Balancing, the five kinds of reaction, ionic compounds in water, the solubility rules, and a precipitation reaction written three ways. The in-class sheet, practice, rounds of ten, and an atom counter for the homework.
Chapter 4, part 1
Chemical equations and precipitation reactions, from slides 2 to 15 of the Chapter 4 partial notes and the first lecture video.
Homework: Writing and Balancing Chemical Equations and Classifying Chemical Reactions, in the Chapter 4 folder.
A chemical equation says what reacts and what forms. The formulas say which substances, the coefficients in front say how many of each, and the states say what form each one is in.
O₂(g) + 2NO(g) → 2NO₂(g)
Reactants on the left, products on the right, and the arrow reads “react to form”.
The 2 in 2NO means two molecules, or two moles. With no number in front, the coefficient is 1.
(s) solid, (l) liquid, (g) gas, (aq) dissolved in water. Heat is a Δ above the arrow.
Write the skeleton equation with the correct formula for every substance. Hydrogen gas reacts with oxygen gas to form steam:
H₂(g) + O₂(g) → H₂O(g)
Balance the atoms with coefficients. Oxygen is 2 on the left and 1 on the right, so put a 2 in front of the water. That makes 4 hydrogens on the right, so put a 2 in front of the hydrogen.
2H₂(g) + O₂(g) → 2H₂O(g)
Make the coefficients whole numbers if they are not, and check every element again: 4 H and 2 O on each side.
Give the states and any conditions, such as Δ above the arrow when heat is needed.
Changing a subscript makes a different substance: H₂O₂ is hydrogen peroxide, not water.
The equation has the substances the reaction has. Only the coefficients move.
Start with the compound with the most atoms and save elements, such as O₂, for last: they change nothing else.
A polyatomic ion found on both sides, such as PO₄³⁻ or SO₄²⁻, can be balanced as one unit.
Ethane burns in oxygen. Carbon and hydrogen first: 2 C means 2CO₂, and 6 H means 3H₂O.
C₂H₆ + O₂ → 2CO₂ + 3H₂O
That leaves 4 + 3 = 7 oxygen atoms on the right, which is 7/2 O₂. A fraction is fine for a moment.
C₂H₆ + 72O₂ → 2CO₂ + 3H₂O
Double everything to clear the fraction, step 3 of the notes.
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
A soluble ionic compound dissociates: it comes apart into its ions, and water molecules gather round each one. That is solvation.
Na₂SO₄(s) → 2Na⁺(aq) + SO₄²⁻(aq)
The 2 in Na₂SO₄ says two sodium ions: 2Na⁺.
Sulfate stays SO₄²⁻. It does not fall apart into sulfur and oxygen.
2(+1) + (−2) = 0, the same as the neutral compound on the left.
There is no way to predict solubility from the charges alone, so the course uses rules drawn from observation: Table 4.1 of the notes.
Soluble
Insoluble
Mercury(I) is the pair Hg₂²⁺, which is why the 2 matters. The Tools tab looks up any pair of ions against these six rules.
Two solutions of soluble ionic compounds are mixed, and an insoluble product forms: the precipitate. Its forming is the driving force, because it takes ions out of the solution.
Note the ions in each solution. Silver nitrate and sodium chromate:
Ag⁺, NO₃⁻ and Na⁺, CrO₄²⁻
Swap partners, as in a double displacement, and write the new compounds with the charges balanced.
Ag₂CrO₄ and NaNO₃
Use the solubility rules. Sodium nitrate dissolves (rules 1 and 2). Chromate is not in the table; silver chromate is the red-brown precipitate.
A balanced ionic or net ionic equation has the same atoms on each side and the same total charge on each side.
Only soluble ionic compounds, written (aq), are written as ions. Solids, liquids and gases stay whole.
If both new combinations are soluble, every ion is a spectator and nothing happens.
Sodium iodide with lead(II) nitrate. Swap partners: lead(II) iodide and sodium nitrate. Iodides are soluble except with Pb²⁺ (rule 3), so lead(II) iodide is the precipitate.
2NaI(aq) + Pb(NO₃)₂(aq) → PbI₂(s) + 2NaNO₃(aq)
Split the three soluble compounds into ions.
2Na⁺(aq) + 2I⁻(aq) + Pb²⁺(aq) + 2NO₃⁻(aq) → PbI₂(s) + 2Na⁺(aq) + 2NO₃⁻(aq)
Cancel the spectators, 2Na⁺ and 2NO₃⁻.
Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)
The questions from the Chemical Equations & Precipitation Reactions in-class sheet. Each part is checked on its own, and the working shows once you have it or ask for it.
A fresh problem every time. Wrong answers get a reason; the working shows when you finish.
Start another round for a fresh set of ten.
Helpers for the homework. They count and look up; the coefficients and the decisions stay yours.
Type any equation, then try coefficients in the boxes. The table counts each element and the charge on both sides as you go, and marks the rows that differ.
Pick a cation and an anion to see the compound they make and which of the six rules decides whether it dissolves.