*REACTION TYPES* –>
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*a chemical reaction is a ‘process’ that leads to the ‘transformation’ of 1 set of ‘chemical substances’ to another*
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‘substrate’ + ‘re-agent’ = ‘product’
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(classically, “chemical reactions” encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation)
(“nuclear chemistry” is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes may occur)
(the substance (or substances) initially involved in a chemical reaction are called “reactants” or “reagents”)
(“chemical reactions” are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the “reactants”)
(reactions often consist of a sequence of individual sub-steps, the so-called “elementary reactions”, and the information on the precise course of action is part of the “reaction mechanism”)
(“chemical reactions” are described with chemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions)
(“chemical reactions” happen at a characteristic reaction rate at a given temperature and chemical concentration)
(typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms)
(“reactions” may proceed in the “forward” or “reverse” direction until they go to “completion” or “reach” equilibrium)
(reactions that proceed in the forward direction to approach equilibrium are often described as “spontaneous”, requiring no input of free energy to go forward)
(“non-spontaneous reactions” require input of free energy to go forward)
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(examples include…)
“charging a battery by applying an external electrical power source”
“photosynthesis driven by absorption of electromagnetic radiation in the form of ‘sunlight'”
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(different chemical reactions are used in combinations during chemical synthesis in order to obtain a desired product)
(in “bio-chemistry”, a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) form metabolic pathways))
(these reactions are often catalyzed by protein enzymes)
(‘enzymes’ increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperatures and concentrations present within a ‘cell’)
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(the general concept of a ‘chemical reaction’ has been extended to ‘reactions’ between ‘entities’ smaller than ‘atoms’, such as…)
*nuclear re-actions*
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*radio-active decays*
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*reactions between ‘elementary particles’*
(as described by ‘quantum field theory’)
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*CHEMICAL EQUATION*
(a chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae, wherein the reactant entities are given on the left-hand side and the product entities on the right-hand side)
(the co-efficients next to the symbols and formulae of entities are the absolute values of the ‘stoichiometric numbers’)
(the first chemical equation was diagrammed by ‘jean beguin’ in ‘1615’)
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*HOW TO WRITE A CHEMICAL EQUATION*
(WIKIHOW)
A good way to think about a chemical reaction is the process of baking cookies.
You mix the ingredients together (flour, butter, salt, sugar, and eggs), bake it, and see that it changes into something new: cookies!
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In chemistry terms…
the equation is the recipe,
the ingredients are “reactants,”
and the cookies are “products.”
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All chemical equations look something like “A + B →C (+ D…),” in which each letter variable is an element or a molecule (a collection of atoms held together by chemical bonds)
The arrow represents the reaction or change taking place. Some equations may have a double-headed arrow (↔), which indicates that the reaction can proceed either forward or backward. To write the equations there are a number of important naming rules that you need to know.
Writing Chemical Formulas of Covalent Compounds
Image titled Write a Chemical Equation Step 1
1
Memorize the prefixes for number of atoms. In naming compounds, Greek prefixes are used to indicate the number of atoms present for each element. Covalent compounds are written out as molecular formulas due to the fact that each compound is a distinct, separate molecule.[1] Covalent compounds have the first element written out completely while the second element is named with the suffix “ide.” For example, diphosphorus trisulfide has a chemical formula of P2S3.[2] Below are the prefixes for 1-10:
1: Mono-
2: Di-
3: Tri-
4: Tetra-
5: Penta-
6: Hexa-
7: Hepta-
8: Octa-
9: Nona-
10: Deca-
Image titled Write a Chemical Equation Step 2
2
Write the chemical symbol for the first element. When a compound has been written out, you must identify the elements and know their chemical symbols. The first element written is “first name” of the compound. Use the periodic table to find the chemical symbol for the element.[3]
For example: Dinitrogen hexafluoride. The first element is nitrogen and the chemical symbol for nitrogen is N.
