Reactions Practice Test

  1. Which pair consists only of substances that can be decomposed chemically?
  1. CaO and Ca
  2. MgO and Mg
  3. CO and Co
  4. CaO and MgO
Show Answer
The correct answer is D!

Compounds can be decomposed chemically, whereas elements cannot.

Both CaO and MgO are compounds (ionic oxides) and can be decomposed, but Ca, Mg, and Co are elements and cannot be chemically decomposed into simpler substances.

Therefore, the pair that consists only of substances that can be decomposed is CaO and MgO.

 

  1. The particles of a substance are arranged in a definite geometric pattern and are constantly vibrating. What phase(s) can this substance be in?
  1. The solid phase only
  2. The liquid phase only
  3. Either the liquid or the solid phase
  4. Neither the liquid nor the solid phase
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The correct answer is A!

A definite geometric pattern with particles “constantly vibrating” describes a solid. In a solid, particles are in fixed positions in a crystal lattice and vibrate about those positions.

Liquids do not have a fixed geometric arrangement. Therefore, the substance is in the solid phase only.

 

  1. Refer to the reaction below. At STP, what is the total number of liters of hydrogen gas produced when 3 mol of hydrochloric acid solution is completely consumed?
\(\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}\)
  1. 11.2 L
  2. 22.4 L
  3. 33.6 L
  4. 44.8 L
Show Answer
The correct answer is C!

From the given reaction, two moles of HCl produce one mole of H2. If 3 mol of HCl are consumed, then \(\frac{3}{2}=1.5\) mol of H2 form.

At STP, one mole of gas occupies 22.4 L, so:

\(1.5\times 22.4=33.6\ \text{L}\)

 

  1. If a system at equilibrium for the reaction below is subjected to a stress, where will change occur?
\(2\text{CO(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{CO}_2\text{(g)}\)
  1. Reactants only
  2. Products only
  3. Both products and reactants
  4. Neither reactants nor products
Show Answer
The correct answer is C!

By Le Châtelier’s principle, when a system at equilibrium is subjected to a stress, the system shifts to counter that stress, causing concentration changes in both reactants and products.

Therefore, change occurs in both products and reactants.

 

  1. What is the concentration of a solution of 10 mol of copper nitrate in 5 L of solution?
  1. 0.5 M
  2. 2 M
  3. 5 M
  4. 10 M
Show Answer
The correct answer is B!

Molarity is \(M=\dfrac{n}{V}\). With 10 mol in 5 L, \(M=\dfrac{10}{5}=2\ \text{M}\).

Therefore, the concentration is 2 M.

 

  1. What is the oxidation number of chlorine in HClO4?
  1. +1
  2. +5
  3. +3
  4. +7
Show Answer
The correct answer is D!

Let the oxidation number of Cl in HClO4 be \(x\). Using \(\text{H} = +1\) and \(\text{O} = −2\), the sum must be zero:

\((+1) + x + 4(-2) = 0\)

\(x – 7 = 0 \Rightarrow x = +7\)

Therefore, the oxidation number of chlorine is +7.

 

  1. Refer to the redox reaction below. Which species acts as the reducing agent?
\(\text{Fe}^{2+}\text{(aq)} + \text{Zn(s)} \gt \text{Zn}^{2+}\text{(aq)} + \text{Fe(s)}\)
  1. Fe2+
  2. Zn(s)
  3. Zn2+
  4. Fe(s)
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The correct answer is B!

A reducing agent is oxidized (loses electrons). In the given reaction, zinc goes from 0 to +2, losing two electrons, so Zn(s) is oxidized and acts as the reducing agent.

 

  1. Refer to the reaction below. Which equation represents the oxidation half-reaction?
\(\text{Zn(s)} + 2\text{HCl(aq)} \gt \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}\)
  1. \(\text{Zn}^{2+} + 2\text{e}^- \rightarrow \text{Zn(s)}\)
  2. \(2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}_2\text{(g)}\)
  3. \(\text{Zn(s)} \rightarrow \text{Zn}^{2+} + 2\text{e}^-\)
  4. \(\text{H}_2\text{(g)} \rightarrow 2\text{H}^+ + 2\text{e}^-\)
Show Answer
The correct answer is C!

Oxidation is loss of electrons. In the given reaction, zinc is the species that is oxidized, and its oxidation half-reaction is \(\text{Zn(s)}\rightarrow \text{Zn}^{2+}+2\text{e}^-\).

 

  1. Which of the following is the strongest Brønsted-Lowry acid?
  1. NH3
  2. HSO3–
  3. H2O
  4. HNO3
Show Answer
The correct answer is D!

