Question
1.0 g of sodium hydroxide, NaOH, was added to 99.0 g of water. The temperature of the solution increased from 18.0 °C to 20.5 °C. The specific heat capacity of the solution is \({\text{4.18 J}}\,{{\text{g}}^{ – 1}}{{\text{K}}^{ – 1}}\). Which expression gives the heat evolved in \({\text{kJ}}\,{\text{mo}}{{\text{l}}^{ – 1}}\)?
A. \(\frac{{2{\text{.}}5 \times 100.0 \times 4.18 \times 1000}}{{40{\text{.}}0}}\)
B. \(\frac{{2{\text{.}}5 \times 100.0 \times 4.18}}{{1000 \times 40{\text{.}}0}}\)
C. \(\frac{{2{\text{.}}5 \times 100.0 \times 4.18 \times 40{\text{.}}0}}{{1000}}\)
D. \(\frac{{2{\text{.}}5 \times 1{\text{.}}0 \times 4{\text{.}}18 \times 40{\text{.}}0}}{{1000}}\)
▶️Answer/Explanation
C
Q=m.c. ΔT
Mass of solution = 1+99 = 100g
c=\({\text{4.18 J}}\,{{\text{g}}^{ – 1}}{{\text{K}}^{ – 1}}\)
ΔT = 20.5-18 = 2.5
Q = 100×4.18×2.5 J
As heat evolved per mole is asked, mass of 1 mole of NaOH = 40g
Heat evolved = 100×4.18×2.5×40 J per mole = \(\frac{{2{\text{.}}5 \times 100.0 \times 4.18 \times 40{\text{.}}0}}{{1000}}\) \({\text{kJ}}\,{\text{mo}}{{\text{l}}^{ – 1}}\)
Question
Which process is endothermic?
A. \({\text{2}}{{\text{C}}_4}{{\text{H}}_{10}}{\text{(g)}} + {\text{13}}{{\text{O}}_2}{\text{(g)}} \to {\text{8C}}{{\text{O}}_2}{\text{(g)}} + {\text{10}}{{\text{H}}_2}{\text{O(g)}}\)
B. \({\text{Na(g)}} \to {\text{N}}{{\text{a}}^ + }{\text{(g)}} + {{\text{e}}^ – }\)
C. \({{\text{H}}_2}{\text{S}}{{\text{O}}_4}{\text{(aq)}} + {\text{2KOH(aq)}} \to {{\text{K}}_2}{\text{S}}{{\text{O}}_4}{\text{(aq)}} + {\text{2}}{{\text{H}}_2}{\text{O(l)}}\)
D. \({\text{N}}{{\text{H}}_3}{\text{(g)}} \to {\text{N}}{{\text{H}}_3}{\text{(l)}}\)
▶️Answer/Explanation
B
In B, An electron is removed from sodium, this is an endothermic process. Energy is always required to remove electron from an atom and hence, ionisation enthalpies are always positive. The process is always endothermic.
A is combustion, so it is exothermic.
C is acid base neutralization. In a neutralization reaction, an acid and a base react to form salt and water. Also, it’s important to understand that during an exothermic reaction, bonds are being made and energy is released to the surroundings. This is what ultimately happens during a neutralization reaction that gives its exothermic character.
In D, The phase transition from a gas to a liquid is an exothermic process.
Question
Which reaction has an enthalpy change equal to the standard enthalpy change of combustion?
A. \({{\text{C}}_3}{{\text{H}}_8}{\text{(g)}} + {\text{5}}{{\text{O}}_2}{\text{(g)}} \to {\text{3C}}{{\text{O}}_2}{\text{(g)}} + {\text{4}}{{\text{H}}_2}{\text{O(g)}}\)
B. \({{\text{C}}_3}{{\text{H}}_8}{\text{(g)}} + {\text{5}}{{\text{O}}_2}{\text{(g)}} \to {\text{3C}}{{\text{O}}_2}{\text{(g)}} + {\text{4}}{{\text{H}}_2}{\text{O(l)}}\)
C. \({\text{2}}{{\text{C}}_4}{{\text{H}}_{10}}{\text{(g)}} + {\text{13}}{{\text{O}}_2}{\text{(g)}} \to {\text{8C}}{{\text{O}}_2}{\text{(g)}} + {\text{10}}{{\text{H}}_2}{\text{O(l)}}\)
D. \({{\text{C}}_5}{{\text{H}}_{12}}{\text{(g)}} + {\text{8}}{{\text{O}}_2}{\text{(g)}} \to {\text{5C}}{{\text{O}}_2}{\text{(g)}} + {\text{6}}{{\text{H}}_2}{\text{O(g)}}\)
▶️Answer/Explanation
B
Standard enthalpy of combustion is defined as the enthalpy change when one mole of a compound is completely burnt in oxygen with all the reactants and products in their standard state under standard conditions (298K and 1 bar pressure).
Water in liquid state and Carbon dioxide in gaseous state is their standard state. Thus, B represents standard enthalpy change of combustion of C3H8.
Question
Which processes are exothermic?
I. \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3}{\text{(g)}} + {\text{5}}{{\text{O}}_2}{\text{(g)}} \to {\text{3C}}{{\text{O}}_2}{\text{(g)}} + {\text{4}}{{\text{H}}_2}{\text{O(g)}}\)
II. \({\text{C}}{{\text{l}}_2}{\text{(g)}} \to {\text{2Cl(g)}}\)
III. \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH(aq)}} + {\text{NaOH(aq)}} \to {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COONa(aq)}} + {{\text{H}}_2}{\text{O(l)}}\)
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
B
A is combustion, so it is exothermic.
C is acid base neutralization. In a neutralization reaction, an acid and a base react to form salt and water. Also, it’s important to understand that during an exothermic reaction, bonds are being made and energy is released to the surroundings. This is what ultimately happens during a neutralization reaction that gives its exothermic character.
In B, the reaction is endothermic as it takes energy to break the Cl2 bond.
Question
Which ionic compound has the most endothermic lattice enthalpy?
A. Sodium chloride
B. Sodium oxide
C. Magnesium chloride
D. Magnesium oxide
▶️Answer/Explanation
D
The lattice energy depends on the attraction between the oppositely charged ions.
The force of attraction F is directly proportional to the charges (q1 and q2) on the two ions and on the distance r between them.
F=kq1q2/r2
The distance between the charges r is the sum of the ionic radii. So the greatest attractions will be between ions that have the smallest radii.
And, since the attraction depends on the product of the charges, we look for ions with the greatest charge.
In both MgCl2 and MgO, the charges are +2 and -2, so the choice is between these two compounds.
The smallest ions will be able to get closest to each other.
They will have the smallest distance between centres and will have the largest lattice energies.
O2– is smaller than Cl–, so MgO has the largest lattice energy.
Question
When some solid barium hydroxide and solid ammonium thiosulfate were reacted together, the temperature of the surroundings was observed to decrease from 15 °C to –4 °C. What can be deduced from this observation?
A. The reaction is exothermic and \(\Delta H\) is negative.
B. The reaction is exothermic and \(\Delta H\) is positive.
C. The reaction is endothermic and \(\Delta H\) is negative.
D. The reaction is endothermic and \(\Delta H\) is positive.
▶️Answer/Explanation
D
Since, temperature of the surroundings was decreased, it means heat was transferred from the surroundings to the system.
Hence, q>0 (positive). Therefore, ΔH will be positive if q is positive and reaction is endothermic.