What’s for dinner?
Various ways of recording recipes have developed over the centuries. The cookbook shown above was written by a woman who probably collected all her own recipes. Later, printed cookbooks became available (even guys had no excuse for not being able to cook). Today we can find recipes on a number of internet sites and can quickly search for information on how to cook anything we want. Reading a recipe sometimes requires we understand a few codes and symbols (what’s the difference between a tsp and a Tsp?), but the information on what we start with and what we end up with is there.
Writing Chemical Equations
Chemical reactions are occurring all around you. Plants use sunlight to drive their photosynthetic process and produce energy. Cars and other vehicles burn gasoline in order to power their engines. Batteries use electrochemical reactions to produce energy and power many everyday devices. Many chemical reactions are going on inside you as well, especially during the digestion of food.
In math class, you have written and solved many mathematical equations. Chemists keep track of chemical reactions by writing equations as well. In any chemical reaction one or more substances, called reactants, are converted into one or more new substances, called products. The general form of the equation for such a process looks like this.
Unlike in a math equation, a chemical equation does not use an equal sign. Instead the arrow is called a yield sign and so the equation is described as “reactants yield products”.
You can describe a chemical reaction by writing a word equation. When silver metal is exposed to sulfur it reacts to form silver sulfide. Silver sulfide is commonly known as tarnish and turns the surface of silver objects dark and streaky black (see Figure below). The sulfur that contributes to tarnish can come from traces of sulfur in the air or from food such as eggs. The word equation for the process is:
The silver and the sulfur are the reactants in the equation, while the silver sulfide is the product.
Another common chemical reaction is the burning of methane gas. Methane is the major component of natural gas and is commonly burned on a gas stove or in a Bunsen burner (Figure below). Burning is a chemical reaction in which some type of fuel is reacted with oxygen gas. The products of the reaction in the burning of methane as well as other fuels are carbon dioxide and water. The word equation for this reaction is:
Word equations can be very useful, but do have one major drawback. They cannot be used for any quantitative work. A word equation does not tell how many moles of each material are needed or how many moles of product are formed.
The reaction between zinc and sulfur can be shown in what is called a chemical equation. In words, we could write the reaction as:
zinc + sulfur → zinc sulfide
The more convenient way to express a chemical reaction is to use the symbols and formulas of the substances involved:
Zn + S → ZnS
The substance(s) to the left of the arrow in a chemical equation are called reactants. A reactant is a substance that is present at the start of a chemical reaction. The substance(s) to the right of the arrow are called products. A product is a substance that is present at the end of a chemical reaction. In the equation above, the zinc and sulfur are the reactants that chemically combine to form the zinc sulfide product.
There is a standard way of writing chemical equations. The reactants are all written on the left-hand side of the equation, with the products on the right-hand side. An arrow points from the reactants to the products to indicate the direction of the reaction:
reactants ==> products
When appropriate, a symbol may be written above or below the arrow to indicate some special circumstance. The symbol "Δ" is often used to indicate that the reaction is to be heated.
The presence of the arrow also indicates that the reaction goes in one direction under the conditions indicated. There are reactions which can be easily reversed, but we will not take those up right now.
There are a wide variety of reactions possible: elements may form compounds (as seen in the reaction above), compounds may form elements (water will break down in the presence of an electric current to form hydrogen gas and oxygen gas) or compounds may combine, break apart, or rearrange to form new materials.
Representation of chemical reactions using 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:
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:
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.
|+||Used to separate multiple reactants or products|
|yield sign; separates reactants from products|
|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)|
|formula written above the arrow is used as a catalyst in the reaction|
|triangle indicates that the reaction is being heated|
- Word equations are used to describe the conversion of reactants to products.
- A chemical equation describes a chemical reaction.
- Reactants are starting materials and are written on the left-hand side of the equation.
- Products are the end-result of the reaction and are written on the right-hand side of the equation.
- Symbols used in chemical equations describe and explain.
Watch the video at the link below and do the examples seen in the video:
Use the link below to answer the following questions:
- Where are reactants written in a chemical equation?
- Where are products written in a chemical equation?
- What chemical information is found in a chemical equation?
- What is a reactant
- What is a product?
- What does a chemical equation do?
- Why would you want to know the physical state of materials?
- What does the symbol mean?
- What symbol is used to represent a gas?
- If I see over the arrow, what will I do?
- In the reaction sodium + water ==> sodium hydroxide + hydrogen,
- what are the reactants?
- what are the products?