Chemical Reaction

(iii) Displacement. Zinc is more reactive than hydrogen, so it displaces hydrogen from HCl and forms ZnCl$_2$.

(b) $3Mg + N_2 \rightarrow Mg_3N_2$. Mg has valency 2 and N has valency 3, so the formula is Mg$_3$N$_2$. Nitrogen is diatomic (N$_2$), and 3 Mg atoms and 2 N atoms balance on both sides.

(i) Reaction (a), $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$, is endothermic because it absorbs heat continuously to proceed.

(ii) When powdered CaCO$_3$ is used, its surface area increases, so more particles are exposed and collide with the other reactant. The rate of reaction increases.

The rate is faster in the first container (with MnO$_2$).

• MnO$_2$ is a positive catalyst. It speeds up the decomposition of hydrogen peroxide without being used up:

• H$_3$PO$_4$ acts as a negative catalyst (inhibitor); it slows down the decomposition, which is why it is added to store H$_2$O$_2$.

The reaction is faster in beaker A (30 °C).

Reason: the rate of a reaction increases with temperature. At a higher temperature the particles have more kinetic energy, move faster and collide more often and more effectively, so more products form per unit time. Beaker B (20 °C) is cooler, so the reaction there is slower.

Any two of the following:

1. Temperature: the rate increases when the temperature is raised.
2. Concentration of reactants: a more concentrated reactant gives a faster reaction.
3. Surface area (size of particles): powder reacts faster than lumps.
4. Catalyst: a positive catalyst speeds up and a negative catalyst slows down the reaction.

(a) The kinetic energy of particles increases when heat increases and particles come in contact faster. Heating makes particles move faster, so collisions are more frequent and energetic and the rate rises.

A negative catalyst decreases (slows down) the rate of a chemical reaction.

A catalyst is used to change (usually increase) the rate of a chemical reaction without itself being consumed. It lets the reaction occur faster or at a lower temperature, saving time and energy.

MnO$_2$ is a positive catalyst, because it increases the rate of decomposition of KClO$_3$ to give oxygen.

Decomposition of hydrogen peroxide using MnO$_2$ as catalyst:

(Another example: $2KClO_3 \xrightarrow{MnO_2,\ \Delta} 2KCl + 3O_2\uparrow$)

(a) Two differences

(b) Factors affecting the rate

• In (i): catalyst (MnO$_2$) and temperature (heat).
• In (ii): temperature (heat) and light (sunlight).

(i) Reaction A, $MgO + H_2O \rightarrow Mg(OH)_2$, is the combination reaction (two substances combine to form a single product).

(ii) Concentrated acid contains more acid particles per unit volume, so collisions with zinc are more frequent and the rate of reaction increases compared with dilute acid. (Note: concentrated H$_2$SO$_4$ is an oxidizing acid, so the gas it gives off with zinc is mainly SO$_2$ rather than H$_2$.)

(i) $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$

(ii) Powder has a much larger surface area than marble chips. More particles are exposed to the acid, so the reaction is faster and CO$_2$ is produced more quickly.

A chemical reaction in which heat energy is absorbed from the surroundings is called an endothermic reaction. The container becomes cold or heat must be supplied continuously.

Example: heating calcium carbonate (limestone):

(i) In beakers A and B, exothermic and C endothermic reaction has occurred. A rise in temperature (A and B) means heat is released, which is exothermic. A fall in temperature (C) means heat is absorbed from the solution, which is endothermic.

(c) displacement reaction. In $2AgNO_3 + BaCl_2 \rightarrow 2AgCl\downarrow + Ba(NO_3)_2$ the two compounds exchange their ions: $Ag^+$ joins $Cl^-$ to form white precipitate of AgCl, and $Ba^{2+}$ joins $NO_3^-$. This is a double displacement (precipitation) reaction, so displacement is the closest option; it is not combination, neutralization or decomposition.

It is a combination reaction: two substances (CaO and H$_2$O) combine to form a single product, Ca(OH)$_2$ (slaked lime). It is also exothermic.

(i) Combination reaction. $MgO + H_2O \rightarrow Mg(OH)_2$; two reactants combine into one product.

Burning of magnesium in oxygen:

(Another: $CaO + H_2O \rightarrow Ca(OH)_2$)

The product is calcium hydroxide, Ca(OH)$_2$:

The white powder is magnesium oxide:

Word equation: magnesium + oxygen $\rightarrow$ magnesium oxide

Formula equation (unbalanced): $Mg + O_2 \rightarrow MgO$

Balancing: there are 2 O atoms on the left and 1 on the right, so put 2 before MgO, then 2 before Mg:

The dust (white ash) is magnesium oxide. It reacts with water to form magnesium hydroxide:

The burning matchstick is extinguished, so the gas in the jar is carbon dioxide, which does not support ordinary combustion. Magnesium is so reactive that it keeps burning in CO$_2$, taking oxygen from it and leaving black carbon:

The valency of Y is 3.

Nitrogen has valency 3. In the formula YN one atom of Y combines with one atom of N, so Y must also have valency 3 (the compound is like AlN).

When carbon dioxide dissolves in water, it forms carbonic acid, a weak acid. The solution turns blue litmus slightly red.

(c) H$_2$CO$_3$: carbonic acid is formed, $CO_2 + H_2O \rightarrow H_2CO_3$.

