Loading the lesson
Loading the lesson
Search AllRounder.ai
Search the courses, subjects, tracks, games and features, or jump straight to a page.
Search the courses, subjects, tracks, games and features, or jump straight to a page.
Learn it your way
A short class with Sarah and her students. The other ways are under More.
Sign up free to listen. Transcripts are free to read.
Magnesium ribbon burns with a brilliant white light and leaves a white powder, magnesium oxide. Who can write the unbalanced equation?
Mg + O₂ → MgO.
Good. Now count the oxygen atoms on each side.
Two on the left and one on the right. Could we just write MgO₂ so that it matches?
No. Changing a subscript changes the substance, so MgO₂ would be a different compound, not magnesium oxide. We may only change coefficients. Where should a 2 go?
In front of MgO. Then magnesium needs a 2 as well, so it is 2Mg + O₂ → 2MgO.
Exactly. Two magnesium atoms and two oxygen atoms on each side, so mass is conserved.
Ethane, C₂H₆, burns to give carbon dioxide and water. Balance carbon and hydrogen first.
Two carbons means 2CO₂, and six hydrogens means 3H₂O.
Now count the oxygen atoms on the right.
Four in the carbon dioxide and three in the water, so seven. That would need three and a half O₂ molecules.
Right. A final equation should use whole numbers, so what can we do?
Double everything: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Perfect. Check it: 4 carbons, 12 hydrogens and 14 oxygens on each side.
Lead(II) nitrate and potassium iodide give a yellow precipitate. Here is the skeleton: Pb(NO₃)₂ + KI → PbI₂ + KNO₃.
Do we count the nitrogen and oxygen atoms separately?
You can, but the nitrate group appears unchanged on both sides, so it is quicker to count it as one unit. How many nitrates are on each side?
Two on the left and one on the right, so we need 2KNO₃.
Then there are two potassiums on the right, so we write 2KI, and the iodine is two on each side too.
Excellent: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃. Treating the ion as a block saved several steps.
subscripts define the substance, so balancing adjusts only the numbers in front of formulas.
balance elements that appear on their own, such as O₂, H₂ or a metal, after the elements in compounds.
a group such as NO₃ or SO₄ that appears unchanged on both sides can be counted as one block.
when atom counts do not divide into each other, aim for their lowest common multiple.
if a half coefficient appears, multiply every coefficient by 2.
count every element on both sides and reduce the coefficients to their lowest whole-number ratio.
Iron burning in chlorine: Fe + Cl₂ → FeCl₃ balances as 2Fe + 3Cl₂ → 2FeCl₃ (Fe 2 = 2, Cl 6 = 6).
Marble chips in hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂, where one coefficient of 2 balances both hydrogen and chlorine.
Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, with carbon and hydrogen balanced first and oxygen last (18 oxygen atoms on each side).
Sodium in water: 2Na + 2H₂O → 2NaOH + H₂, where the H₂ molecule forces an even number of sodium atoms and water molecules.
Change the big number in front, never the small one below; count each atom left and right, and balanced it will show.
A lab technician packs atoms into boxes for delivery. She may send more boxes (coefficients) but must never repack what is inside a box (subscripts). The order is only accepted when every atom that left the warehouse arrives at the customer.
MINHO gives a useful balancing order: Metals, then Ions (polyatomic groups), then other Non-metals, then Hydrogen, and Oxygen last.
C.L.I.C.K. - Coefficients only, Leave free elements last, Ions as units, Clear any fractions, Keep checking the atom count.
Term
Why can't you change H₂O to H₂O₂ to balance an equation?
Click to flip
Term
Which elements are usually balanced last?
Click to flip
Term
Balance C₂H₆ + O₂ → CO₂ + H₂O
Click to flip
Term
What do you do when a coefficient comes out as 7/2?
Click to flip
Term
Balance KClO₃ → KCl + O₂
Click to flip
Term
Is 4H₂ + 2O₂ → 4H₂O acceptable as a final answer?
Click to flip
A whole number written in front of a formula in an equation; it multiplies every atom in that formula.
A small number inside a formula showing how many atoms of an element are in one unit; it is never changed when balancing.
A charged group of atoms, such as nitrate (NO₃⁻) or sulfate (SO₄²⁻), that often stays together during a reaction.
A reaction of a substance with oxygen that releases energy; complete combustion of a hydrocarbon produces carbon dioxide and water.
An insoluble solid that forms when two solutions are mixed.
A reaction in which one substance breaks down into two or more simpler substances.
The labels (s), (l), (g) and (aq) showing whether a substance is a solid, a liquid, a gas or dissolved in water.
The simplest set of whole-number coefficients that still balances the equation.
This lesson balances chemical equations through worked examples of increasing difficulty, from magnesium burning in oxygen to the combustion of ethane and reactions that contain polyatomic ions.
Balancing is a skill you learn by doing. Here the four steps from the previous lessons (write the unbalanced equation, adjust the coefficients, check the atom count, simplify the ratio) are applied to four worked problems: a metal burning, a precipitation reaction, a hydrocarbon combustion and a decomposition. Along the way you meet the strategies that make harder equations manageable: balancing free elements last, treating polyatomic ions as single units, using the lowest common multiple and doubling every coefficient to clear a fraction.
A balanced equation has the same number of atoms of each element on both sides, as the law of conservation of mass requires. Only the coefficients (the numbers in front of formulas) may change. The subscripts inside a formula are fixed, because changing them changes the substance: H₂O is water, but H₂O₂ is hydrogen peroxide.
Worked problem 1: magnesium burning in oxygen Word equation: magnesium + oxygen → magnesium oxide
Worked problem 2: a polyatomic ion that stays together Lead(II) nitrate solution and potassium iodide solution react to form a yellow precipitate of lead(II) iodide.
Worked problem 3: combustion of ethane and the doubling trick Ethane burns completely in oxygen to form carbon dioxide and water.
Worked problem 4: a decomposition reaction Heating potassium chlorate produces potassium chloride and oxygen.
Strategies these problems show