Question

TWhen a calorimeter is used, no heat is exchanged between the contents of the container and the outside air. If an exothermic reaction is conducted inside a calorimeter filled with water, which of the following statements best describes what occurs during the reaction?
A. The total energy inside the calorimeter is conserved.
B. The thermometer will show a decrease in temperature.
C. The water decreases in temperature as the reaction absorbs heat.
D. The potential energy of the reactants is lower than that of the reactants.

Answer 1

Answer: option A. The total energy inside the calorimeter is conserved.

Explanation:

Since no heat is exchanged between the contents of the container and the outside air, If an exothermic reaction is conducted inside a calorimeter filled with water, you can do the following analysis:

• Since an exothermic reaction releases heat energy (some times also light), the temperature inside the calorimeter will increase. So, as per the option B, the thermometer will not show a decrease in temperature but an increase. As per the option C, the water inside the calorimeter will absorb this heat, and its temperature will increase. So, the options B. and C. are wrong.

• Since in an exothermic reaction energy is released (in the form of heat or light) there is a negative change of enthalpy, this is the chemical potential energy of the products will be lower than that of the reactants, which is exactly the opposite of what is stated by the option D.

• Conservation of energy is a universal law. It cannot be violated. So, if, as stated, no heat is exchanged between the contents of the container and the outside air, you must conclude that the total energy inside the calorimeter is conserved (option A).

The law of conservation of energy for the calorimeter, where no heat is exchanged between the contents of the container and the outside air, states that:

• heat generated by the chemical reaction = heat absorbed by the calorimeter and the water

Answer 2

An exothermic reaction is one that releases energy from the system to its surroundings, usually in the form of heat, but also in the form of light, electricity, or sound. Since the water gives off heat, the temperature of the water should be lower, so the answer might be B) The thermometer will show a decrease in temp.