Daily Olympiad: Science - Chemical Reactions [20260521]

Solution

Correct: B

The unbalanced equation is Fe(s) + H₂O(g) → Fe₃O₄(s) + H₂(g). 1. Balance Fe atoms: There are 3 Fe atoms on the right (Fe₃O₄), so place a coefficient of 3 in front of Fe on the left: 3Fe(s) + H₂O(g) → Fe₃O₄(s) + H₂(g). 2. Balance O atoms: There are 4 O atoms on the right (Fe₃O₄), so place a coefficient of 4 in front of H₂O on the left: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + H₂(g). 3. Balance H atoms: Now there are 4x2 = 8 H atoms on the left (4H₂O). Place a coefficient of 4 in front of H₂ on the right: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g). The equation is now balanced.

Solution

Correct: B

Oxidation is the loss of electrons (increase in oxidation number), and reduction is the gain of electrons (decrease in oxidation number). Let's analyze the oxidation numbers: - In MnO₂: Oxygen is -2, so Manganese (Mn) is +4. - In HCl: Hydrogen is +1, so Chlorine (Cl) is -1. - In MnCl₂: Chlorine is -1, so Manganese (Mn) is +2. - In Cl₂ (elemental form): Chlorine (Cl) is 0. - Manganese changes from +4 (in MnO₂) to +2 (in MnCl₂). This is a decrease in oxidation number, so MnO₂ is reduced. - Chlorine changes from -1 (in HCl) to 0 (in Cl₂). This is an increase in oxidation number, so HCl is oxidized. Therefore, HCl is the substance that gets oxidized.

Solution

Correct: D

A decomposition reaction is one where a single compound breaks down into two or more simpler substances. Since this decomposition is brought about by heat (implied by 'heated'), it is specifically a thermal decomposition reaction. The reaction is: 2Pb(NO₃)₂(s) --(heat)--> 2PbO(s) + 4NO₂(g) + O₂(g).

Solution

Correct: B

While a change in state can occur in both physical and chemical changes (e.g., melting ice is physical), specific indicators for chemical reactions include: a change in color, the evolution of a gas (seen as bubbles), and the formation of a precipitate (an insoluble solid). These observations suggest that new substances with different properties have been formed. Change in state alone is not a definitive indicator of a chemical reaction.

Solution

Correct: B

Exothermic reactions are chemical reactions that release energy, typically in the form of heat or light, into the surroundings. This release of energy leads to an increase in the temperature of the surroundings. Endothermic reactions, conversely, absorb heat from the surroundings, causing the surroundings' temperature to decrease.

Solution

Correct: B

Let's analyze each factor: I. Decrease in temperature: Lowers the kinetic energy of particles, reducing the frequency and energy of collisions, thus decreasing the reaction rate. II. Increase in concentration of reactants: Leads to more particles per unit volume, increasing the frequency of effective collisions, and thus increasing the reaction rate. III. Addition of a catalyst: Provides an alternative reaction pathway with a lower activation energy, speeding up the reaction without being consumed. IV. Decrease in surface area of solid reactants: Reduces the exposed area for reaction, leading to fewer collisions and a slower reaction rate. Therefore, increasing the concentration of reactants and adding a catalyst generally increase the rate of a chemical reaction.

Solution

Correct: C

The reaction between dilute hydrochloric acid (HCl) and silver nitrate solution (AgNO₃) is a double displacement reaction: HCl(aq) + AgNO₃(aq) → AgCl(s) + HNO₃(aq) Silver chloride (AgCl) is an insoluble ionic compound, which forms the white precipitate. Nitric acid (HNO₃) remains dissolved in the solution.

Solution

Correct: B

For a metal to displace another metal from its salt solution, the displacing metal must be more reactive than the metal in the salt. According to the reactivity series (from most reactive to least reactive): Potassium > Sodium > Calcium > Magnesium > Aluminum > Zinc > Iron > Lead > Hydrogen > Copper > Silver > Gold. Zinc (Zn) is more reactive than Iron (Fe). Therefore, zinc can displace iron from iron(II) sulfate solution: Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s) Copper, Silver, and Gold are all less reactive than iron and cannot displace it.

Solution

Correct: B

Rancidity is the process where fats and oils containing unsaturated fatty acids react with oxygen in the air (undergo oxidation), leading to unpleasant smell and taste. This is why food items rich in fats and oils are often packaged in nitrogen gas or contain antioxidants to prevent or slow down oxidation.

