chemistry - chemical reactions

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Other Chemical Reactions

OTHER CHEMICAL REACTIONS

You will learn:

• Combination reaction

• Combustion reaction

• Thermal decomposition

OTHER CHEMICAL REACTIONS

Elements can react in many ways to form new compounds.

The new compound formed will have totally different properties from the starting elements.

Two elements can react to form a new compound in a combination reaction.

Example 1:

Two elements can react to form a new compound in a combination reaction.

Example 1:

elementelement

sodium(soft silvery

metal)

sodium(soft silvery

metal)

elementelement

chlorine(greenish-yellow poisonous gas)

chlorine(greenish-yellow poisonous gas)

compoundcompound

sodium chloride /

common salt(white solid)

sodium chloride /

common salt(white solid)

Combination Reaction

Example 2Example 2

Formation of copper chlorideFormation of copper chloride

elementelement

copper(reddish-brown

solid)

copper(reddish-brown

solid)

elementelement

chlorine(greenish-

yellow poisonous gas)

chlorine(greenish-

yellow poisonous gas)

compoundcompound

copper chloride(green solid)

copper chloride(green solid)

Combustion Reaction

-When an element burns in oxygen

Example 1:Burning of magnesium strip

magnesium + oxygen magnesium oxide

magnesium

Combustion ReactionExample 2

hydrogen + oxygen water

Video: http://www.youtube.com/watch?v=k5MJhuaXUlc&feature=related

http://www.youtube.com/watch?v=e6ejjNsJegk&feature=related

H2 O2

Breaking up a Compound by Heating-Thermal DecompositionBreaking up a Compound by Heating-Thermal Decomposition

A compound may be broken down into simpler substances. Heating is needed.A compound may be broken down into simpler substances. Heating is needed.

New compound 1New compound 1

Copper oxide

(black powdery solid)

Copper oxide

(black powdery solid)

New compound 2New compound 2

Carbon dioxide

( colourless gas)

Carbon dioxide

( colourless gas)

Starting compound

Starting compound

copper carbonate

(green powder)

copper carbonate

(green powder)

Video: http://www.youtube.com/watch?v=FYUWY39ikcI&feature=related

WRITING CHEMICAL EQUATIONS

Writing Chemical Equations

You need to know:1. The correct chemical formula of elements (metals e.g Na. Zn, Al, Mg etc)

(gases: e.g Cl2, H2, O2 etc)

2. Chemical formula of compounds (based on your knowledge of valency)

Example: chemical formula of

magnesium chloride: Mg Cl2

Writing Chemical Equations

3. Balancing the equation

This is to ensure that the no of atoms of each elements is the same before and after the reaction.

Example : Reaction of hydrogen and chlorine gas

Is this chemical equation balanced?

H2 + Cl2 2 HCl

H H ClCl Cl

H

ClH

Balancing Chemical Equations

• All chemical equations must be balanced.

• The number of atoms of an element before and after the reaction must be the same as the Law of conservation of Matter states that:

Atoms cannot be created nor destroyed in a chemical reaction.

How to Balance?

H2 + Cl2 HCl

Left hand side Right hand side

2H atoms 2 Cl atoms 1H atom and 1 Cl atom

This equation is not balanced.

How to Balance?Balancing can be achieved by changing the

number in front of the formula

H2 + Cl2 HCl

Left hand side Right hand side

2H atoms 2 Cl atoms 1H atom and 1 Cl atom

H2 + Cl2 2 HCl

Left hand side Right hand side

2H atoms 2 Cl atoms 2H atoms and 2 Cl atoms

Balanced!

More Examples

Writing Chemical Equations

Combination ReactionExample 1

Sodium + chlorine sodium chloride

Step 1: Write the chemical formula

Na + Cl2 NaCl

Step 2

Balancing to ensure atoms of each elements are the same on both side.

Why the need to balance chemical Equations?Law of Conservation of Matter:

Atoms cannot be created nor destroyed

Step 2: Balancing

Na + Cl2 NaCl

2 NaCl

2 2 NaCl

First try not balanced

Second tryBalanced!

Na + Cl2

Na + Cl2

Combination ReactionExample 2

Copper + chlorine copper (II) chloride

Step 1: Write the chemical formula

Cu + Cl2 CuCl2

Step 2 :Balancing to ensure atoms of each elements are the same on both side

Cu + Cl2 CuCl2

No need to balance

Combustion ReactionExample 1

Magnesium + oxygen magnesium oxide

Step 1: Write the chemical formula

Mg + O2 MgO

Step 2 :Balancing to ensure atoms of each elements are the same on both side

Need to balance?

Mg + O2 MgO22

Combustion ReactionExample 2

Hydrogen + oxygen water

Step 2 :Balancing to ensure atoms of each elements are the same on both side

Step 1: Write the chemical formula

H2 + O2 H2O

Need to balance?

H2 + O2 H2O22

Thermal Decomposition ReactionExample 1

Copper copper + carbon

Carbonate oxide dioxide

Step 2 :Balancing to ensure atoms of each elements are the same on both side

Step 1: Write the chemical formula

Cu CO3 CuO + CO2

Need to balance?

It is already balanced

Summary

• A chemical reaction can be represented by a word or chemical equation.

• Word equations can be converted into a chemical equations by using chemical formulas.

• All chemical equations must be balanced.

Part II

Chemical Equations for

Reactions of Acids

Reactions of Acids

Complete the worksheet given on writing words and chemical equations

Note:

You must know the

*chemical formula of acids and alkalis

* chemical formula of the salts