Chem Rxns: 19 MCQs with answers

Rusting of iron

Rusting of iron is a chemical change where iron reacts with oxygen and moisture to form a new substance (iron oxide). The other options are physical changes.

2

The coefficient is the number before the chemical formula. In 2H₂, the coefficient is 2, indicating 2 molecules of hydrogen gas.

2Mg + O₂ → 2MgO

In a combination reaction, two or more reactants combine to form a single product. Magnesium and oxygen combine to form magnesium oxide.

All of the above

Chemical reactions can be identified by various observations including change in state, color, evolution of gas, or formation of a precipitate.

Decomposition reaction

Water (H₂O) breaks down into hydrogen and oxygen gases, which is a decomposition reaction (specifically, electrolysis of water).

Zn + 2HCl → ZnCl₂ + H₂

Zinc (Zn) displaces hydrogen from hydrochloric acid (HCl) to form zinc chloride and hydrogen gas. A more reactive element displaces a less reactive one.

CaO

Quicklime is calcium oxide, with the chemical formula CaO. It is produced by the decomposition of limestone (CaCO₃).

True

Balancing ensures that atoms are neither created nor destroyed, which is the law of conservation of mass.

False

In a decomposition reaction, a single compound breaks down into two or more simpler products. Combination reactions involve reactants combining.

2H₂ + O₂ → 2H₂O

In 2H₂ + O₂ → 2H₂O, the number of atoms of each element is equal on both sides: 4 hydrogen atoms and 2 oxygen atoms on each side.

To satisfy the law of conservation of mass

Balancing ensures that the number of atoms of each element is the same on both sides, obeying the law of conservation of mass.

Decomposition reaction

A single compound (CaCO₃) breaks down into two or more simpler products (CaO and CO₂), which is a decomposition reaction.

Copper

Iron (Fe) is more reactive than copper (Cu), so it displaces copper from its salt solution (CuSO₄). Copper is the element displaced.

AgNO₃ + NaCl → AgCl + NaNO₃

In a double displacement reaction, two compounds exchange ions. Here, Ag⁺ and Na⁺ exchange places to form AgCl and NaNO₃.

A yellow precipitate of lead iodide is formed

Lead nitrate reacts with potassium iodide to form a yellow precipitate of lead iodide (PbI₂) and potassium nitrate. This is a double displacement reaction.

CuO + H₂ → Cu + H₂O

In CuO + H₂ → Cu + H₂O, copper is reduced (Cu²⁺ to Cu) and hydrogen is oxidized (H₂ to H₂O). This is a redox reaction.

4Fe + 3O₂ → 2Fe₂O₃

Balancing iron and oxygen: 4 iron atoms on the left and 4 on the right (2×2), and 6 oxygen atoms on the left (3×2) and 6 on the right (2×3). Thus, 4Fe + 3O₂ → 2Fe₂O₃ is balanced.

+3

In Fe₂O₃, oxygen has an oxidation number of -2. The total for 3 oxygen atoms is -6. To balance, the two iron atoms must have a total of +6, so each iron is +3.

2Na + 2H₂O → 2NaOH + H₂

To balance, we need 2 sodium atoms on both sides, 2 hydrogen atoms from water on the left, and 2 hydrogen atoms on the right (1 in NaOH and 1 in H₂). Thus, 2Na + 2H₂O → 2NaOH + H₂ is correct.