Image titled Write a Chemical Equation Step 3
3
Add the number of atoms as a subscript. To identify the number of atoms present for each element, you simply need to look at the prefix of the element. Memorizing the Greek prefixes will help you to be able to write chemical formulas quickly without looking anything up.[4]
For example: Dinitrogen has a the prefix “di-“ which means 2; therefore, there are 2 atoms of nitrogen present.
Write dinitrogen as N2.
Image titled Write a Chemical Equation Step 4
4
Write the chemical symbol for the second element. The second element is the “last name” of the compound and will follow the first element. For covalent compounds, the element name will have a suffix of “-ide” instead of the normal ending of the element.[5]
For example: Dinitrogen hexafluoride. The second element is fluorine. Simply replace the “ide” ending with the actual element name. The chemical symbol for fluorine is F.
Image titled Write a Chemical Equation Step 5
5
Add the number of atoms present as a subscript. As you did with the first element, identify the number of atoms present in the second element by reading the prefix. Using this prefix, write the number of atoms as a subscript to the right of the chemical symbol.[6]
For example: Hexafluoride has a prefix of “hexa-“ which means 6; therefore, there are 6 atoms of fluorine present.
Write hexafluoride as F6.
The final chemical formula for dinitrogen hexafluoride is N2F6.
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6
Practice with some examples. When first learning chemistry, there is a lot of memorization involved. It is kind of like learning a new language. The more examples you practice with, the easier it will be to decipher chemical formulas in the future and learn the language of chemistry.
Sulfur dioxide: SO2
Carbon tetrabromide: CBr4
Diphosphorus pentoxide: P2O5
Writing Chemical Formulas of Ionic Compounds
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1
Identify the chemical symbols for the cations and anions. All chemicals have what you can call a first and last name. The first name is the cation (positive ion) while the last name is the anion (negative ion). Cations are written as the element name while anions are the element name ending with the suffix “ide.”[7]
The chemical symbol for each element can be found on the periodic table.
Unlike covalent compounds, Greek prefixes are not used to indicate the number of atoms of each element. You have to balance the charges of the elements to determine the atoms.
For example: Lithium oxide is Li2O.
Image titled Write a Chemical Equation Step 8
2
Recognize polyatomic ions. Sometimes the cation or anion is a polyatomic ion. These are molecules that have two or more atoms with ionic groups. There’s no good trick to remembering these, you just need to memorize them.[8]
There are only 3 cation polyatomic ions and they are ammonium (NH4+), hydronium (H3+), and mercury(I) (Hg22+). They all have a +1 charge (though, technically, 2 mercury atoms are bonded together, which creates a 2+ charge, with each mercury cation containing a 1+ charge).
The rest of the polyatomic ions have negative charges ranging from -1 to -4. Some common ones are carbonate (CO32-), sulfate (SO42-), nitrate (NO3-), and chromate (CrO42-).
Image titled Write a Chemical Equation Step 9
3
Determine the valence charge of each element. The valence charge can be determined by looking at the position of the element on the periodic table. There are a few rules to keep in mind that help you identify the charges:[9]
All group 1 elements at +1.
All group 2 elements are +2.
Transition elements will have Roman numerals in parentheses to indicate their charge.
Silver is 1+, zinc is 2+, and aluminum is 3+.
Group 17 elements are 1-.
Group 16 elements are 2-.
Group 15 elements are 3-.
Remember, when working with polyatomic ions, use the charge of the complete polyatomic ion, rather than the individual ions.
Image titled Write a Chemical Equation Step 10
4
Balance the positive and negative charges of the ions. Once you have identified the charge of each element (or polyatomic ion), you will use these charges to determine the number of atoms present of each element. You want the charge of the compound to equal zero so you will add atoms to balance the charges.[10]
For example: Lithium Oxide. Lithium is a group 1 element and has a +1 charge. Oxygen is a group 16 element and has a 2- charge. In order to balance the 2- charge of the oxygen, you need 2 atoms of lithium; therefore, the chemical formula of lithium oxide is Li2O.
