Science Buddies: "Ask an Expert"

I am testing when you combine certain substances if they are endothermic or exothermic. The project says to measure the temperature at starting and then in thirty seconds after adding the solid substance, and then after 2 minutes. On both substances , citric acid and baking soda, vinegar and baking soda, the temperature increased after 30 seconds showing exothermic energy , but after 2 min there was a decrease in temperature showing endothermic energy. All the research states that they should both be exothermic after 2 min. Can you explain? Thank you.

Hi smartin1716,

What is your source of information that says baking soda/vinegar and baking soda/citric acid reactions are exothermic? Both these reactions are endothermic, not exothermic. This article from the American Chemical Society explains the energy changes in chemical reactions quite nicely: http://www.middleschoolchemistry.com/le ... r6/lesson7.

That being said, when measuring temperature changes in chemical reactions, you want all your reactants, containers, and thermometer to be at the same temperature.It's a good idea to let the thermometer sit in the liquid for a few minutes so that their temperatures can equilibrate before starting the experiment. Once everything is at the same temperature, then start the reaction. It is also important that the entire thermometer bulb be immersed in the solution. I suspect there was some temperature contrast between the reagents and thermometer when you started the reaction.