Chemical Reactions — Science

In a chemical change no atoms are created or destroyed — the mass you start with must equal the mass of everything you end with, gases included.

The starting solid weighed 8.4 g and the solid left behind weighs 5.3 g. The 'missing' mass didn't vanish — it left as gas.

Escaped gas = starting mass − solid remaining = 8.4 − 5.3.

3.1 g

Conservation of mass: total starting mass = total ending mass (solid left + gas released).

The 8.4 g of NaHCO₃ becomes 5.3 g of solid Na₂CO₃ plus the escaping gases, so gas mass = 8.4 − 5.3.

Mass of gas escaped = 3.1 g.

Balance the metal and the oxygen so each element has equal atoms on both sides; oxygen is trickiest because O₂ comes in pairs while Fe₂O₃ holds 3 oxygens.

Try 2 Fe₂O₃ on the right: that needs 4 Fe atoms and 6 O atoms, and six O atoms come from 3 O₂.

The balanced equation is 4Fe + 3O₂ → 2Fe₂O₃; now add 4 + 3 + 2.

9

Put a 2 in front of Fe₂O₃ to make the oxygen count even: 2Fe₂O₃ has 6 O atoms and 4 Fe atoms.

Supply 6 O atoms with 3 O₂ and 4 Fe atoms with 4 Fe: 4Fe + 3O₂ → 2Fe₂O₃ (check: Fe 4 = 4, O 6 = 6).

Sum of coefficients = 4 + 3 + 2 = 9.

Balance carbon and hydrogen first (each appears in only one product) and leave oxygen for last, since O₂ is easy to adjust.

One C₄H₁₀ gives 4 CO₂ and 5 H₂O — but that needs an odd number of oxygen atoms, so double the whole equation to keep the coefficients whole.

With 2C₄H₁₀ you need 8 CO₂ and 10 H₂O; count the oxygen atoms on the right, then divide by 2.

13

One butane has 4 C and 10 H, needing 4 CO₂ and 5 H₂O — that requires (4×2) + (5×1) = 13 O atoms, or 6.5 O₂, which isn't whole.

Double everything: 2C₄H₁₀ → 8CO₂ + 10H₂O. Right-side oxygen = (8×2) + (10×1) = 26 atoms.

O₂ supplies 2 atoms each, so you need 26 ÷ 2 = 13. The balanced equation is 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O, so the coefficient of O₂ is 13.

In a single-replacement reaction a lone element and a compound react, and the element swaps places with part of the compound — look for a free element on the left AND a different free element on the right.

Synthesis combines two things into one; decomposition breaks one into two; double-replacement swaps partners between two compounds.

Which reaction shows one pure element displacing another element out of its compound, leaving a new element free?

Zn + 2HCl → ZnCl₂ + H₂

Choice A combines two elements into one product (synthesis); choice B breaks one compound into two (decomposition); choice D swaps ions between two compounds (double-replacement).

In choice C the element zinc displaces hydrogen from HCl, freeing H₂ — a lone element in, a different lone element out.

The single-replacement reaction is Zn + 2HCl → ZnCl₂ + H₂ (choice C).

Every reaction is a two-part energy budget: breaking old bonds costs energy (counts as positive), forming new bonds pays energy back (counts as negative).

ΔH = (energy to break bonds) − (energy released forming bonds) = 620 − 780.

If more energy comes out than went in, ΔH is negative and the reaction is exothermic.

-160 kJ

Energy in to break bonds = +620 kJ; energy out forming bonds = 780 kJ.

ΔH = 620 − 780 = −160 kJ; the negative sign means more energy is released than absorbed.

ΔH = −160 kJ (exothermic).

Balance the chlorine first: it appears as CuCl₂ (2 Cl) on the left and AlCl₃ (3 Cl) on the right, so find the least common multiple of 2 and 3.

The LCM of 2 and 3 is 6, so you need 3 CuCl₂ (giving 6 Cl) and 2 AlCl₃ (also 6 Cl).

Then balance Al (2 AlCl₃ ⇒ 2 Al) and Cu (3 CuCl₂ ⇒ 3 Cu): 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu.

3

Chlorine sets the pace: CuCl₂ has 2 Cl and AlCl₃ has 3 Cl, so use 6 Cl total — that means 3 CuCl₂ and 2 AlCl₃.

Now Al: 2 AlCl₃ needs 2 Al. Cu: 3 CuCl₂ gives 3 Cu. Check: Al 2 = 2, Cu 3 = 3, Cl 6 = 6.

The balanced equation is 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu, so the coefficient of CuCl₂ is 3.

The magnesium oxide is made only of the magnesium plus the oxygen that joined it, so its mass is the sum of those two.

By conservation of mass: mass of Mg + mass of O₂ = mass of MgO.

Mass of oxygen = mass of MgO − mass of Mg = 20.0 − 12.0.

8 g

Conservation of mass: the product MgO weighs as much as the magnesium plus the oxygen that reacted with it.

So mass of oxygen = 20.0 g MgO − 12.0 g Mg.

Mass of oxygen = 8.0 g.