Image titled Write a Chemical Equation Step 11
5
Practice with some examples. The best way to learn formula writing is to practice with lots of examples. Use examples in your chemistry book or look for practice sets online. Do as many as you can until you feel comfortable writing chemical formulas.
Calcium Nitride: Symbol for calcium is Ca and symbol of nitrogen is N. Ca is a group 2 element and has a charge of +2. Nitrogen is a group 15 element and has a charge of 3-. To balance this, you need 3 atoms of calcium (6+) and 2 atoms of nitrogen (6-): Ca3N2.
Mercury(II) Phosphate: Symbol for Mercury is Hg and phosphate is the polyatomic ion PO4. Mercury has a 2+ charge as indicated by the Roman numeral II next to it. Phosphate has a 3- charge. In order to balance them, you will need 3 atoms of mercury (6+) and 2 molecules of phosphate (6-): Hg3(PO4)2.
Determining the Products Given Reactants
Image titled Write a Chemical Equation Step 12
1
Identify all of the cations and anions in the reactants. In a basic double replacement equation you will have 2 cations and 2 anions. The general equation takes the form of AB + CD → AD + CB, where A and C are cations and B and D are anions. You also want to determine the charges of each ion.[11]
For example: AgNO3 + NaCl → ?
The cations are Ag+1 and Na+1. The anions are NO31- and Cl1-.
Image titled Write a Chemical Equation Step 13
2
Switch the ions to build the products. Once you have identified all of the ions and their charges, rearrange them so that the first cation is now paired with the second anion, and the second cation is now paired with the first anion. Remember the equation: AB + CD → AD + CB.[12]
Remember to balance the charges when forming new compounds.
For example: AgNO3 + NaCl → ?
Ag+1 now pairs with Cl1- to form AgCl.
Na+1 now pairs with NO31- to form NaNO3.
Image titled Write a Chemical Equation Step 14
3
Write the full equation. After writing the products that will form in the equation, you can write the whole equation with both products and reactants. Keep the reactants on the left side of the equation and write the new products on the right side with a plus sign between them.[13]
For example: AgNO3 + NaCl –> ?
AgNO3 + NaCl –> AgCl + NaNO3
Image titled Write a Chemical Equation Step 15
4
Balance the equation. Once you have written the equation and have all of the products and reactants you need to make sure everything is balanced. An equation is balanced only when you have the same number of atoms of every element present on both sides.[14]
For example: AgNO3 + NaCl –> AgCl + NaNO3
Count the number of atoms on each side: 1 Ag left, 1 Ag right; 1 N left, 1 N right; 3 O left, 3 O right; 1 Na left, 1 Na right; 1 Cl left, 1 Cl right
This equation is balanced because there are equal numbers of atoms on both the left and right side of the equation.
5
Note the states of matter. It’s important to indicate the states of matter for both the reactants and the products. There is a designated letter for each state of matter which goes in parentheses. Put this information after the formula of the substance it is describing.[15]
Use “(g)” to indicate a gas, “(s)” to indicate a solid, “(l)” to indicate a liquid, and “(aq)” to indicate a substance dissolved in water.
Image titled Write a Chemical Equation Step 16
6
Practice with some examples. The only way to get better at writing chemical equations is to actually do it. Work your way through these examples to make sure you really understand the process.
NiCl2 + (NH4)2S → ?
Cations: Ni2+ and NH4+
Anions: Cl1- and S2-
Recombine ions to make new products: NiS + NH4Cl
Write the equation: NiCl2 + (NH4)2S → NiS + NH4Cl
Balance the equation: NiCl2 + (NH4)2S → NiS + 2NH4Cl
Community Q&A
Add New Question
Question
In a formula, is it necessary for us to arrange them in order from the element with least atoms to the element with the most atoms?
