Writing Chemical Equations

Writing Chemical Equations

by Monica DaJose

Poll

How comfortable are you currently with writing the formulas of a compound based on the name? 

Ex: What is the formula of Calcium nitride? 

• I am confident in my abilities. 

• I can do the easy stuff, but I struggle with the harder stuff. 

• I remember parts of it, but I need help. 

• I can't do that at all. 

Multiple Choice

Magnesium sulfide

• Mg

• Mg₂S₂

• MgSO₄

• Mg₄SO

Multiple Choice

What is the formula for water?

• HO

• OH₂

• H₂O

• I don't know

​States of Matter Symbols

Chemical equations can be used to show the states of matter of the different elements and compounds in an equation.

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(s) - solid

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​(l) - liquid

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​(g) - gas

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​(aq) - aqueous

​Mg (s)

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​H₂O (l)

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​O₂ (g)

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​NaCl (aq)

​Diatomic Elements

​Diatomic elements -- elements that always appear as a molecule of 2 atoms bonded together.

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The diatomic elements are N₂ , O_{2 ,} F₂ , Cl₂ , Br₂ , I₂ , H₂

​Writing a chemical equation from a written description

​1. Identify all the reactants and products. If products are not given, use the reaction type to figure out the products.

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1. Synthesis -- usually an ionic compound. Write the ionic compound formed by the two elements

3. Decomposition -- products are (usually) the elements in the compound. If not, they will be listed for you

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1. Single replacement -- switch your free element and the metal/nonmetal it would switch with. Make sure you make a real compound.

2. Double replacement -- switch the cations. Make sure you make real compounds.

3. Combustion

   1. Combustion of a hydrocarbon forms CO₂ and H₂O

   2. Combustion of a metal forms the ionic compound between that metal and oxygen. 

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​2. Write the formulas for all the reactants and products.

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​3. Balance the reaction.

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​(Optional) 4. Use symbols to indicate the states of matter of each compound or element involved.

Multiple Choice

Solid elemental phosphorous (P₄) reacts with fluorine gas to produce gaseous phosphorus pentafluoride. 

Which of the following has the formulas for all the reactants and products written correctly? 

• P₄ + F --> PF

• P₄ + F₂ --> PF₅

• P₄ + F --> PF₄

• P₄ + F₂ --> PF

​Things to note about the correct answer

​Fluorine is a diatomic element -- If it is by itself, write it as F₂

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​We had prefixes to indicate subscripts.

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​Phosphorus pentafluoride --> PF₅

Multiple Choice

Solid elemental phosphorous (P₄) reacts with fluorine gas to produce gaseous phosphorus pentafluoride. 

P₄ + 10 F₂ --> 4 PF₅

Which of the following has the correct states of matter written in? 

• P₄ (s) + 10 F₂ (g) --> 4 PF₅ (g)

• P₄ (g) + 10 F₂ (s) --> 4 PF₅ (s)

• P₄ (g) + 10 F₂ (aq)    --> 4 PF₅ (l)

• P₄ (s) + 10 F₂ (g) --> 4 PF₅ (l)

P₄ +     F₂ -->     PF₅

Multiple Choice

Solid elemental phosphorous (P₄) reacts with fluorine gas to produce gaseous phosphorus pentafluoride. 

Which of the following has the correct coefficients written in? 

• 5 P₄ + 10 F₂ --> 4 PF₅

• 2 P₄ + 5 F₂ --> 2 PF₅

• P₄ + F₂ --> PF₅

• P₄ + 10 F₂ --> 4 PF₅

​Final Equation:

P₄ (s) + 10 F₂ (g) --> 4 PF₅ (g)

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