All Free Chemistry MCQs with Answers

111. When balancing a redox equation by the ion electron method, the two half equations must be multiplied so that

• A. the number of atoms of oxygen is the same in both
• B. the number of electrons lost equals the number gained
• C. the charges on both sides are zero
• D. the coefficients are all whole numbers greater than one

Explanation: Electrons cannot appear in the final equation, so the two half equations are scaled until the electrons cancel exactly on addition, which is simply a statement that charge is conserved. Oxygen and hydrogen are balanced separately by adding water and hydrogen ions in acidic solution. The total charge on each side need not be zero, only equal.

112. In balancing a redox equation in acidic medium, oxygen atoms are balanced by adding

• A. hydroxide ions
• B. oxygen gas
• C. water molecules to the side deficient in oxygen
• D. hydrogen gas

Explanation: Water supplies the missing oxygen, and hydrogen ions are then added to the opposite side to balance the hydrogen introduced, which is valid because both are abundant in an acidic solution. In alkaline medium hydroxide ions are used instead, since hydrogen ions cannot be present in quantity. Adding elemental oxygen or hydrogen would change the chemistry rather than balance the equation.

113. The electrode potential of the standard hydrogen electrode is assigned a value of

• A. 0.00 volts by convention
• B. 1.00 volt
• C. minus 1.00 volt
• D. 0.76 volts

Explanation: No single electrode potential can be measured in isolation, so the hydrogen electrode is defined as zero and every other potential is quoted relative to it. The value 0.76 volts is the standard potential of the zinc electrode measured against it, which is why it appears as a distractor. The conditions specified are 1 molar hydrogen ions, 1 atmosphere of hydrogen gas and 298 K.

114. The standard hydrogen electrode consists of hydrogen gas at 1 atmosphere bubbled over

• A. a zinc rod in zinc sulphate solution
• B. platinised platinum foil dipping in 1 molar hydrogen ion solution
• C. a copper rod in copper sulphate solution
• D. a carbon rod in pure water

Explanation: Platinum is used because it is chemically inert and does not itself take part in the reaction, while the finely divided platinum black coating provides a large surface that catalyses the equilibrium between hydrogen gas and hydrogen ions. The solution must be exactly 1 molar in hydrogen ions and the temperature 298 K for the potential to be standard. Any reactive metal would corrode and destroy the reference.

115. In an electrochemical cell, oxidation always takes place at the

• A. cathode
• B. salt bridge
• C. anode
• D. voltmeter

Explanation: The anode is by definition the electrode at which oxidation occurs and reduction occurs at the cathode, and this holds for both galvanic and electrolytic cells. What changes between the two is the sign, since the anode is negative in a galvanic cell but positive in electrolysis. Remembering the definitions by process rather than by sign avoids the usual confusion.

116. The function of the salt bridge in a galvanic cell is to

• A. allow electrons to pass between the two solutions
• B. complete the circuit by allowing ions to migrate and keeping both solutions electrically neutral
• C. prevent the cell from producing any current
• D. act as a catalyst for the reaction

Explanation: As the cell runs, positive charge builds up in one half cell and negative charge in the other, which would stop the reaction almost immediately, so ions flow through the bridge to cancel this build up. Electrons travel through the external wire, not through the bridge, which is the point of the first distractor. A saturated potassium chloride or potassium nitrate solution is normally used because its ions move at similar rates.

117. A metal with a large negative standard electrode potential is

• A. a strong reducing agent that is easily oxidised
• B. a strong oxidising agent
• C. chemically unreactive
• D. unable to form ions

Explanation: A large negative value means the metal releases electrons readily and prefers to exist as its ion, so it is easily oxidised and therefore acts as a powerful reducing agent, as lithium and potassium do. Metals at the positive end of the series, such as gold and silver, hold their electrons tightly and are unreactive. The electrochemical series is simply the reactivity series expressed in volts.

118. The oxidation number of chromium in the dichromate ion, Cr2O7 2-, is

• A. plus 3
• B. plus 12
• C. plus 6
• D. plus 7

Explanation: Seven oxygens contribute minus 14, and the overall charge is minus 2, so the two chromium atoms must total plus 12, giving plus 6 each. Quoting plus 12 forgets to divide between the two atoms, which is the commonest slip. Acidified dichromate is reduced to the green chromium three ion, and that colour change is used as a test for a reducing agent.

119. During the electrolysis of molten sodium chloride, sodium metal is produced at the cathode because sodium ions

• A. lose electrons there
• B. gain electrons and are reduced there
• C. are attracted to the anode
• D. react with chlorine there

Explanation: Positive sodium ions migrate to the negative cathode, each accepting one electron to become a sodium atom, which is reduction. Chloride ions travel to the anode, give up electrons and are oxidised to chlorine gas. This is the Down's process, and the salt must be molten because the ions in the solid cannot move.

120. In the reaction 2Na + Cl2 gives 2NaCl, chlorine acts as

• A. a reducing agent because it gains electrons
• B. neither an oxidising nor a reducing agent
• C. a catalyst
• D. an oxidising agent because it accepts electrons from sodium

Explanation: Each chlorine atom accepts one electron and falls from zero to minus 1, so chlorine is reduced and is therefore the oxidising agent, while sodium is oxidised and acts as the reducing agent. The confusion in the first option comes from linking gaining electrons with the word reducing, when in fact the species that gains electrons is the oxidising agent. Naming the agent always means describing what it does to the other substance.