Tag Archives: chemical reactions

Calculate Theoretical Yield of a Chemical Reaction – Theoretical Yield Example Problem

Gas Collection Woodcut

Source: Elements of Modern Chemistry, Charles Adolphe Wurtz, 1887.

The theoretical yield of a chemical reaction is the amount of product you expect to get after a chemical reaction takes place from the reagents you have available. To calculate the theoretical yield of a reaction, you must first know the reaction. Let’s look at the following reaction where heating potassium chlorate (KClO3) produces oxygen gas (O2).

2 KClO3 (s) → 3 O2 (g) + 2 KCl (s)

where KCl is potassium chloride. This reaction is fairly common in school laboratories since it is a relatively inexpensive method of obtaining oxygen gas.

The balanced reaction shows that 2 moles of KClO3 will produce 3 moles of O2 and 2 moles of KCl. To calculate the theoretical yield, you use these ratios as a conversion factor.  Here is a typical example problem.

Calculate Theoretical Yield Example Problem

Question: How many moles of oxygen gas will be produced from heating 735.3 grams of KClO3?

Answer:

Step 1. We need to know the amount of KClO3 in moles to use the conversion, so the first step is to convert grams KClO3 to moles KClO3. To make this easier, know the molecular mass of KClO3 is 122.55 g/mol.

Theoretical Yield Example Step 1

Theoretical Yield Example Step 2

Theoretical Yield Example Step 3

6 = x moles KClO3

Step 2: Use the chemical equation to relate moles KClO3 to moles O2.

From above, we see 2 moles of KClO3 will produce 3 moles of O2 gas.

Theoretical Yield Example Problem Step 4

Theoretical Yield Example Problem Step 5

Theoretical Yield Example Problem Step 6

x moles O2 = 3 x 3 moles O2
x moles O2 = 9 moles O2

6 moles of KClO3 (735.3 grams of KClO3) will produce 9 moles of O2 gas.

Soap in a Tube Saponification Reaction

Test Tube Soap Bubbles (Jason Hickey)

Test Tube Soap Bubbles (Jason Hickey)

Soap in a tube? Why not! Typically when people make soap, they make a lot of it. However, you can make a small amount of soap in a test tube as a chemistry demonstration. This is a terrific example of the saponification reaction, plus it results in a product you can use.

Soap Materials

  • sodium carbonate (a home canning chemical)
  • calcium oxide (used to control humidity, sold in home stores)
  • sodium chloride (table salt)
  • butter or lard
  • water
  • 2 test tubes (or small heat-safe cups, if you don’t have test tubes)
  • candle or alcohol lamp
  • teaspoon or other measure

Make Soap in a Tube

  1. Add two scoops of sodium carbonate and 3 scoops of calcium oxide into a test tube.

    Once you master soap in a tube, upscale and try making homemade bars of soap. (kahvikis)

    Once you master soap in a tube, upscale and try making homemade bars of soap. (kahvikis)

  2. Fill the test tube about 3/4 full with water.
  3. Heat the test tube in the flame, but don’t bring the solution to the boiling point. As a safety measure, point the open end of the test tube away from your face. This is a good lab practice when heating any chemicals.
  4. Remove the test tube from heat and allow it to cool.
  5. When the liquid has cleared, add a pea-sized amount of butter or lard.
  6. Return the test tube to the flame. First, the butter or lard will melt. When it starts to mix with the water, add 6 scoops of sodium chloride.
  7. Carefully shake the test tube to mix the ingredients. The soap will rise to the top of the test tube.
  8. Allow the tube to cool and then remove the soap. If you rub it between your fingers, you’ll feel the characteristic slippery texture of your creation.

Examples of Chemical and Physical Properties

Flammability is a chemical property. (David Lindes)

Flammability is a chemical property. (David Lindes)

A chemical property is a characteristic of matter that can be observed and measured only when a chemical reaction occurs. Contrast this with a physical property, which is a characteristic that may be observed and measured without altering the chemical composition of a sample.

Here is a list of several examples of chemical and physical properties.

Chemical Properties

In order to observe a chemical property, the chemical composition of a sample must be changed by a chemical process or reaction.

