Question
\(HCl(aq)+NH_3(aq)→NH_4^+(aq)+Cl^−(aq)\)
The chemical reaction between HCl(aq) and \(NH_3\)(aq) is represented above. A student combines equimolar amounts of HCl(aq) and \(NH_3\)(aq), both solutions initially at 24°C, in a coffee-cup calorimeter. The student observes that the mixture reaches a temperature of 28°C . Based on the experimental results, which of the following can be concluded about the reaction?
A It is an endothermic process, because energy is released by the reaction and is gained by the reaction mixture.
B It is an endothermic process, because energy is absorbed by the reaction and is lost from the reaction mixture.
C It is an exothermic process, because energy is released by the reaction and is gained by the reaction mixture.
D It is an exothermic process, because energy is absorbed by the reaction and is lost from the reaction mixture.
▶️Answer/Explanation
Ans: C
In this case, the system can be defined as the reactants. As the reaction occurs, heat is released and is absorbed by the surroundings. As the mixture absorbs heat, the temperature reading increases. Therefore, the reaction is an exothermic process.
Question
When pellets of NaOH(s) are added to a flask of water, it is observed that the temperature of the water increases as the pellets dissolve. Which of the following claims about the observed dissolution of NaOH(s) in water is most accurate?
A It is an endothermic process because heat energy is absorbed by the water as the NaOH(s) dissolves in it.
B It is an endothermic process because heat energy is released by the water as the NaOH(s) dissolves in it.
C It is an exothermic process because heat energy is absorbed by the water as the NaOH(s) dissolves in it.
D It is an exothermic process because heat energy is released by the water as the NaOH(s) dissolves in it.
▶️Answer/Explanation
Ans:C
The dissolution of NaOH(s) is an exothermic process. The heat energy released as the NaOH(s) dissolves is absorbed by the water, resulting in an increase in the temperature of the water.
Question
An advertisement for a commercial hand warmer claims that the hand warmer works because a chemical reaction in the hand warmer draws out the body’s own natural heat, causing a warming effect. Which of the following states the accuracy of the claim in the advertisement and best provides a correct scientific justification of the claim?
A The advertisement’s claim is inaccurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was endothermic, which would cause the hands to feel colder.
B The advertisement’s claim is inaccurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was exothermic, which would cause the hands to feel colder.
C The advertisement’s claim is accurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was endothermic, which would cause the hands to feel warmer.
D The advertisement’s claim is accurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was exothermic, which would cause the hands to feel warmer.
▶️Answer/Explanation
Ans: A
The advertisement’s claim is inaccurate because if heat flowed from the hands to the warmer, the hands would get colder. The chemical reaction is exothermic, and the direction of heat flow is from the warmer to the hands, which causes the hands to feel warmer.
Question
\(2H_{2}O(l)\rightleftharpoons H_{3}O^{+}(aq)+OH^{-}(aq)\)
The autoionization of water is represented by the equation above. Values of \(pK_{w}\), at various temperatures are listed in the table below.
Based on the information above, which of the following statements is true?
(A) The dissociation of water is an exothermic process.
(B) The pH of pure water is 7.00 at any temperature.
(C) As the temperature increases, the pH of pure water increases.
(D) As the temperature increases, the pH of pure water decreases.
▶️Answer/Explanation
Ans:D
Question.
The first ionization energy of an element is the energy required to remove an electron from a gaseous atom of the element (i.e.,\( X(g) → X^{+}(g) + e ^-)\). The values of the first ionization energies for the third-row elements are shown in the graph above. On the basis of the information given, which of the following reactions is exothermic?
(A) \(Cl(g) + Mg+(g) → CI^{+}(g) + Mg(g)\)
▶️Answer/Explanation
Ans:B