Sunday, November 21, 2010

Balancing Chemical Equations (Introductory Stoichiometry)

A chemical equation describes what happens in a chemical reaction. The equation identifies the reactants (starting materials) and products (resulting substance), the formulas of the participants, the phases of the participants (solid, liquid, gas), and the amount of each substance. Balancing a chemical equation refers to establishing the mathematical relationship between the quantity of reactants and products. The quantities are expressed as grams or moles.

It takes practice to be able to write balanced equations. There are essentially three steps to the process:

1.Write the unbalanced equation.

•Chemical formulas of reactants are listed on the lefthand side of the equation.

•Products are listed on the righthand side of the equation.

•Reactants and products are separated by putting an arrow between them to show the direction of the reaction. Reactions at equilibrium will have arrows facing both directions.

2.Balance the equation.

•Apply the Law of Conservation of Mass to get the same number of atoms of every element on each side of the equation. Tip: Start by balancing an element that appears in only one reactant and product.

•Once one element is balanced, proceed to balance another, and another, until all elements are balanced.

•Balance chemical formulas by placing coefficients in front of them. Do not add subscripts, because this will change the formulas.

3.Indicate the states of matter of the reactants and products.

•Use (g) for gaseous substances.

•Use (s) for solids.

•Use (l) for liquids.

•Use (aq) for species in solution in water.

•Write the state of matter immediately following the formula of the substance it describes.

Worked Example Problem

Tin oxide is heated with hydrogen gas to form tin metal and water vapor. Write the balanced equation that describes this reaction.

1.Write the unbalanced equation.

SnO2 + H2 → Sn + H2O

Magic Rocks in Space

Have you ever wondered how a chemical crystal garden, like Magic Rocks, would grow in space? These photos show the difference between a silicate chemical garden grown on the International Space Station and one grown in full gravity on Earth.

Mercury Beating Heart

The Mercury Beating Heart is a popular chemistry demonstration/experiment in which a blob of mercury is made to pulsate, resembling a beating heart. The mercury beating heart requires only a few materials and is easy to set up, though it's a little tricky to get going.

Mercury Beating Heart Materials

•drop of mercury

•dilute sulfuric acid (battery acid strength works)

•potassium permanganate or potassium dichromate solution or crystals

•iron wire or nail

•watch glass or petri dish

Perform the Mercury Beating Heart Demonstration

1.Place a drop of mercury into a dish.

2.Cover the mercury with sulfuric acid.

3.Add a small amount of potassium permanganate or potassium dichromate.

4.Slowly move the iron wire or nail so that the tip is near the mercury, but not quite touching it. The distance depends on the ratio of the concentration of the acid and oxidizer, so you'll need to play with this to find the sweet spot.

5.Once you have the distance right, the mercury will contract into a rounded ball and then spread out to touch the iron and contract again. The 'beating heart' will beat for about 20 seconds.

Learn how the Mercury Beating Heart works..