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Chemical Equations

Vocabulary

  • chemical equation
  • reactant
  • product
  • Law of Conservation of Mass
  • Coefficient
  • Balanced chemical equation

Andy Goldsworthy, 2009

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Quick Note: Diatomic Elements

  • The following elements in their elemental form (not bonded to another element to form a compound) are diatomic.
  • This means they bond to themselves to form molecules.
  • You must know this as we get into chemical reactions

​

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

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What do you need to make water?

  • You need hydrogen and oxygen!
  • Are hydrogen and oxygen diatomic elements?
  • Yes!

H2

+

O2

→

H2O

This is a chemical equation

reactants

products

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Question

  • Do you see any problems with the equation below?

H2

+

O2

→

H2O

reactants

products

Where did the oxygen go?

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Question

  • This violates the Law of Conservation of Mass (Matter): Matter cannot be created or destroyed, only rearranged.

H2

+

O2

→

H2O

reactants

products

Where did the oxygen go?

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Question

  • Let’s do some accounting:

H2

+

O2

→

H2O

H

H

O

O

H

H

O

What happens to this oxygen?

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Question

It reacts with another hydrogen molecule to make another water molecule!

H2

+

O2

→

H2O

H

H

O

O

H

H

O

H

H

H

H

O

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Question

Now how many hydrogen and oxygen atoms are on the left of the equation? On the right?

H2

+

O2

→

H2O

H

H

O

O

H

H

O

H

H

H

H

O

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Question

So how many hydrogen molecules do you need?

How many oxygen molecules do you need?

How many water molecules are created?

H2

+

O2

→

H2O

H

H

O

O

H

H

O

H

H

H

H

O

2

coefficient

1

2

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Question

This is called a balanced equation because the number of atoms in the reactants equals the number of atoms in the products.

+

O2(g)

→

H2O(l)

2 H2(g)

​

2

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Question

This is called a balanced equation because the number of atoms in the reactants equals the number of atoms in the products.

The states of matter can also be placed in the equation. (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous-dissolved in water.

+

O2(g)

→

H2O(l)

2 H2(g)

​

2

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Practice 1

Balance the following

K2S

+

F2

→

KF

2

2 K

+

S

1 K

2 x

1 S

1 S

2 F

2 F

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Practice 2

Balance the following

Al

+

Cl2

→

AlCl3

2

2

1 Al

1 Al

2 Cl

3 Cl

3x

2x

3

2x

2x

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Practice 2

Balance the following

Rb NO3

+

→

Rb2S

2

1 Rb

2 Rb

2 NO3

2 NO3

2x

MgS

+

Mg(NO3)2

( )

1 Mg

1 Mg

1 S

1 S

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Practice 2

Balance the following

RbNO3

+

→

Rb2S

2

MgS

+

Mg(NO3)2

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Practice 3

Balance the following

Sr(BrO)2

+

→

Na BrO

1 Sr

1 Sr

2 BrO

1 BrO

Na2O

+

SrO

( )

2 Na

2 Na

1 O

1 O

2x

2

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Practice 3

Balance the following

Sr(BrO)2

+

→

NaBrO

Na2O

+

SrO

2

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Practice 4

Balance the following

Al2(C2O4)3

+

→

2 Al

1 Al

3 C2O4

1 C2O4

Cs3PO4

+

Cs2 C2O4

AlPO4

3 Cs

6 Cs

2 PO4

2 PO4

3x

2

2x

( )

3

2x

2

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