Meredith Juncker, PhD
PhD in Biochemistry & Molecular Biology Candidate, Louisiana State University
Meredith Juncker is a PhD candidate in Biochemistry and Molecular Biology at Louisiana State University Health Sciences Center. Her studies are focused on proteins and neurodegenerative diseases.
PhD in Biochemistry & Molecular Biology Candidate, Louisiana State University
Expert Answer
No, it is not necessary to do this. As long as the reactants remain on the left side of the arrow and the products remain on the right side, the compounds can be written in any order, regardless of how many atoms there are in each element.
Question
Can balancing be done with a quicker method than the algebraic one?
Meredith Juncker, PhD
PhD in Biochemistry & Molecular Biology Candidate, Louisiana State University
Meredith Juncker is a PhD candidate in Biochemistry and Molecular Biology at Louisiana State University Health Sciences Center. Her studies are focused on proteins and neurodegenerative diseases.
PhD in Biochemistry & Molecular Biology Candidate, Louisiana State University
Expert Answer
Unfortunately, no. Balancing equations is often a trial and error process. A good tip to follow is to start by balancing an element that appears in only one reactant and product and then work from there. Make sure the coefficients you use to balance are in their most reduced form (if the only coefficients you have in the equation are 2 and 6, you would write them as 1 and 3.)
Question
Is it necessary to write down the status of the element under it?
It depends upon the situation, but it’s usually not necessary. The status is usually written just to make the equation more informative. If you are asked to write it for an assignment or examination, however, then you should write it down.
Question
What is the valency of bromine?
Fred Deng
Community Answer
Bromine has 7 valence electrons, therefore is an ionic compound. It is able to gain an electron to form a bromide ion with a -1 charge. It is capable of making one covalent bond.
Question
Can you explain why “2” is used to balance the equation in the last step of the chemical equation?
Jerinjames
Community Answer
So no extra atoms of any element are added, and so the number of atoms are equal on the left and right sides.
Question
What product is created when marsh gas and oxygen react?
Marsh gas contains several chemical components. The primary one is methane. It reacts with oxygen when ignited. There are two products of this reaction, carbon dioxide and water–CH4 + 2 O2 –> CO2 + 2 H2O. Hydrogen sulfide can also be present in marsh gas in minor quantities. When ignited, its reaction with oxygen produces water and sulfur dioxide–2 H2S + 3 O2 –> 2 H2O + 2 SO2. If not ignited, it could react slowly by this equation–2 H2S + O2 –> 2 H2O + 2 S–producing water and sulfur.
Question
What is a balanced equation?
A balanced equation is an equation in which the number of atoms in each reacting element is equal to the number of atoms in each product element.
Question
What is the difference between ferric and ferrous?
Ferric refers to the iron oxidation state of +3 while ferrous refers to the iron oxidation state of +2.
Question
Why is respiration an exothermic reaction?
The release of gas makes it exothermic. Respiration is not just inhalation, but exhalation, too.
Question
What is a saturated bond?
Saturated bond is a chemical compound that has a chain of carbon atoms linked together by single bonds.
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Put the coefficients in front of the ions, rather than adding them as subscripts, which would change the formula.
Sample Equations and Periodic Table
About This Article
Article SummaryX
If you want to write a chemical equation, start by writing the chemical formulas of each reactant. Use the prefixes, such as mono-, di-, tri-, and tetra-, to figure out the number of atoms present for each element, and write this number as a subscript for each element. For example, dihydrogen monoxide would be more easily written as H2O. Once you have the names of the reactants written, add a plus in between them and draw an arrow to indicate the product of the reactants. For help with writing the chemical formulas of ionic compounds, keep reading!
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www.wikihow.com /Write-a-Chemical-Equation
How to Write a Chemical Equation
Co-authored by Meredith Juncker, PhD Updated: January 10, 2020
19-24 minutes
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