The Practical Notebook with Four Equations
During a Class 10 science practical, a student performs four reactions and writes the equations in her notebook.
Tube A: a cleaned magnesium ribbon is burnt in air; dense white smoke and a white powder are produced. Tube B: zinc granules are added to dilute sulphuric acid; brisk effervescence occurs and the gas burns with a pop. Tube C: green copper(II) carbonate is heated in a dry boiling tube; it turns black and the gas produced turns limewater milky. Tube D: sodium sulphate solution is added to barium chloride solution; a thick white precipitate forms.
Her notebook contains: A: Mg + O2 → MgO B: Zn + H2SO4 → ZnSO4 + H2 C: CuCO3 → CuO + CO2 D: Na2SO4 + BaCl2 → BaSO4 + NaCl
The teacher says two equations are not correctly written and that the observations alone do not decide the type of reaction. The student started with 2.40 g of magnesium and obtained 3.90 g of white powder; the school balance reads to 0.1 g.
1. Identify which two equations are incorrect and write the correct balanced equations. For each of the four reactions, state its type (combination, decomposition, displacement or double displacement) and give the observation that supports your choice.
Read the worked answer
An equation is correct only when every element has the same number of atoms on both sides.
A: Left has 1 Mg and 2 O; right has 1 Mg and 1 O. Multiply MgO by 2: 2Mg + O2 → 2MgO. This is a combination reaction - two elements combine to form one compound - supported by a silvery ribbon turning into a white powder.
B: 1 Zn, 2 H, 1 S and 4 O on each side, so it is already balanced. It is a displacement reaction: zinc, being more reactive, displaces hydrogen from the acid. Zinc dissolves and the gas burns with a pop.
C: 1 Cu, 1 C and 3 O on each side, so it is balanced. It is a decomposition reaction: one reactant gives two products. The green solid turns black and the gas turns limewater milky.
D: Left has 2 Na and 2 Cl, right has 1 Na and 1 Cl. Write Na2SO4 + BaCl2 → BaSO4 + 2NaCl. This is a double displacement reaction - ions are exchanged - supported by the thick white precipitate of BaSO4.
2. The teacher says a gas is produced in both B and C, yet only C is a decomposition. Explain, using the number and nature of reactants and products, how reactions are classified. How would you confirm the gas in B and in C?
Read the worked answer
Classification depends on what goes in and what comes out, not on whether a gas is seen.
In B, the reactants are an element (Zn) and a compound (H2SO4). Zinc takes the place of hydrogen in the compound, so one element displaces another: this fits A + BC → AC + B. It is a displacement reaction. Hydrogen happens to be a product, but that does not make the reaction a decomposition.
In C, there is a single reactant, CuCO3, which breaks into two products, CuO and CO2. This fits A → B + C, the defining pattern of decomposition. The gas is only a symptom, not the criterion.
The gases are different. Gas from B is hydrogen: colourless, burns with a pop. Gas from C is carbon dioxide: it does not burn and turns limewater milky. Passing each gas through limewater separates them at once.
3. Using Mg = 24 and O = 16, calculate the theoretical mass of MgO from 2.40 g of magnesium. The student obtained 3.90 g. Is this acceptable? Give two reasons for the difference and state whether the law of conservation of mass is violated.
Read the worked answer
Moles of Mg = 2.40/24 = 0.10 mol. From 2Mg + O2 → 2MgO, 2 mol Mg give 2 mol MgO, so 0.10 mol Mg gives 0.10 mol MgO.
Mass of MgO = 0.10 × (24 + 16) = 0.10 × 40 = 4.0 g.
The student's 3.90 g is 97.5% of this - acceptable for a school balance reading to 0.1 g, and normal for an open-air experiment. Two reasons for the shortfall: (i) some white MgO smoke escaped with the air current before it could be collected; (ii) a little magnesium remained unburnt, or product was left sticking to the tube or spatula. In air a small amount of magnesium nitride, Mg3N2, may also form; it gives less product per gram of magnesium than MgO does.
The law of conservation of mass is not violated. Atoms are neither created nor destroyed; the missing product simply went into the surroundings and was not weighed. In a closed vessel, total mass before and after would be equal.