Atomic Ratios
Getting Started
Having students take a small amount of baking soda and creme of tartar, add some water. Sodium bicarbonate is a molecule that contains 3 elements, Sodium, carbon, and oxygen. When we add water the reaction occurs we see bubbles. The bubbles are carbon dioxide gas. Help students think about the model of carbon dioxide gas by showing a diagram of the atomic structure:
The red orbs represent the atoms of oxygen and the grey, carbon.
This is the model for baking soda, sodium bicarbonate. The red, oxygen, the grey, carbon, the white, a hydrogen atom, and the purple, a sodium atom. NaHCO3
We can see the carbon dioxide in the baking soda molecule. Notice that this model shows a white ball to represent the atom sodium. Notice also that there is an extra oxygen atom.
Exploration
Give students opportunity to use gum balls and toothpicks to represent atoms and molecules. Colors can represent different types of atoms and number of toothpicks can represent the number of bonds.
Build a model of Sodium Bicarbonate and then remove the C02. What is left? Have students experiment with the spice balls to remove the CO2 from the Sodium bicarbonate.
Chemical Reaction
When we add an acid to baking soda the CO2 is released. The acid we will use is a molecule of Tartaric Acid (Creme of tartar)
Here we see 4 carbon atoms (grey), 6 oxygen atoms (red), and 6 hydrogen atoms (white)
Here is a gum drop model
Exploration
Our task now is to determine how much tartaric acid we need to release the carbon dioxide. What we want students to do is determine what the ideal ratio is between the two substances to understand the requirements for trading. Here is how we can explore this relationship:
1) Assemble a gas generating station as shown below.
2) Fill a graduate cylinder or test tube with water and invert in a water bath.
3) Insert a tube from a gas generating bottle as shown. What will happen that any gas produced in the bottle or vial will then displace the water in the graduate and then we can report the volume of the gas. We will have to read our graduate upside down, but will be able to report gas volume in milliliters.
4) Prepare a mixture of baking soda and creme of tartar.
1/8 tsp of Creme of tartar 1/8 tsp of baking soda (1:1) ratio
5) Place the mixture in the dry gas generating tube
6) One person should hold the vial and cap and another pour in about 10 ml of water. Immediately cap the bottle so all gas will now go through the tubing and not be lost.
7) Gently swirl the mixture and watch to see that all materials are thoroughly dissolved and that the reaction is allowed to be completed.
8) Record the volume of gas produced for the 1:1 ratio.
9) Repeat the above process, this time use
1/4 tsp creme of tartar and 1/8 tsp of baking soda (2:1) ratio
Repeat the procedure and collect the gas. Report the result.
10) A final time, repeat the process, this time us:
1/8 tsp of creme of tartar and 1/4 tsp of baking soda (1:2) ratio
Repeat the procedure and collect the gas. Report the result.
Reporting-out
Collect all the data from the teams and chart the results on the board. What ratio yielded the most gas? How many molecules of each are necessary to write the equation?
There is an ideal ratio. The best outcome should be a 1:2 ratio of creme of tartar to baking soda. Let's look at the formula:
1 molecule Tartaric Acid + 2 molecules of Baking soda = 2 molecules of Carbon dioxide + a new molecule, sodium tartrate + some water
symbolically,
C4H6O6 + 2 NaHCO3 → Na2C4H4O6 + 2 CO2 + 2 H2O
In this chemical reaction, the acid takes apart the baking soda and creates a new compound and releases carbon dioxide and water. The atoms shift around and replace one another to form new structures. If we have the correct ratio, the molecules can trade correctly. Otherwise there is some of either chemical left over, unreacted.
Besides C02 we now have also replaced some atoms on the Tartaric Acid molecule. Now the sodium is attached to it and it has now become sodium tartrate.
Students can use their gum balls to assemble this new molecule.
Sodium tartrate is used as a food additive to remove water and helps to emulsify substances in water.