Video Transcript
A student pours a sodium hydroxide
solution into a beaker of hydrochloric acid, which contains a thermometer showing
room temperature. Which of the following is the
correct observation? (A) The thermometer shows no change
in temperature. (B) The thermometer shows a rise in
temperature, so it is an endothermic reaction. (C) The thermometer shows a rise in
temperature, so it is an exothermic reaction. (D) The thermometer shows a fall in
temperature, so it is an endothermic reaction. (E) The thermometer shows a fall in
temperature, so it is an exothermic reaction.
Sodium hydroxide is an example of a
metal hydroxide. If we add a metal hydroxide to a
beaker of acid, we’ll observe that the beaker gets hot. The products of the reaction
occurring in the beaker is a salt, sodium chloride, and water. One thing we need to determine to
answer this question is if we would observe a change in temperature on a thermometer
during this reaction. Since this reaction causes the
beaker to get hot, we can assume there will be a rise in temperature, so we can
eliminate answer choices (A), (D), and (E). We also need to figure out if this
reaction is endothermic or exothermic.
During an endothermic reaction,
energy is absorbed from the environment in the form of heat. So an endothermic reaction will
cause the temperature to decrease. An exothermic reaction releases
energy into the environment in the form of heat. As energy is released, an
exothermic reaction will cause the temperature to increase. Since the reaction between sodium
hydroxide and hydrochloric acid causes a rise in temperature, this reaction must be
exothermic. So the correct observation when
sodium hydroxide is added to hydrochloric acid is answer choice (C). The thermometer shows a rise in
temperature, so it is an exothermic reaction.
