Video Transcript
Why will the measured change in energy always be lower than the total energy transferred? (A) Not all of the reactants will be used up. (B) The thermometers will have an error associated with them. (C) The water will contain impurities that will absorb some of the heat energy. (D) There will always be some amount of heat energy lost to the surroundings. Or (E) some of the energy gets destroyed during the reaction.
This question is asking about two different amounts of energy: the energy change that we measure and the energy change that is actually transferred. When we carry out a reaction, we can measure the amount of heat that is transferred between the reaction system and the surroundings. To understand this question, we need to know how does temperature change relate to energy change.
As a simple example, we can consider a neutralization reaction, where hydrogen ions combine with hydroxide ions. When these ions combine to form a water molecule, energy is released in the form of heat. That release of energy will increase the energy level and the temperature of the surrounding substance. A change in temperature involves a transfer of thermal energy from one substance to another.
In the experimental setup we’ve drawn above, the thermometer is measuring the temperature of the solution in the beaker. However, the thermal energy does not only get transferred to the solution. There is an increase in temperature of the solution, the beaker that contains the solution, and even the air surrounding the beaker. If we measure just the temperature change of the solution, we won’t be accounting for the entirety of the energy transfer of this process. The answer choice that best fits this description is choice (D). There will always be some amount of heat energy lost to the surroundings.
We often insulate reactions to minimize this heat loss so that the measured change in energy is closer in value to the total energy transferred. However, no matter how well we insulate the reaction, we won’t be able to measure the temperature in a way that accounts for all of the energy transferred during the reaction.
To be thorough, let’s look at the other answer choices as well. Answer (A) is incorrect. If not all of the reactants are used up, that would decrease the measured change in energy. But it would also decrease the total energy transfer. It would not cause a change between these two values. Answer (B) is incorrect as well. While thermometers may have errors associated with them, they are just as likely to overestimate as underestimate. So we can’t use this as a reason for why the measured energy change would be lower than the total energy change. Answer (C) is also incorrect. A thermometer placed in a solution will measure the temperature change of the solution as a whole, including its impurities. So our measured temperature change does account for the energy transferred to those impurities. Lastly, choice (E) is also incorrect, as energy cannot be created or destroyed. It can only be transferred or converted from one form to another.
So why will the measured change in energy always be lower than the total energy transferred? Because there will always be some amount of heat energy lost to the surroundings, choice (D).