  • flammability
  • toxicity
  • enthalpy of formation
  • heat of combustion
  • oxidation states
  • pH
  • half-life
  • coordination number
  • surface tension
  • reactivity
  • hygroscopy

Physical Properties

A physical property may be observed without changing the chemical nature of a sample. Any mechanical property you can name is a physical property, including:

  • mass
  • volume
  • density
  • color
  • temperature
  • melting point
  • boiling point
  • reflectivity
  • elasticity
  • luster
  • permeability
  • ductility
  • pressure
  • viscosity
  • strength
  • solubility
  • electrical charge
  • opacity
  • hardness

5 Ways To Make a Volcano

Vinegar and Baking Soda Volcano

The vinegar and baking soda volcano is the classic science fair model volcano. (Anne Helmenstine)

There is more than one way to make a chemical volcano. Actually, there are several methods. Here are some of the best, from the tried-and-true baking soda and vinegar volcano to the most exotic dry ice volcano.

Make the Volcano Cone

You can use a bottle or can or really any container for your volcanic eruption, but it’s easy to make the volcano shape by coating your container with clay or papier mache. Here is a simple recipe for a homemade clay volcano:

  • 6 cups flour
  • 2 cups salt
  • 2 cups water
  • 4 tablespoons cooking oil
  • 2 cups water
  1. Mix the ingredients together in a large bowl. It’s easiest if you stir the flour, salt, and oil together first and then mix in the water. You can add more water if needed. You want a firm, smooth dough.
  2. Stand an empty soda bottle or can in a pie tin or baking pan (so your ‘lava’ won’t make a mess) and mold the dough into a volcano shape. Be sure you don’t drop dough into the bottle or cover the opening.
  3. If you want to paint the volcano, wait until the dough is dry.

Now for the recipes! Most use common ingredients that you have at home.

Baking Soda and Vinegar Volcano

This is the classic science fair project volcano. The baking soda (sodium bicarbonate) reacts with the vinegar (weak acetic acid) to produce carbon dioxide gas. The detergent traps the gas, which is heavier than air, so it flows down the side of the volcano.

  • warm water
  • liquid dishwashing detergent
  • red or orange food coloring
  • baking soda
  • vinegar
  1. Pour warm water into the volcano until it is 1/2 to 3/4 of the way full.
  2. Add several drops of food coloring.
  3. Add a squirt of detergent. This helps the ‘lava’ foam up and flow.
  4. Add a couple of spoonfuls of baking soda.
  5. When you are ready to start the eruption, pour vinegar into your volcano.
  6. You can recharge the volcano with more baking soda and vinegar.

Note: If you don’t have vinegar, you can use another acidic liquid, like lemon juice or orange juice.

Yeast and Peroxide Volcano

  • packet of quick-rise yeast
  • hydrogen peroxide (3% sold in stores or can use 6% from beauty supply stores)
  • food coloring
  1. Pour the hydrogen peroxide solution into the volcano until it is nearly full. The 3% household peroxide is safe to handle, but wear gloves and use extreme caution if you use the 6% peroxide, which can give you chemical burns!
  2. Add several drops of food coloring for your lava.
  3. When you are ready for the eruption, add the packet of yeast to the volcano.

Ketchup and Vinegar Volcano

Ketchup Volcano

The ketchup volcano erupts with a thick red lava. (Anne Helmenstine)

This volcano bubbles and oozes lava. The eruption is not so dramatic, but is interesting and long-lasting. The acidity of the vinegar and tomatoes in the ketchup reacts with the baking soda to produce carbon dioxide gas, which gets trapped as bubbles by the detergent.

  • ketchup
  • warm water
  • dishwashing liquid
  • baking soda
  1. Mix together ketchup, warm water, and a squirt of detergent to make lava.
  2. Pour the mixture into the volcano so it is nearly full.
  3. When you are ready for the eruption, add baking soda.

Mentos and Diet Soda Volcano

Mentos and Tonic Water

The quinine in tonic water causes it to glow blue under black light. (Anne Helmenstine)

This volcano erupts instantly and spectacularly. For a truly memorable volcano, use diet tonic water instead of diet cola and shine a black light on the volcano. This produces a vivid blue glowing eruption!

  • diet soda (regular soda works too, but produces a sticky mess)
  • Mentos candies
  1. Fill the volcano full of soda (or you could have molded the volcano around a full soda bottle.
  2. When you are ready for the eruption, drop all of the Mentos candies into the mouth of the bottle at once. One easy way to do this is to roll a sheet of paper around the candies, put your finger beneath them to hold them in place, and release the candies over the hole. Be prepared for a major splash!

Dry Ice Volcano

This volcano appears to smoke, releasing a cascade of bubble lava.

  • warm water
  • dishwashing liquid
  • dry ice
  1. Fill the volcano with warm water.
  2. Add a bit of dishwashing liquid.
  3. When you are ready to start the eruption, use gloves or tongs to drop a piece of dry ice into the volcano.