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Writing Chemical Equations

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Chemical Equations
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How do you make Shrimp gumbo?

Shrimp gumbo is one of many enjoyable dishes that are part of the Cajun culture of Louisiana. It’s a spicy dish that needs careful control of all the ingredients so that it has a “kick”, but is not overwhelming. Recipes tell not only what is in the preparation, but describes how much of each ingredient and details of how to cook the meal. We need this type of information in order to carry out chemical reactions successfully and safely.

Chemical Equations

Word equations are time-consuming to write and do not prove to be convenient for many of the things that chemists need to do with equations. A chemical equation is a representation of a chemical reaction that displays the reactants and products with chemical formulas. The chemical equation for the reaction of methane with oxygen is shown:

\text{CH}_4+\text{O}_2 \rightarrow \text{CO}_2+\text{H}_2\text{O}

The equation above, called a skeleton equation, is an equation that shows only the formulas of the reactants and products with nothing to indicate the relative amounts. The first step in writing an accurate chemical equation is to write the skeleton equation, making sure that the formulas of all substances involved are written correctly. All reactants are written to the left of the yield arrow, separated from one another by a plus sign. Likewise, products are written to the right of the yield arrow, also separated with a plus sign.

It is often important to know the physical states of the reactants and products taking part in a reaction. To do this, put the appropriate symbol in parentheses after each formula: ( s ) for solid, ( l ) for liquid, ( g ) for gas, and ( aq ) for an aqueous (water-based) solution. The previous reaction becomes:

\text{CH}_4(g) + \text{O}_2(g) \rightarrow \text{CO}_2(g) + \text{H}_2\text{O}(l)

The table below shows a listing of symbols used in chemical equations. Some, such as the double arrow which represents equilibrium, and the use of a catalyst in a reaction, will be treated in detail in other concepts.

Symbols Used in Chemical Equations
Symbol Description
+ Used to separate multiple reactants or products
\rightarrow yield sign; separates reactants from products
\rightleftarrows replaces the yield sign for reversible reactions that reach equilibrium
( s ) reactant or product in the solid state
( l ) reactant or product in the liquid state
( g ) reactant or product in the gas state
( aq ) reactant or product in an aqueous solution (dissolved in water)
\overset{Pt}{\rightarrow} formula written above the arrow is used as a catalyst in the reaction
\overset{\Delta}{\rightarrow} triangle indicates that the reaction is being heated

Summary

  • A chemical equation is described.
  • Symbols used in chemical equations are described and explained.

Practice

Watch the video at the link below and do the examples seen in the video:

http://www.youtube.com/watch?v=lSoRj_iBwYc

Review

  1. What does a skeleton equation tell you?
  2. Why would you want to know the physical state of materials?
  3. What does the symbol  \rightarrow mean?
  4. If I see  \Delta over the arrow, what will I do?

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