All Free Chemistry MCQs with Answers

121. The standard electrode potential of a half cell is measured under conditions of

• A. 1 molar solution, 298 K and 1 atmosphere pressure
• B. any concentration at 273 K
• C. 0.1 molar solution at 373 K
• D. pure water at room temperature

Explanation: Standard conditions fix the concentration of every solution at 1 molar, the pressure of any gas at 1 atmosphere and the temperature at 298 K, so that values from different laboratories can be compared. Departing from these conditions changes the measured potential, which the Nernst equation quantifies. Note that 298 K is 25 degrees Celsius, not the 273 K of standard temperature for gases.

122. Which of the following is NOT a redox reaction?

• A. Zn + 2HCl gives ZnCl2 + H2
• B. 2H2 + O2 gives 2H2O
• C. NaOH + HCl gives NaCl + H2O
• D. 2Mg + O2 gives 2MgO

Explanation: In neutralisation no element changes its oxidation number, since hydrogen stays at plus 1, oxygen at minus 2 and both sodium and chlorine keep their values throughout, so only ions are rearranged. In the other three reactions a metal or hydrogen changes from zero to a positive state while another element is reduced. Checking oxidation numbers on both sides is the reliable test.

123. In the electrochemical series, a metal higher in the series will

• A. displace a metal lower in the series from its salt solution
• B. be displaced by a metal lower in the series
• C. never react with acids
• D. always be a poor conductor

Explanation: A more reactive metal gives up electrons more readily, so it reduces the ions of a less reactive metal and takes their place in solution, which is why zinc displaces copper from copper sulphate but copper cannot displace zinc. The same order predicts which metals react with dilute acid to release hydrogen, namely those above hydrogen in the series. Displacement reactions are always redox reactions.

124. The oxidation number of an element in its free, uncombined state is

• A. plus 1
• B. equal to its group number
• C. minus 1
• D. zero

Explanation: An uncombined element has not gained or lost any electrons, so its oxidation number is zero, and this holds for atoms such as helium and for molecules such as O2, P4 and S8 alike. This rule is the starting point for every oxidation number calculation and for recognising which species has been oxidised in a reaction. The group number gives the usual oxidation state in compounds, not in the free element.

125. The rate of a chemical reaction is defined as the change in

• A. concentration of a reactant or product per unit time
• B. temperature of the mixture per unit time
• C. mass of the catalyst per unit time
• D. total pressure of the system

Explanation: Rate measures how quickly reactants are consumed or products formed, so its units are usually moles per cubic decimetre per second. It is always positive, which is why a minus sign is written in front of the reactant term. Rate is greatest at the start, when reactant concentration is highest, and falls as the reaction proceeds.

126. Which of the following will NOT increase the rate of a reaction between a solid and a solution?

• A. Raising the temperature
• B. Powdering the solid
• C. Increasing the volume of solvent without adding more solute
• D. Adding a suitable catalyst

Explanation: Adding solvent alone dilutes the solution, so the concentration of the reacting species falls and collisions become less frequent, which slows the reaction rather than speeding it. Heating, increasing surface area and adding a catalyst all raise the frequency or the effectiveness of collisions. Note that a larger volume gives more total reactant but a lower concentration, and rate depends on concentration.

127. According to collision theory, a reaction occurs only when colliding molecules

• A. are of the same size
• B. collide with energy equal to or greater than the activation energy and with the correct orientation
• C. collide at low temperature
• D. collide exactly head on, regardless of energy

Explanation: Most collisions are fruitless because the molecules either rebound with too little energy to break existing bonds or meet at the wrong angle for new bonds to form. Only collisions that satisfy both conditions are effective, and the fraction that do so is what determines the rate. Raising the temperature increases both the collision frequency and, far more importantly, the fraction with sufficient energy.

128. A rise of 10 degrees Celsius roughly doubles the rate of many reactions mainly because

• A. the activation energy of the reaction falls
• B. the concentration of the reactants increases
• C. the molecules become larger
• D. a much larger fraction of molecules acquire energy equal to or above the activation energy

Explanation: The Maxwell Boltzmann distribution shifts to the right on heating, and because the activation energy lies out in the tail of the curve, even a small temperature rise multiplies the number of molecules beyond it. The increase in collision frequency is real but accounts for only a few per cent of the effect. Activation energy itself is a property of the reaction and is not changed by heating.

129. A catalyst increases the rate of a reaction by

• A. increasing the kinetic energy of the reacting molecules
• B. raising the temperature of the reaction mixture
• C. providing an alternative pathway with a lower activation energy
• D. increasing the concentration of the reactants

Explanation: By offering a route with a smaller energy barrier, a catalyst allows a far larger proportion of collisions to be effective at the same temperature. It is recovered chemically unchanged at the end and it speeds the forward and reverse reactions equally, so the equilibrium position is untouched. A catalyst neither supplies energy nor alters concentration.

130. The order of a reaction with respect to a particular reactant

• A. is always equal to its coefficient in the balanced equation
• B. is the power to which its concentration is raised in the experimentally determined rate equation
• C. can never be zero
• D. is always a whole number

Explanation: Order is an experimental quantity and must be measured, because the balanced equation describes the overall stoichiometry while the rate depends only on the slow, rate determining step. This is why orders can be zero, fractional or even negative, none of which a coefficient can be. Only for a genuinely single step reaction do order and coefficient happen to agree.