The strongest Brønsted–Lowry acid listed is nitric acid, HNO3, which dissociates completely in water.

 

  1. What is the pH of a solution with a hydronium ion concentration of 0.01 moles per liter?
  1. 1
  2. 2
  3. 10
  4. 14
Show Answer
The correct answer is B!

pH is \(-\log[\text{H}_3\text{O}^+]\). With \([\text{H}_3\text{O}^+]=0.01=10^{-2}\), \(\text{pH}=2\).

 

  1. Which terms describe a substance that has a low melting point and poor electrical conductivity?
  1. Covalent and metallic
  2. Covalent and molecular
  3. Ionic and molecular
  4. Ionic and metallic
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The correct answer is B!

Substances with low melting points and poor electrical conductivity are typically molecular covalent compounds (held by intermolecular forces, not ions or metallic bonding).

 

  1. Refer to the reaction below. What is the maximum number of moles of H2O that can be produced when 2 mol of NH3 are completely reacted?
\(4\text{NH}_3 + 3\text{O}_2 \rightarrow 2\text{N}_2 + 6\text{H}_2\text{O}\)
  1. 1
  2. 2
  3. 3
  4. 6
Show Answer
The correct answer is C!

Using the given reaction, four moles of NH3 yield six moles of H2O. Therefore, two mol of NH3 produce \(6\times \frac{2}{4}=3\) mol of H2O.

 

  1. Which compound is a salt?
  1. Na3PO4
  2. H3PO4
  3. CH3COOH
  4. Ca(OH)2
Show Answer
The correct answer is A!

A salt is an ionic compound formed from a cation and an anion. Na3PO4 (sodium phosphate) is an ionic salt.

H3PO4 is an acid, CH3COOH is an acid, and Ca(OH)2 is a base.

 

  1. At 1 atm and 298 K, which of the Ka values listed below represents the strongest acid?
  1. \(1.1 \times 10^{-7}\)
  2. \(1.8 \times 10^{-5}\)
  3. \(5.6 \times 10^{-11}\)
  4. \(4.6 \times 10^{-4}\)
Show Answer
The correct answer is D!

Larger Ka means a stronger acid. Among the choices, \(4.6\times 10^{-4}\) is the largest value, so it represents the strongest acid.

 

  1. Which compound conducts an electric current when dissolved in water?
  1. NaOH
  2. C2H5OH
  3. C6H12O6
  4. C12H22O11
Show Answer
The correct answer is A!

An aqueous solution conducts electricity if it contains ions (electrolytes). NaOH is a strong base that dissociates completely to Na+ and OH–. The other compounds listed are non-electrolytes.

 

  1. Which half-reaction correctly represents the oxidation of Sn2+?
  1. \(\text{Sn}^{2+} + 2\text{e} \gt \text{Sn}^0\)
  2. \(\text{Sn}^{4+} + 2\text{e}^- \gt \text{Sn}^{2+}\)
  3. \(\text{Sn}^0 \rightarrow \text{Sn}^{2+} + 2\text{e}^-\)
  4. \(\text{Sn}^{2+} \rightarrow \text{Sn}^{4+} + 2\text{e}^-\)
Show Answer
The correct answer is D!

Oxidation is loss of electrons (OIL RIG). The given half-reaction shows tin’s oxidation state increasing (losing electrons), so it represents oxidation.

 

  1. What does the reducing agent do in a redox reaction?
  1. Loses electrons and is reduced
  2. Loses electrons and is oxidized
  3. Gains electrons and is reduced
  4. Gains electrons and is oxidized
Show Answer
The correct answer is B!

The reducing agent loses electrons (and is oxidized) while causing the other species to be reduced.

 

  1. A reaction will be spontaneous if it results in products that have…
  1. Lower potential energy and less randomness
  2. Lower potential energy and more randomness
  3. Greater potential energy and less randomness
  4. Greater potential energy and more randomness
Show Answer
The correct answer is B!

Processes that are spontaneous at given conditions typically move toward lower potential energy (enthalpy) and greater randomness (entropy).

 

  1. Which of the following does not have a strong Ka value?
  1. Boric acid
  2. Nitric acid
  3. Sulfuric acid
  4. Chloric acid
Show Answer
The correct answer is A!

A strong Ka value implies a strong acid. Boric acid is a weak acid, whereas nitric, sulfuric, and chloric acids are strong. Therefore, boric acid does not have a strong Ka.

 

  1. If pH is greater than 7, what color will litmus paper become?
  1. Red
  2. Orange
  3. Neutral
  4. Blue
Show Answer
The correct answer is D!

Litmus turns red in acidic solutions (pH < 7) and blue in basic solutions (pH > 7).