Solution

Correct: C

A neutralization reaction is a specific type of double displacement reaction where an acid reacts with a base to form a salt and water. For example, Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H₂O).

Solution

Correct: B

This is a double displacement reaction where ions are exchanged: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) Barium sulfate (BaSO₄) is insoluble in water, and therefore it forms a white precipitate. Sodium chloride (NaCl) remains dissolved in the aqueous solution.

Solution

Correct: C

Highly reactive metals like Potassium (K) and Sodium (Na) react vigorously and exothermically with cold water, producing hydrogen gas and their respective hydroxides. The reaction for sodium is: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Heat. Magnesium reacts slowly with cold water but faster with hot water/steam. Iron reacts only with steam. Copper does not react with water.

Solution

Correct: C

This is a displacement reaction. Copper is more reactive than silver (Cu > Ag) in the reactivity series. Therefore, copper will displace silver from its salt solution. Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s) As a result, silver metal will be deposited on the copper coin (often appearing as a shiny white coating), and the solution, initially colorless (silver nitrate), will turn blue due to the formation of copper(II) nitrate.

Solution

Correct: C

Let's determine the oxidation states of sulfur in each compound: - In H₂S: Hydrogen (H) is +1. Since there are two H atoms, 2(+1) + S = 0, so S = -2. - In SO₂: Oxygen (O) is -2. Since there are two O atoms, S + 2(-2) = 0, so S = +4. - In S(s) (elemental sulfur): S = 0. Comparing the changes: - For H₂S: Sulfur goes from -2 to 0. This is an increase in oxidation number, which means H₂S is oxidized. - For SO₂: Sulfur goes from +4 to 0. This is a decrease in oxidation number, which means SO₂ is reduced. Therefore, H₂S is oxidized, and SO₂ is reduced.

Solution

Correct: B

A combination reaction (also known as a synthesis reaction) is a reaction in which two or more reactants combine to form a single, more complex product. - A) CaCO₃(s) → CaO(s) + CO₂(g) is a decomposition reaction. - B) 2Mg(s) + O₂(g) → 2MgO(s): Magnesium combines with oxygen to form magnesium oxide. This fits the definition of a combination reaction. - C) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) is a displacement reaction. - D) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) is a double displacement reaction.

Solution

Correct: C

When a decomposition reaction is caused by light energy, it is known as photolytic decomposition (or photochemical decomposition). Silver bromide (AgBr) is famously used in black and white photography, where light energy causes its decomposition: 2AgBr(s) --(light)--> 2Ag(s) + Br₂(g).

Solution

Correct: D

Sodium hydroxide (NaOH) is a strong base, and sulfuric acid (H₂SO₄) is a strong acid. When a strong acid and a strong base react, they undergo a neutralization reaction, forming a salt and water. If the amounts of acid and base are stoichiometric (i.e., they completely neutralize each other), the resulting solution will be neutral, and its pH will be approximately 7.

Solution

Correct: C

A chemical change involves the formation of new substances with different chemical properties. Physical changes alter the form or appearance but not the chemical composition. - A) Melting of ice: Water changes from solid to liquid. It is still H₂O. (Physical change) - B) Dissolving sugar in water: Sugar molecules disperse in water but do not change chemically. (Physical change) - C) Burning of a candle: The wax (a hydrocarbon) reacts with oxygen from the air to produce carbon dioxide, water, light, and heat. New substances are formed. (Chemical change) - D) Boiling of water: Water changes from liquid to gas. It is still H₂O. (Physical change)

Solution

Correct: A

The coefficients in a balanced chemical equation represent the molar ratios of reactants and products. From the given balanced equation: 2 moles of H₂(g) react with 1 mole of O₂(g) to produce 2 moles of H₂O(l). Therefore, the ratio of moles of hydrogen (H₂) to moles of water (H₂O) produced is 2:2, which simplifies to 1:1.

Solution

Correct: B

An oxidizing agent is a substance that causes another substance to be oxidized and itself gets reduced. A reducing agent is a substance that causes another substance to be reduced and itself gets oxidized. Let's analyze the changes: - CuO loses oxygen to become Cu. Losing oxygen is reduction. Therefore, CuO is reduced, and it acts as the oxidizing agent. - H₂ gains oxygen to become H₂O. Gaining oxygen is oxidation. Therefore, H₂ is oxidized, and it acts as the reducing agent.