂O₃ (steel wool burning), which substances are the reactants?1 pt

A. Fe₂O₃ only
B. Fe and O₂
C. O₂ and Fe₂O₃
D. Fe only

2.In the same equation, how many oxygen atoms are in 3O₂, and how many are in 2Fe₂O₃? Show the multiplication for each.2 pts

3.In 2NH₃ (two ammonia molecules), the small 3 is a:1 pt

A. coefficient; it counts molecules
B. subscript; it counts hydrogen atoms in each molecule
C. subscript; it counts nitrogen atoms in each molecule
D. coefficient; it counts atoms in total

4.True or false: During a chemical reaction, atoms break apart into smaller pieces, and the pieces build the products.1 pt

○ True ○ False Explain your thinking:

5.Tally the atoms in 2Mg + O₂ → 2MgO (magnesium burning; MgO is an extended structure counted in repeating units).3 pts

Atoms before (reactants)	Atoms after (products)	Same? (yes/no)
Mg
O
Total

6.Sort each reaction by how its atoms regroup: breaking apart, joining together, or swapping partners. (a) 2H₂O₂ → 2H₂O + O₂ (b) 2Mg + O₂ → 2MgO (c) AgNO₃ + NaCl → AgCl + NaNO₃. Give a one-phrase reason for each.3 pts

7.A fizzing reaction runs inside a sealed rigid bottle on a balance. Before: 240.0 g. What does the balance read after the fizzing stops? Explain in one sentence.2 pts

8.The same reaction runs in an open dish. Before: 85.6 g. After: 84.9 g. How much gas left the dish? Show the subtraction and name what carried the mass away.2 pts

9.Steel wool reads 2.00 g before burning and 2.14 g after. The best explanation for the extra 0.14 g is:1 pt

A. heat from the flame was stored in the metal
B. oxygen atoms from the air bonded to the iron atoms
C. the balance drifted while the metal was hot
D. iron atoms were created by the reaction

10.Nitrogen and hydrogen react to form ammonia: N₂ + 3H₂ → 2NH₃. Draw a BEFORE box and an AFTER box. Draw every atom as a labeled circle (N or H) with lines for bonds: nitrogen gas is two N atoms bonded together, hydrogen gas is two H atoms bonded together, and each ammonia molecule is one N atom bonded to three H atoms. Write the tally (N = ___, H = ___) under each box.4 pts

11.A process uses 300 kg of reactants per batch and produces 120 kg of the wanted product; everything else is waste. (a) How much waste is made per batch? (b) What is the efficiency, as the percentage of reactant mass that becomes wanted product? Show both calculations.2 pts

12.One sentence states the law of conservation of matter. Another sentence explains why it is true. Write both sentences and label which is the law and which is the explanation.2 pts

Part B: Synthesis (6 points)

13.A cup of vinegar and baking soda on a balance lost 1.1 g while it fizzed. A pad of steel wool on a balance gained 0.14 g while it burned. Explain both readings using one rule about atoms. Your answer must: say what crossed the boundary in each case and in which direction; state what would happen to each reading inside a sealed container; and explain why a chemical engineer who wants less waste cares about this same rule.6 pts

Before you start

Counting rule: multiply the big number in front (coefficient) by the small number after a symbol (subscript); a symbol with no subscript counts as 1. Add every element across the whole side of the arrow before you compare sides. Reading rule: in a sealed, rigid container the balance reading does not change; in an open one, the reading falls if a gas leaves and rises if atoms from the air join a solid. Efficiency rule: wanted product mass ÷ reactant mass × 100.

Write the hardest fair item

Write one quiz item of your own that a classmate could only answer by using the atom model, not by memory. It must include an equation (already correct), a sealed or open container, and a balance reading, and it must have a single defensible numerical answer. Write the full answer key for it, including the calculation and one wrong answer a student might give with the misconception that produces it.