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Name: Kelsi Status: student Grade: 4-5 Country: USA Date: Winter 2012-2013

Question:
I am doing a project determining the rate of reaction when combining acetic acid (table vinegar) and sodium bicarbonate. The more vinegar I use the faster the reaction when a constant amount of sodium bicarbonate is used. Why is the reaction faster and why is it cold to the touch the more vinegar is used?

Replies:
So, I see that you are in the 4th or 5th grade.

Let us start with the second question. If it feels cold, then this is an endothermic reaction. This means that the reaction is absorbing heat energy. When you touch the container in which the reaction is taking place, it feels cold because your skin in supplying some of the heat energy being absorbed. When heat energy leaves your skin, you feel cooler.

Now to the first question. Understanding rate of reaction is pretty complex. To a chemist, the "rate of reaction" is not so simple as the time it takes for a reaction to reach the completion point. Your question is what we in science education call a model for the behavior that you have observed. That word MODEL is very important in science education. The model for your experiment has a lot to do with something called chemical kinetics. In any event, it would probably be better (at least from a chemist's point of view) to refer to your results as determining the time to completion. Time to completion is an important part of determining the reaction rate but determining reaction rate takes a lot more math.

So, here is my explanation of a model for the behavior you are asking about in your 1st question. The chemical units that are reacting together are acetic acid (vinegar is mostly water into which acetic acid is dissolved), and sodium bicarbonate (baking soda). Baking soda is also called bicarbonate of soda or sodium bicarbonate. It is a salt that creates a basic solution when dissolved in water. When you mix vinegar with baking soda, the baking soda is dissolved in the water that is in your vinegar. Now, acids sort of behave like salts when in water. Both acetic acid and sodium bicarbonate are broken up into ions. Acetic acid breaks up into acetate ions and hydrogen ions (sometimes called protons). Sodium bicarbonate breaks up into sodium ions and bicarbonate ions. When you mix a greater amount of vinegar you could be dissolving the sodium bicarbonate faster, allowing the ions to react faster OR you could be getting more acetic acid ions to get to the sodium bicarbonate ions faster, OR it could be a little of both. That last choice is my guess and it would take a different set of experiments to find out which of the 1st 2 choices is the most important.

Why is this about kinetics? Kinetics is about movement and the model is about getting the chemical units (the ions from vinegar and baking soda) to run into each other and react.

Good luck on your experiment and presentation. Keep up the good work!

Ray Tedder, NBCT


Kelsi,

First of all, you need to know that table vinegar is a mixture of water and acetic acid. The water does not really react with sodium bicarbonate, only the acetic acid does. So think of what is happening at the molecular level when you combine vinegar with sodium bicarbonate. The water will certainly touch the molecules of sodium bicarbonate, but the acetic acid - which is moving around in the water, might not. But, in order for the acetic acid and the sodium bicarbonate to react, they have to touch each other - they have to come into contact.

So we need to think of what factors can increase the amount of contact of acetic acid with sodium bicarbonate.

Now imagine that you are walking in the hallway of your school. If there are 10 boys and 10 girls walking around in the hallway, the chances of you bumping against a boy will be less than if there were 20 boys and 0 girls in the hallway. This should tell you that the "concentration" of boys matter in the number of times you have contact with a boy. If on the other hand instead of 10 boys and 10 girls, there are 100 boys and 100 girls, the ratio of boys to girls is still the same, your chances of contact with a person, boy or girl, will be higher, but the chances of contact with a boy as opposed to a girl will be the same as before. If the concentration is not changed (the ratio of boys to girls) then the amount of contact with a boy as opposed to a girl will be the same.

So there are two things to think about: (1) the increased contact (and increased number of reactions) that is due to the increase in the amount of the vinegar used, and (2) the fact that if you increase the amount, you are not really changing the concentration of the acetic acid in the solution so there will be a point that the reaction will not get any faster because the amount of contact with vinegar as opposed to water does not increase.

Greg (Roberto Gregorius) Canisius College


Hi Kelsi,

Thanks for the question. We teach our students that if there are more reactants present then the reaction is faster. This is one of the principle of chemistry and it is valid in your situation. So, you are correct to expect a faster reaction and more bubbles when there is more vinegar and sodium bicarbonate. Since there is more reactants present, they bump into each other more and the reaction happens at a faster rate.

The mixture gets cold since the reaction is "endothermic" which means that it takes heat from the outside. The molecules need heat in order to form the bubbles.

I hope this helps. Please let me know if you have more questions. Thanks Jeff Grell


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