ChemThink; Chemical Reactions

Starting materials in a chemical reaction are called reactants. The materials you end up with in a chemical reaction are called products. An arrow indicates that a chemical change has taken place.

All reaction have on thing in common: there is a rearrangement of chemical bonds. Chemical reactions always involve breaking old bonds, forming new bonds, or both. In all reactions we still have all of the same atoms at the end that we had at the start.     In every reaction there can never be any missing atoms or new ones. Chemical reactions only rearrange the the bonds in the atoms that are already there.

To make water out of hydrogen and oxygen, If you only use 2 Hydrogen (H2) and 2 Oxygen (O2) we would end up  with a water molecule and one O molecule left over.  We must have the same atoms present before and after the reaction. In a balanced chemical reaction the same # is present before and after. To make the equation for H2O balanced we need 4 Hydrogen atoms (2 H2) and 2 Oxygen atoms (O2) to make 2 H2O molecules. Which is the same as:

# of atoms in Reactants	Element	# of atoms in product
4	H	4
2	O	2

This idea is called the law of conservation of mass. There must be the same atoms and the same # of atoms before and after the reaction.
To make the following equation balanced (_Cu + _O ---> _CuO) there must be 2 Copper atoms and 1 Oxygen atom to make 2 CuO molecules. In the unbalanced equation there are 2 Cu atoms and  2 O atoms -----> 2 Cu atoms and  1 O atom.  We need to add CuO atoms to the product because this equation doesn’t make lone O atoms. Now there are enough O atoms but there aren’t enough Cu atoms. Therefore we must add more Cu atoms.This makes the balanced equation for this reaction 2 Cu + O2 -----> 2 CuO, which is the same thing as saying the reactants have 2 Cu atoms and so does the product. The reactants have 2 O atoms and so does the product.
The balanced equation for _ CH4 + _ O2 ---> _H2O + _ CO2 is 1Ch4 + 2 O2 -----> 2H2O + 1 CO2.
The balanced equation for _N2 + _H2 --->_NH33 is 1 N2 + 3 H2 ------> 2 NH3.
The balanced equation for _ KClO3 ---> _ KCl + _ O2 is 2 KCIO3 -------> 2 KCI  + 3 O2.
The balanced equation for _ Al + _O2 --->_ Al2O3 is 4 Al + 3 O2 --------> 2 Al2O3.

In summary, chemical reactions always involve breaking bonds, making bonds, or both. The law of conservation of mass says chemical reactions can rearrange bonds but there must be the same amount of atoms present before and after the change. To balance a chemical equation you change the coefficients in front of each atom or molecule until there are the same # of each type of atom before and after the reaction.

Chemical Reactions Demonstration

Today Ms. Leland decided to show us some chemical reactions.

My favorite experiment was the ethanol experiment. Ms. Leland poured about a tablespoon of ethanol in to a liter bottle. Then she screwed on the cap and shook it around until it coated the inside of the bottle. The bottle was unscrewed, the excess ethanol was dumped, and the bottle set on its side. The lighter was lit and she held it right next to the bottle mouth. Blue flames spread throughout the bottle, but the bottle stayed in the same place. The second time the bottle moved about three inches. The third time we didn't dump the ethanol and the bottle didn't move, but it set the table on fire. Ok, I'm exagerating, but it did dump the extra ethanol on the table when we picked it up, and the ethanol was on fire.

The second experiment was with baking soda and vinegar. We put some baking soda into a flask and then poured some vinegar into it. We put the mouth of the flask next to the two lit candles. It promptly blew the candles out because it was emitting carbon-dioxide.

The last experiment was an experiment with Hydrochloric acid (the stuff in your pool) and zinc. She poured about 3/4 of a cup of hydrochloric acid into a beaker. Ms. Leland showed us the zinc, and then dropped a few in the beaker. The lights were turned off and we saw whitish smoke drifting up. Ms Leland would later explain that it was Hydrogen gas. She lit the gas with a lighter and all the gas in that area burst into flame. It burned down into the beaker and feasted on the gas the combination was emitting. It put itself out when there was no longer enough gas being emitted to sustain the flame. Ms. Leland had to use a pair of tongs to take the beaker out of the room so that we wouldn't become nauseous from the smell/gas. :)

Overall I thought it was a wonderful demonstration. I enjoyed the science behind it and thought it was a lot of fun.

