All Free Chemistry MCQs with Answers

141. In the reaction between marble chips and hydrochloric acid, the rate can be followed by measuring

• A. the mass of the flask, which falls as carbon dioxide escapes
• B. the colour of the acid
• C. the temperature of the marble
• D. the mass of the flask, which rises as gas is produced

Explanation: Carbon dioxide leaves the open flask, so the total mass decreases steadily and the loss per unit time is a direct measure of the rate. The alternative method is to collect the gas in a syringe and record its volume at fixed intervals. Mass could never increase, since nothing is entering the flask from outside.

142. Which statement about a catalyst is INCORRECT?

• A. It remains chemically unchanged at the end of the reaction
• B. It is needed only in a small amount
• C. It shifts the equilibrium towards the products
• D. It lowers the activation energy of both the forward and reverse reactions

Explanation: Because a catalyst lowers the barrier equally in both directions, the two rates rise by the same factor and the equilibrium position and the value of Kc are completely unaffected. What changes is how quickly equilibrium is reached. The other three statements are all true, and the small quantity needed follows from the catalyst being regenerated in each cycle.

143. For a reaction A gives products, doubling the concentration of A leaves the rate unchanged. The reaction is

• A. first order in A
• B. second order in A
• C. third order in A
• D. zero order in A

Explanation: If concentration has no effect on rate then the exponent of that term must be zero, since anything raised to the power zero equals one. Had the rate doubled the reaction would be first order, and had it quadrupled it would be second order, which is the reasoning used to find orders experimentally. Zero order behaviour usually signals a saturated catalyst or enzyme.

144. The Arrhenius equation shows that the rate constant increases when

• A. the activation energy increases
• B. the temperature falls
• C. the activation energy decreases or the temperature rises
• D. the concentration of reactants rises

Explanation: The equation contains an exponential term in which activation energy appears with a negative sign and temperature in the denominator, so lowering the barrier or raising the temperature both increase k, and the dependence is exponential rather than linear. This is why a catalyst and a temperature rise produce such large changes in rate. Concentration does not appear in the equation at all.

145. Across a period from left to right, the atomic radius of the elements generally

• A. decreases, because nuclear charge increases while electrons enter the same shell
• B. increases, because more electrons are added
• C. stays the same
• D. increases, because the nuclear charge decreases

Explanation: Each successive element adds one proton and one electron, but the electron enters the same outermost shell, so shielding hardly changes while the effective nuclear charge rises and the shell is pulled inwards. Down a group the opposite happens, since a whole new shell is added and radius increases. Atomic radius is the single trend from which ionisation energy and electronegativity mostly follow.

146. The first ionisation energy is the energy required to

• A. add one electron to a gaseous atom
• B. remove one mole of electrons from one mole of gaseous atoms, forming gaseous unipositive ions
• C. break one mole of covalent bonds
• D. convert one mole of a solid into a gas

Explanation: The definition specifies the gaseous state so that no lattice or intermolecular forces interfere, and it is always endothermic because an electron must be pulled away from an attracting nucleus. Adding an electron to a gaseous atom is electron affinity, the reverse process. Second ionisation energy is always larger than the first, since the electron is being removed from a positively charged ion.

147. Which element has the highest electronegativity?

• A. Oxygen
• B. Chlorine
• C. Fluorine
• D. Nitrogen

Explanation: Fluorine is the most electronegative element on the Pauling scale at 4.0, because it is small and has a high effective nuclear charge, so it attracts bonding electrons more strongly than any other atom. Oxygen at 3.5 is second and chlorine and nitrogen follow at 3.0. Electronegativity increases across a period and decreases down a group, so the maximum lies at the top right, excluding the noble gases.

148. Down group I from lithium to caesium, the reactivity of the metals

• A. decreases, because atomic radius increases
• B. stays constant
• C. decreases, because ionisation energy falls
• D. increases, because the outer electron is further from the nucleus and more easily lost

Explanation: Each element down the group has an extra shell, so the single outer electron is further from the nucleus and better shielded, ionisation energy falls and the metal loses that electron more readily. This is why potassium ignites the hydrogen it releases from water while lithium merely fizzes. In the halogens the same reasoning gives the opposite trend, since there reactivity depends on gaining an electron.

149. The elements of group I are called the alkali metals because they

• A. are found free in nature
• B. react with water to form hydroxides that are strongly basic
• C. are all radioactive
• D. form acidic oxides

Explanation: Sodium and its neighbours react with water to give hydrogen and a hydroxide solution that is strongly alkaline, which is the origin of the family name. Because they are so reactive they are never found uncombined in nature and are stored under oil. Their oxides are basic, not acidic, which is typical of metals.

150. When sodium reacts with excess oxygen on burning, the main product is

• A. sodium oxide, Na2O
• B. sodium superoxide, NaO2
• C. sodium peroxide, Na2O2
• D. sodium hydroxide, NaOH

Explanation: Lithium forms the simple oxide, sodium forms mainly the peroxide and potassium, rubidium and caesium form superoxides, the larger cations being better able to stabilise the larger anions. Sodium peroxide reacts with water to give hydrogen peroxide and alkali. Sodium hydroxide is the product of reaction with water rather than with oxygen.