Unit 4 · Topic 4.1 Beta

Introduction for Reactions

A chemical reaction rearranges atoms into new substances.

Practice 1: Models and RepresentationsPractice 5: Mathematical Routines

Question set for this topic

Part 1 · Hook

Why this matters

Strike a match and the head flares, smokes and leaves a black stub. The match is lighter afterward, yet not one atom has been destroyed. The atoms were rearranged into new substances, and some of them drifted away as gases. Every chemical equation you write in this unit is a record of that bookkeeping.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. How many atoms are in one molecule of C₂H₆O?

  1. 9
  2. 3
  3. 6
  4. 8
Show the answer

Add the subscripts: 2 C + 6 H + 1 O = 9 atoms.

  • Correct: 9:
  • 3:
  • 6:
  • 8:

2. What is the molar mass of water, H₂O (H 1.008, O 16.00)?

  1. 18.02 g/mol
  2. 17.01 g/mol
  3. 16.00 g/mol
  4. 34.02 g/mol
Show the answer

2(1.008) + 16.00 = 18.02 g/mol.

  • Correct: 18.02 g/mol:
  • 17.01 g/mol:
  • 16.00 g/mol:
  • 34.02 g/mol:

3. What does the label (aq) after a formula mean?

  1. Dissolved in water
  2. A pure liquid
  3. A gas
  4. A solid
Show the answer

Aqueous means dissolved in water.

  • Correct: Dissolved in water:
  • A pure liquid:
  • A gas:
  • A solid:

Part 4 · See it

See it first

The equation N₂(g) + 3H₂(g) → 2NH₃(g). Labels point to the coefficients 3 and 2 (how many particles or moles; change these to balance), to a subscript (never change it) and to a formula (it names the substance). Reactants are on the left and the product on the right. The atom count is 2 N and 6 H on each side, so mass is conserved.
The parts of a balanced equation. Coefficients count particles (and moles); subscripts are part of each formula and never change. Each side holds the same atoms: 2 N and 6 H. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Bonds break in the reactants and new bonds formthe same atoms end up arranged into new substances
  2. Atoms are rearranged, never created or destroyedeach kind of atom has the same count on both sides of the equation
  3. Every atom keeps its massthe total mass before a reaction equals the total mass after, if nothing escapes
  4. Subscripts say what each substance isyou balance with coefficients only

Part 6 · Key ideas

Key ideas

  • In a chemical reaction, bonds break and form so the atoms of the reactants end up in new substances, the products.
  • Evidence (a gas, a new solid, a color or temperature change) suggests a reaction; only a new substance proves one.
  • A balanced chemical equation has the same number of each kind of atom on both sides. Change coefficients, never subscripts.
  • Coefficients count particles and moles, not grams. Conservation of mass: total mass is unchanged when nothing can escape.

Part 7 · Misconception

A common mistake

The wrong idea: If the products weigh less than the reactants, matter was destroyed.

What actually happens: Matter is conserved in every reaction. A drop in measured mass means a product, usually a gas, left the container. Seal the container and the mass stays the same.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Particle view

Before and after a reaction

HHHHNNHHHHHHNNHHBeforeNHHHNHHHNHHHNHHHAfter

Key: large blue circle labeled N, nitrogen atom; small white circle labeled H, hydrogen atom.

1. Which balanced equation matches the change shown in the two boxes?

  1. N₂ + 3H₂ → 2NH₃
  2. N₂ + H₂ → NH₃
  3. 2N₂ + 6H₂ → 4NH₆
  4. N + 3H → NH₃
Show the answer

The boxes show 2 N₂ and 6 H₂ becoming 4 NH₃. Dividing by 2 gives the smallest whole-number ratio, 1 : 3 : 2, so N₂ + 3H₂ → 2NH₃. Each side has 2 N and 6 H.

  • Correct: N₂ + 3H₂ → 2NH₃: Right: 2 N and 6 H on each side, in the smallest whole-number ratio.
  • N₂ + H₂ → NH₃: Unbalanced: 2 N and 2 H on the left but 1 N and 3 H on the right, so atoms would appear and vanish.
  • 2N₂ + 6H₂ → 4NH₆: Changes the formula of ammonia to NH₆ to make the atoms fit. Subscripts are fixed by what the substance is; only coefficients may change.
  • N + 3H → NH₃: Writes nitrogen and hydrogen as single atoms, but the diagram shows them as two-atom molecules, N₂ and H₂.