When CO$_2$ is passed for a short time, lime water turns milky because insoluble calcium carbonate forms:

When CO$_2$ is passed for a long time, the milkiness disappears and the solution becomes clear again, as soluble calcium bicarbonate forms:

Zinc is more reactive than hydrogen, so it reacts with dilute HCl (a displacement reaction). The zinc of the bottle wall dissolves as zinc chloride and hydrogen gas is released, so the wall becomes thinner:

X is zinc oxide (ZnO), a metal oxide that reacts with an acid to give salt and water.

Unbalanced: $ZnO + HCl \rightarrow ZnCl_2 + H_2O$

Balanced (ZnCl$_2$ has 2 Cl on the right, so 2 HCl is needed on the left):

(b) NaCl + H$_2$O. Hydrochloric acid and sodium hydroxide undergo neutralization, giving a salt and water:

(d) Na$_2$SO$_4$. Sulphuric acid neutralizes NaOH to give sodium sulphate and water: $H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$.

X is sodium hydroxide, NaOH.

The products are a salt and water, so it is neutralization with a base containing Na.

Formula equation: $H_2SO_4 + Ca(OH)_2 \rightarrow CaSO_4 + H_2O$

Balancing H and O: the left side has 4 H, so put 2 before H$_2$O.

Formula equation: $MgO + HCl \rightarrow MgCl_2 + H_2O$

There are 2 Cl on the right, so put 2 before HCl; then H and O balance.

Unbalanced: $KBr + Pb(NO_3)_2 \rightarrow KNO_3 + PbBr_2$

PbBr$_2$ has 2 Br, so put 2 before KBr. Then 2 K needs 2 KNO$_3$.

Ammonium chloride reacts with calcium hydroxide to give calcium chloride, ammonia and water.

Unbalanced: $NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + NH_3 + H_2O$

Balanced:

(a) A neutralization reaction (an acid-base reaction): the weak base NH$_4$OH reacts with the acid HCl to give a salt and water.

(b)

Silver chloride decomposes in sunlight (used in photography):

Unbalanced: $AgCl \xrightarrow{light} Ag + Cl_2$

Balancing Cl (2 atoms in Cl$_2$): put 2 before AgCl and 2 before Ag.

The gases are ammonia, NH$_3$ (molecular weight $14 + 3 = 17$), and carbon dioxide, CO$_2$ (molecular weight $12 + 32 = 44$). They react under high temperature and pressure to form urea and water:

In the preparation of oxygen from hydrogen peroxide, acetanilide (or phosphoric acid) acts as a negative catalyst because it slows down the decomposition of $H_2O_2$. (Manganese dioxide is the positive catalyst.)

Iron displaces copper from copper sulphate solution: $Fe + CuSO_4 \to FeSO_4 + Cu$

1. New substances (products) with properties different from the reactants are formed.
2. There is an energy change: heat, light or electricity is given out or absorbed. (Colour change, evolution of gas or formation of a precipitate may also occur.)

Powdered limestone reacts faster because it has a larger surface area, so more particles are exposed to the acid and collide with it.

$CaCO_3 + 2HCl \to CaCl_2 + H_2O + CO_2$

OR The salt formed is calcium chloride ($CaCl_2$).

(a) The salt formed in the first step is sodium chloride, which is soluble in water and colourless, so nothing visible happens. In the second step the $Cl^-$ ions react with silver nitrate to form silver chloride, which is insoluble in water, so it appears as a curdy white precipitate.

(b) $NaCl + AgNO_3 \to AgCl\downarrow + NaNO_3$

(c) Sodium reacts violently (and exothermically) with hydrochloric acid, forming sodium chloride and releasing hydrogen gas, which may catch fire: $2Na + 2HCl \to 2NaCl + H_2\uparrow$

(a) Reaction B is a displacement reaction. Zinc displaces hydrogen from sulphuric acid.

(b) Reaction A is a decomposition reaction with a single reactant that breaks down into simpler substances on heating. Reaction B has two reactants, no heat is needed, and one element replaces another in a compound.

(c) Magnesium carbonate also decomposes on heating, giving magnesium oxide and carbon dioxide: $MgCO_3 \xrightarrow{Heat} MgO + CO_2$

The reaction is fastest in flask C.

Reason (from the figure: A 25 °C, 1.0 mol/L, powder; B 50 °C, 1.0 mol/L, a piece; C 50 °C, 2.0 mol/L, powder):

• Flask C has powdered limestone, which has a large surface area, so more particles are exposed to the acid.
• It has the highest temperature (50 °C) (same as B), so particles have more kinetic energy.
• It has the highest concentration (2.0 mol/L) of acid, so more acid particles collide with the limestone.

Compared with A (low temperature and concentration) and B (a lump with small surface area), C combines all three factors that increase the rate.

(a) Catalyst. X is MnO$_2$, which is written over the arrow as a catalyst to speed up the decomposition of KClO$_3$. Heat ($\Delta$) is written below the arrow.

(d) $XY + AB \rightarrow XB + AY$. In a double displacement reaction two compounds exchange their ions/radicals to form two new compounds.

Zinc is more reactive than copper, so it displaces copper from copper sulphate solution. Blue CuSO$_4$ is used up and colourless ZnSO$_4$ forms, so the blue colour fades. Zinc dissolves into the solution, so holes appear in the plate. Copper deposits as a brown solid.

(c) HCl. $NaOH + HCl \rightarrow NaCl + H_2O$ is a neutralization reaction between a base and an acid.