Section 2: Chemical Formulas and Equations

Year 10 Science

The Language of Chemistry Early chemists used symbols for elements and

compounds derived from alchemy, a ‘mystical chemistry’.

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The Language of Chemistry

Currently used symbols for the elements are those found on the Periodic Table.

Elements are represented by symbols, compounds by formulas and reactions by equations.

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Formulas Molecular(covalent) formulas are

those made up of non-metals. They are listed on the data sheet of the booklet.

There are some flashcards on the website to help you learn them

Ionic formulas are those for metals joined to non-metals. You need to be able to write these given the ions on the data sheet.

There are activities in the booklet and the website to help you revise them.

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The Language of Chemistry In a chemical reaction, at least one

new substance is formed. Chemical equations are a shorthand

way to represent long, complex reactions.

Chemical equations take the form:

Reactants Products

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Conservation of mass

Brainpop: Conservation of mass

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Conservation of mass The Law of conservation of mass states that

matter can neither be destroyed or created.

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Rearranging atoms During chemical reaction the bonds in

the reactants are broken. The atoms are rearranged and new bonds form.

Reactants → Products

A + B → C + DA → B + CA + B → C, etc..

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Some Common Reaction types Precipitation

Decomposition Combustion Corrosion Acid-base Metal displacement..

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We will be studying•Precipitation•Some reactions of acids

Word Equations Chemical reactions can be represented by

word equations.

Photosynthesis can be represented as:Carbon dioxide + water glucose + oxygen

Reactants: carbon dioxide, water Products: glucose, oxygen

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Experiment 2:Describing Chemical Reactions

Remember to record:

What the reactants look like before the experiment

What the products look like at the end of the experiment.

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A. Magnesium in hydrochloric acidHow do we know that a chemical reaction has occurred?Describe this reaction using a word equation.

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B. Copper sulfate + sodium hydroxideHow do we know that a chemical reaction has occurred?Describe this reaction using a word equation.

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C. Copper in silver nitrate solutionHow do we know that a chemical reaction has occurred?Describe this reaction using a word equation.

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Word Equations Complete the report for experiment 2.

Text: Read pages 88-89 and complete questions 1-7 page 89.

Exercise 4 page 11 booklet.

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Text page 89

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Chemical Equations Brainpop: Chemical Equations

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Chemical EquationsA chemical equation gives the chemical formulas of the reactants on the left of the arrow and the products on the right.

Reactants Products

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C(s)

O2 (g) CO2 (g)

Symbols Used in Equations Symbols used in

chemical equations show:

-the states of the reactants.

-the states of the products.

-the reaction conditions.

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Can you write a word equation for this reaction?What about the Law of Conservation of Mass?

Subscripts vs CoefficientsSubscripts tell Subscripts tell

you how many you how many atoms of a atoms of a particular particular element are in a element are in a substance. The substance. The coefficient tells coefficient tells you about the you about the number of number of molecules or molecules or ion pairs of the ion pairs of the substance.substance.

Learning Check How many oxygen atoms are present in:

NaOH

3NaOH

H2O

2H2O

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1

3

1

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Learning CheckHow many oxygen atoms are present in:

Cu(OH)2

23Cu(OH)2

62Na3PO4

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Chemical Equations are Balanced In a chemical

reaction atoms are neither gained nor lost.

In a balanced chemical equation the number of reactant atoms is equal to the number of product atoms.

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A Balanced Chemical Equation In a balanced chemical equation: -there must be the same number of each type of atom

on the reactant side and on the product side of a balanced equation.

-numbers called coefficients are used in front of one or more formulas.

Al + S Al2S3

Not Balanced

2Al + 3S Al2S3

Balanced

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Learning Check State the number of atoms of each

element on the reactant side and the product side for each of the following balanced equations.

A. P4(s) + 6Br2(l) 4 PBr3(g)4 P 4 P

12 Br 12 Br

B. 2Al(s) + Fe2O3(s) 2Fe(s) + Al2O3(s)2 Al 2 Al

2 Fe 2 Fe 3 O 3 O

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Learning CheckDecide if each equation is balanced or not.

A. Na(s) + N2(g) Na3N(s)No. 2 N on reactant side, 1 N on product side.

1 Na on reactant side, 3 Na on product side.

B. C2H4(g) + H2O(l) C2H5OH(l) Yes. 2 C = 2 C 6 H = 6 H

1 O = 1 O

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Steps in Balancing an Equation

To balance the following equation, Fe3O4(s) + H2(g) Fe(s) + H2O(l)

Work on one element at a time. Use only coefficients in front of formulas. Do not change any subscripts.Fe: Fe3O4(s) + H2(g) 3Fe(s) + H2O(l)

O: Fe3O4(s) + H2(g) 3Fe(s) + 4H2O(l)

H: Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

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Balancing Chemical Equations

1. Write the equation with the correct formulas. NH3(g) + O2(g) NO(g) + H2O(g)

2. Determine if the equation is balanced.No, not all atoms are balanced.

3. Balance with coefficients in front of formulas. 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

4. Check that atoms of each element are equal in reactants and products.

4 N (4 x 1 N) = 4 N (4 x 1 N) 12 H (4 x 3 H) = 12 H (6 x 2 H) 10 O (5 x 2 O) = 10 O (4 O + 6 O)

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Balancing with Polyatomic IonsMgCl2(aq) + Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s)

Balance PO43- as a single unit

MgCl2(aq) + 2Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s)

2 PO43- = 2

PO43-

Balance Mg and Cl

3MgCl2(aq) + 2Na3PO4(aq) 6NaCl(aq) + Mg3(PO4)2(s)

3 Mg2+ = 3 Mg2+

6 Na+ = 6 Na+

6 Cl- = 6 Cl-

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Balancing Chemical Equations When balancing equations you must

follow a simple set of rules:

1. Atoms cannot appear from nowhere nor can they disappear. There must be the same number of each atom of each element on either side of the chemical equation.

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Balancing Chemical Equations2. You cannot change the small subscript

numbers in a formula. 3. For example, H2O is water but H2O2 is

hydrogen peroxide, a type of bleach that would be incredibly dangerous to wash with or drink.

4. Change the subscript and you change the chemicals.

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Balancing Chemical EquationsYou can only change the number in front of each chemical formula (the coefficient). For example, if you want to double the number of oxygen atoms in an equation, do not change O2 into O4. O4 does not exist in that form and you just can’t go about creating things that don’t exist! Instead, write 2O2.

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Balancing Chemical EquationsIf you place a coefficient in front of a

compound like Al2(CO3)3 then you have multiplied all the atoms in the formula by that number. For example:

2 Al2(CO3)3 contains:2 X 2 = 4 Al atoms2 X 1 X 3 = 6 C atoms2 X 3 X 3 = 18 O atoms

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Balancing Chemical Equations Complete Exercise 6 page 12 booklet

Read page 90-91 text

Complete Q1-3 page 91 text

Now try: Balancing equations worksheet in class files

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Text page 91

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Online activities Chembalancer

Creative chemistry

It's elemental

Tutorial 1

Tutorial 2

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I want more… Chemistry!!!

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