Question
Which describes an exothermic reaction?
| Heat transfer | Enthalpy |
A | from surroundings to system | reactants > products |
B | from surroundings to system | products > reactants |
C | from system to surroundings | products > reactants |
D | from system to surroundings | reactants > products |
▶️Answer/Explanation
Ans: D
The flow of heat (q) at constant pressure in a process equals the change in enthalpy based on the following equation,
ΔH=q.
If the heat is released, the reaction being an exothermic reaction, it means the heat is transferred from the system to the surroundings. Hence, q<0 (negative). Therefore, ΔH will be negative if q is negative.
Since, the ΔH of a reaction is calculated by subtracting the sum of the enthalpies of the reactants from the sum of the enthalpies of the products, therefore, enthalpy of reactants > products in case of exothermic reaction.
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.
Question
Some water is heated using the heat produced by the combustion of a particular sample of magnesium magnesium metal. Which values are needed to calculate the enthalpy change of reaction?
I. The mass of magnesium
II. The mass of the water
III. The change in temperature of the water
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
D
Heat produced associated with the combustion of the particular sample of magnesium depend on the mass of magnesium in that particular sample.
Enthalpy change of heating of water depends on mass of the water heated and the change in temperature of water heated.
Question
What is the energy, in kJ, released when 1.00 mol of carbon monoxide is burned according to the following equation?
\[\begin{array}{*{20}{l}} {{\text{2CO(g)}} + {{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2C}}{{\text{O}}_{\text{2}}}{\text{(g)}}}&{\Delta {H^\Theta } = – 564{\text{ kJ}}} \end{array}\]
A. 141
B. 282
C. 564
D. 1128
▶️Answer/Explanation
B
According to the given reaction, the change is enthalpy is negative, which means reaction is exothermic and heat is released.
Hence,
when 2 moles of carbon monoxide is burned, 564 kJ energy is released.
So, if 1.00 mol of carbon monoxide is burned, 282 kJ of energy would be released.
Question
The specific heat of iron is \({\text{0.450 J}}\,{{\text{g}}^{ – 1}}{{\text{K}}^{ – 1}}\). What is the energy, in J, needed to increase the temperature of 50.0 g of iron by 20.0 K?
A. 9.00
B. 22.5
C. 45.0
D. 450
▶️Answer/Explanation
D
Q=m.c. ΔT
Q = 50×.450×20 = 450 J
Question
Which is true for a chemical reaction in which the products have a higher enthalpy than the reactants?
A. Endothermic reaction.
B. Exothermic reaction.
C. Spontaneous reaction.
D. Reversible reaction.
▶️Answer/Explanation
A
Since, the ΔH of a reaction is calculated by subtracting the sum of the enthalpies of the reactants from the sum of the enthalpies of the products, therefore, enthalpy of reactants < enthalpy of products in case of endothermic reaction.