Chemical Reactions and Heat Lab

In this lab we got to explore reactants and products. We also got to see how catalysts effect them.

We changed the temperature (catalyst) to see how it effected the time it took to dissolve an alka-seltzer tablet in water.
This table shows the comparison of the experiments depending on the temperature.

  This is the graph for the room temperature test.

  This is the graph for the cold water test.

 This is the graph for the hot water test.

The following diagrams depict our setup and the amount of product produced at an instantaneous time.

Cold = Very few bubbles

Room = Some bubbles

Hot = Lots of bubbles!!!

 What is a catalyst? A catalyst is a particle of matter that changes the rate of reaction. Some xamples of catalysts are temperature and concentration.

 or

 or

alka-seltzer + water =  CO2 bubbles

My hypothesis was that the higher the temperature the faster the reaction would happen. My hypothesis was correct and it was fun to test.

If I were to do this test again I would do all three experiments at once to see if I was physically able to do that many experiments on time and perfectly. It probably won't happen but its a fun idea.

Freezing and Melting Water Lab

Problem: What is the relationship between the freezing and melting points of water?

Hypothesis: They are far apart on the temperature scale.

Materials:
-ring stand

-safety goggles
-test tube

-400 mL beaker
-water
-10 mL graduated cylinder ice
-salt
-temperature probe
-Logger Pro
-verneir computer interface
-computer
-stirring rod 
-spoon
-paper cup

Procedure: Collect the materials listed above. Put on safety goggles. Connect the temperature probe to the lab quest mini and insert the probe in to the test tube. Plug the lab quest mini into the computer. Open the Logger Pro file called 02 Freeze Melt Water. Fill the beaker 1/3 full of ice and add 100 mL. Measure 5 spoonfuls of salt into the paper cup. Put 5 mL of water in the test tube. Put the beaker under the ring stand and select the collect button on the computer. Lower the test tube in the beaker and move the probe around inside the test tube. Add the 5 spoonfuls of salt to the water in the beaker and use the stirring rod to dissolve it. When the data collection table reaches ten minutes, lean the test tube against the inside of the beaker and let it be until the 15 minutes on the data collection table is up. Remove the test tube and hold it so that the probe inside doesn't move at all. Select the flat part of the graph and hit the statistics button. What ever the listed median is is the freezing point of water. Write this down on a chart.
Dispose of the ice water and fill the beaker with normal tap water. Hit the collection button. Set the flask inside the beaker, after 12 minutes, on the ring stand. Select the flat area of the graph and click the statistics button. The median is the melting point of water.

Results: We got to use a temperature probe to find out what the freezing and melting points are.

We realized that it wasn't the boiling point we were looking for. We were looking for the melting and freezing points, which was when we realized that they would be close together. We found that the melting point was .7063˚C and the freezing point was .6433˚C. The water temperature dropped really low when we put it in the ice water bath and it spiked near the end of the warm tap water. The freezing temperature is close in temp to that of the melting point. The melting point is higher. Clean up lab area and materials. Take goggles off.

Does the Kinetic energy stay the same, increase or decrease...

when the temperature is changing at the beginning and end of Part I 
The kinetic energy is decreasing.
when the temperature remains constant in Part I
The kinetic energy stays the same or there is none. The potential energy is increasing.
when the temperature is changing at the beginning and end of Part II 
the kinetic energy is increasing.
when the temperature remains constant in Part II 
it stays the same or there is none. The potential energy is increasing.


Conclusion: If I were to do this experiment again I would like to have a utility clamp to hold the test tube in place instead of having team members take turns holding it. I would also liked to have had more use for a graduated cylinder. If we could put a twist on it we could use a freezer to speed up the process.
I now realize my mistake in thinking we were talking about freezing and boiling points. I think that was the  main reason that my hypothesis was wrong. I now know that I must make sure to read about what we are doing carefully.