4.3 Chemical Equations

8/13/2019 4.3 Chemical Equations

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You Should Be Able To…

1. Define and explain the law of conservation of mass

2. Represent chemical reactions and the conservationof atoms, using molecular models

3. Write and balance (using the lowest whole number

coefficients) chemical equations from formulae, wordequations, or descriptions of experiments





(c) McGraw Hill Ryerson 2007

• Chemical reactions result in chemicalchanges.

– Chemical changes occur when new substances

are created. – The original substance(s), called reactants,

change into new substance(s) called products.

See pages 202 - 203

Reactants Products



(c) McGraw Hill Ryerson 2007 See pages 202 - 203

Reactants Products




• Chemical reactions can be written in differentways.

– A word equation:

• Nitrogen monoxide + oxygen nitrogen dioxide

– A symbolic equation:

• 2NO( g ) + O2( g ) 2NO2( g ) STATE OF MATTER

- Letters indicate the state of

each compound.

(aq) = aqueous/dissolved in

water

( s ) = solid

( ) = liquid

( g ) = gas

COEFFICIENTS

- Indicates how many of each

molecule there is.

-Ie: there are 2 molecules of NO.




• When a chemical reaction occurs, new compounds are

created, BUT… – No new matter is created or destroyed; atoms are

just rearranged as the atoms change partners to form

new compounds.

– If there are 3 atoms of oxygen in the reactants, thereMUST be 3 atoms of oxygen in the products.

– Number of each atom in reactants = number of each

atom in products.

• The law of conservation of mass: – Mass of reactants = mass of products

If you could collect and measure all of the exhaust from

this car, you would find that mass of reactants (gas + O2) =

mass of products (exhaust).



• The simplest form of chemical equation is a word

equation. – Potassium metal + oxygen gas potassium oxide

• A skeleton equation shows the formulas of theelements/compounds. – A skeleton equation shows which atoms are involved, but

not how many molecules are involved.• K + O2 K2O



• A balanced chemical equation shows all

atoms and the coefficients tells us how many

molecules (and atoms) there are.

– Balancing ensures that the number of each

atom is the same on both sides of the reactionarrow.

4K + O2 2K2O

K

K

K

K

K

K

K

K

O O O

O




• Using the law of conservation of mass, we

can count atoms to balance the number of

atoms in chemical equations.

– Word equation: methane + oxygen water +

carbon dioxide – Skeleton equation: CH4 + O2 H2O + CO2

• To balance the compounds, take note of how many

atoms of each element occur on each side of the

reaction arrow.

See Page 207




Skeleton equation: CH4 + O2 H2O + CO2

Carbon = 1 Carbon = 1Hydrogen = 4 Hydrogen = 2

Oxygen = 2 Oxygen = 3

Balanced equation: CH4 + 2O2 2H2O + CO2 Carbon = 1 Carbon = 1

Hydrogen = 4 Hydrogen = 4

Oxygen = 4 Oxygen = 4

The same number

of atoms must be on

each side.

See Page 207



Balancing Equations

Reactants Products

H 2 2

hydrogen + oxygen water

O2 H2OH2 +

O 2 1



Balancing Equations

H2

+ O2

H2

O2

Reactants Products

H 2 2

O 2 2

hydrogen + oxygen hydrogen peroxide

YOU C NNOT CH NGE THE SUBSCRIPTS



Balancing Equations

H2 + O2 H2O

Reactants Products

H 2 2

O 2 1


2



Balancing Equations

H2 + O2 H2O

Reactants Products

H 2 4

O 2 2


2



Balancing Equations

2 H2 + O2 H2O

Reactants Products

H 4 4

O 2 2


2(g) (g) (l)




• Balance chemical equations by following these steps: –

Trial and error will work but can be very inefficient.• USE A TABLE (write atoms underneath reactants and products)

• If they look the same on both sides of the equation, treatpolyatomic ions (such as SO4

2 –) as a group & balance them assuch.

• If ‘OH’ and H2O are in the equation, write water as HOH.

• Balance one compound at a time & rewrite the # of atoms inthe chart as things change.

• Only add coefficients; NEVER change subscripts!!!

• If H and O appear in more than one place, attempt to balancethem LAST.

– Balance everything that isn’t ‘H’ or ‘O’ 1st.

– Balance the ‘H’s 2nd to last.

– Balance the ‘O’s last.

– Always double-check after you think you are finished.

– CHECK YOUR ANSWERS!!! See pages 209 - 211



http://www.authorstream.com/Presentation/kellymdeters-86103-section-

2-7-balancing-chemical-equations-education-ppt-powerpoint/

http://www.authorstream.com/Presentation/kellymdeters-86103-section-2-7-balancing-chemical-equations-education-ppt-powerpoint/

























• Balance the following:

– Fe + Br 2 FeBr 3

– Sn(NO2)4 + K3PO4 KNO2 + Sn3 (PO4)4

– C2H6 + O2 CO2 + H2O



____Ba + ____H2O ____Ba(OH)2 + ____H2

____CO2 + ____H2O ____H2CO3

____Fe2O3 + ____C ____Fe + ____CO

____Fe + ____H2O ____H2 + ____Fe2O3




• If you don’t transform your word into askeleton equation properly, you won’tbe able to balance the equationcorrectly. – Change chemical names into chemical

formulas. 4 types:• Simple ionic compounds

• Multivalent ionic compounds

• Ionic compounds with polyatomic ions

• Covalent compound

• Be careful of diatomic elements --remember the special seven!!

H2, N2, O2, F2, Cl2, Br2, I2

See page 208




See page 208

Several common covalent molecules containing hydrogen have

common names that you should know and MEMORIZE!!

methane = CH4

glucose = C6H12O6

ethane = C2H6

ammonia = NH3




See page 208

Example #1:

Word Equation: Solutions of lead nitrate react with potassium iodide to

produce solid lead iodide and a solution of potassium nitrate.

Skeleton Equation: Pb(NO3)2(aq) + KI(aq) PbI2(s) + KNO3(aq)

Balanced Equation: Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)

Example #2:

Word Equation: Copper reacts with hydrogen nitrate to produce copper (II)

nitrate plus hydrogen.

Skeleton Equation: Cu + H(NO3) Cu(NO3)2 + H2

Balanced Equation: Cu + 2H(NO3) Cu(NO3)2 + H2

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4.3 Chemical Equations