2. Count the hydrogen atoms in the After box.

Type a number in atoms.

Show the answer

Four NH₃ molecules × 3 H atoms each = 12 H atoms, the same as the 6 H₂ × 2 = 12 H atoms before. Atoms are conserved.

  • Answer: 12 atoms

3. Which quantities are the same in the Before box and the After box? Select all that apply.

  1. The number of nitrogen atoms
  2. The total number of molecules
  3. The total mass of the contents
  4. The kinds of molecules present
Show the answer

Atoms are rearranged, not created or destroyed, so the number of each kind of atom and the total mass stay the same. The molecules themselves change, and so does their count (8 before, 4 after).

  • Correct: The number of nitrogen atoms: Right: 4 N atoms before and 4 after.
  • The total number of molecules: There are 8 molecules before and 4 after. Molecules are not conserved; atoms are.
  • Correct: The total mass of the contents: Right: the same atoms are present, so the mass is the same.
  • The kinds of molecules present: N₂ and H₂ are replaced by NH₃: new substances form, which is what makes this a reaction.

4. A student says the reaction must have destroyed matter, because the After box holds fewer particles. What is the best reply?

  1. All 16 atoms are still present, now joined into fewer, larger molecules
  2. Matter is lost as energy, which is why the After box has fewer particles
  3. Half the hydrogen escaped as a gas, which the diagram leaves out
  4. The student is right, because each molecule counts as one unit of matter
Show the answer

Count atoms, not molecules: 4 N + 12 H = 16 atoms before and 16 after. The atoms regrouped into four larger molecules.

  • Correct: All 16 atoms are still present, now joined into fewer, larger molecules: Right: the same 16 atoms are there in fewer, larger molecules.
  • Matter is lost as energy, which is why the After box has fewer particles: Ordinary reactions do not turn measurable mass into energy; every atom is still in the box.
  • Half the hydrogen escaped as a gas, which the diagram leaves out: Every hydrogen atom from the Before box is inside an NH₃ molecule after; none is missing.
  • The student is right, because each molecule counts as one unit of matter: Molecules are not the unit that is conserved; atoms are, and their number is unchanged.

Data table

Weighing a reaction

A student mixes 2.00 g of solid sodium hydrogen carbonate, NaHCO₃, with 50.0 mL of vinegar on an electronic balance. Bubbles form and the container feels slightly cold. Trial 1 uses an open flask. In Trial 2 the solid sits in a small cup inside a stiff plastic bottle; the bottle is capped tightly, weighed, and then tipped so the solid falls into the vinegar.

Mass of container and contents
TrialContainerMass before mixing (g)Mass after bubbling stops (g)
1Open148.62147.58
2Capped bottle151.07151.07

5. What mass of gas left the open flask in Trial 1?

Type a number and its unit.

Show the answer

148.62 g − 147.58 g = 1.04 g. When you subtract, keep the decimal places of the measurements (two here), which gives three significant figures.

  • Answer: 1.04 g

6. Which claim do the two trials best support?

  1. Mass is conserved; the open flask lost mass because a gas product escaped
  2. The reaction destroys about 1 g of matter whenever the flask is left open
  3. The cap added mass to Trial 2, which hid a loss of mass
  4. The reactants weigh more than the products, as Trial 1 shows
Show the answer

In the capped bottle, nothing could leave and the mass did not change. The open flask lost 1.04 g because the gas product (carbon dioxide) left the flask. Both trials agree with conservation of mass once every product is counted.

  • Correct: Mass is conserved; the open flask lost mass because a gas product escaped: Right: the capped trial shows no change; the open trial lost only the escaping gas.
  • The reaction destroys about 1 g of matter whenever the flask is left open: Whether a flask is open cannot change whether matter is conserved; it changes whether a product can leave the balance.
  • The cap added mass to Trial 2, which hid a loss of mass: The cap was on the bottle for both weighings in Trial 2, so it adds the same mass before and after.
  • The reactants weigh more than the products, as Trial 1 shows: Trial 1 did not weigh all the products: the gas left. Trial 2, which kept everything, shows no difference.

Part 9 · Summary

Summary

A chemical reaction rearranges atoms into new substances. Because atoms are conserved, a balanced equation has the same atoms on both sides and the total mass does not change; balance with coefficients, which count particles and moles, never with subscripts.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections