Is this endothermic or exothermic
https://prezi.com/6uirn5n8gvuu/endothermic-baking-soda-and-vinegar/
http://www.hawkaia.com/oldhawkaiasite/endothermic/
The basic definitions are easy to understand:
- Endothermic: a reaction or process which absorbs heat from its surroundings.
- Exothermic: a reaction or process which gives off heat to its surroundings.
The problem comes in trying to state which specific examples of reactions are which, because usually a lot of things are going on at the same time.
The Confusing Experiment and the Difficult Question
Here was the experiment which confused her: Mix baking soda and vinegar, stick a thermometer in the solution and watch it go down. This concludes our demonstration of an endothermic reaction.
Here was the question my daughter asked: “How can you be adding heat to something and its temperature be going down? If the temperature is going down, it must be giving off heat, and that is exothermic!”
Now this is a VERY good question, and no one she talked to could give her a good explanation as to just how you could be adding heat to the solution, and yet its temperature be dropping. Here is how we (me, along with the help of four Purdue engineering graduates!) were finally able to resolve her confusion.
Basic Concepts
First- we must define the system. The vinegar and baking soda mixed together to form a solution are the system. EVERYTHING else – the container, the air, the people doing the experiment- are the surroundings. Focus on just our little system, and don’t let everything else going on around the system add confusion.
Second– we must understand an important concept about the conservation of energy. Everything is trying to get to the same temperature. Pour a cold drink into a glass and what happens after about an hour? Assuming you didn’t drink it, the temperature of the drink is now the same as its surroundings.
Third– to change from one state to another (from liquid to solid, from solid to gas, etc.) takes energy. To change from one molecular structure to another (from separate substances mixed to form a solution) takes energy.
Demonstarting the Concepts
Put some ice cubes in a container, and apply heat.
- What happens? The ice melts.
- Is this endothermic or exothermic? This is an endothermic reaction, because the system – the ice and water – are absorbing heat.
- What temperature is the ice? Roughly 32 degrees F. (0 degrees C.)
- What temperature is the water formed right as the ice begins to melt? Roughly 32F. (0 C.)
- What happened to the heat energy we applied to the ice? It takes energy to convert the ice to water, so at the instant that the ice melts and the water forms, both are exactly 32 degrees, and the heat energy is used by the ice to change its state into water.
- What does the temperature of the ice and water have to do with determining whether this is an endothermic or exothermic reaction? NOTHING! The only thing you need to know is that the ice absorbed the heat from its surroundings, therefore it is endothermic.
So, in this endothermic example, the temperature of the ice and water stayed the same at the exact instant of the reaction. As long as heat continues to be applied after the change from ice to water, either from our burner flame, or even just from sitting in a warm room, the temperature of the water goes up.
Continue to apply heat to the melting water, until the water begins to boil.
- What happens? The water boils.
- Is this endothermic or exothermic? This is an endothermic reaction, because the system – the water – is absorbing heat.
- What temperature is the water just before it boils? Roughly 212 degrees F. (100 degrees C.)
- What temperature is the gas formed right as the water begins to boil? Roughly 212F. (100 C.)
- What happened to the heat energy we applied to the water? It takes energy to convert the water to a gas (steam), so at the instant that the water boils and steam forms, both are exactly 212 degrees, and the heat energy is used by the water to change its state into steam.
- What does the temperature of the water and steam have to do with determining whether this is an endothermic or exothermic reaction? NOTHING! The only thing you need to know is that the water absorbed the heat from its surroundings, therefore it is endothermic.
So, in this endothermic example, the temperature of the water and steam stayed the same at the exact instant of the reaction. As the hot gas is realeased into its surroundings, the water vapor will cool and possibly condense on some surface (like on the outside of that cold drink glass we talked about before). Then it will sit around for a while longer until it all becomes the same temperature as the room. None of this has anything to do with determining whether the boiling water was an endothermic reaction. The water absorbed heat – that is all we need to know.
What’s going on with the Vinegar and Baking Soda?
We mix vinegar and baking soda, and observe the temperature of the solution. This is a much more complex example than the water we used before, because we are mixing two substances which react with each other to form a solution.
- What happens? The vinegar and baking soda bubble, forming a gas, and a soupy white goop.
- What happens to the temperature of the solution? It goes down
- Are the bubbles giving off any heat? No, they are roughly the same temperature as the soupy white goop – just like the boiling water example.
- Is the solution absorbing heat energy from its surroundings? Yes, it absorbs it from the surrounding air. This is an endothermic reaction.
- How can you be adding heat to it, and the temperature goes down? The vinegar and baking soda are changing from their individual molecular structures to a new molecular structure – the soupy white goop. This molecular change requires energy (heat) which it absorbs from the room. The problem is, it can’t absorb it fast enough. It needs more heat than it is getting, therefore the temperature goes down. Eventually, if the soupy white goop sits around long enough, it will become the same temperature as the room. Another way to think of it is that the goop is trying to stay the same temperature, but it can’t, so its temperature drops. If you add more heat to the solution, the temperature will rise again. It wants to be the same temperature as the room – but during that exact instant when the solution reacts, it just can’t get enough heat.
- What does the temperature of the solution have to do with determining whether this is an endothermic or exothermic reaction? NOTHING! The only thing you need to know is that the reaction absorbed the heat from its surroundings, therefore it is endothermic. The dropping temperature of the solution is just evidence that it is absorbing heat, and not giving it off. If you could somehow measure the temperature of the room to 64 decimal places, you would note that the temperature of the room also dropped during the reaction, which would be just as good evidence that it was an endothermic reaction.
Advanced Explanation – Potential and Kinetic Energy.
The molecular structure of the vinegar and baking soda combined and reacting form kinetic energy. Heat energy from the surroundings is required because in the process, they are also combining to form a new molecular structure which has potential energy (the goop). The change from kinetic energy to potential energy requires heat. So much heat is needed that the temperature drops because it can’t get the heat fast enough. This explanation did not help my daughter as well as the previous examples, but is a little more exact explanation as to how the temperature can be dropping, even though you are adding heat.
Keep it Simple
Understanding exothermic reactions is pretty easy. Most reactions are exothermic. A log burns, it gives of heat. An atom splits, it gives off heat. The key to understanding endothermic reactions is to stay focused on the one question – is the system absorbing heat from its surroundings? Stay focused on the little system, and don’t let everything else going on around the system add confustion.
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