Chemical Reactions Review (IP3/IPC3)

•When substances change their properties such as change of state, solubility, shape, or form, these changes are called physical changes.

•When a new substance is formed with different compounds and properties, this change is called a chemical change.

•And, the reaction that makes this change is called a chemical reaction.

Physical changesChemical changes
No new substances are producedNew substances are produced
The chemical properties of the compounds and substances remain the same although their shapes change.New substances have entirely different chemical properties and compounds.
It is possibly reverted to its original form.It is difficult to be reversed to its original form.

Chemical Reactions

A substance before a change is called a reactant.

A substance produced after a chemical reaction is called a product.

      A + B -> C + D

Reactants -> Products

2H2 (g) + O2 (g) -> 2H2O (l)

2H2O (l) -> 2H2 (g) + O2 (g)

      2H2 (g)         +          O2 (g)         ->             2H2O (l)

Hydrogen gas                   Oxygen gas                 Water

2 hydrogen molecules      1 oxygen molecule      2 water molecules

4 hydrogen atoms            2 oxygen atoms           H=4 atoms, O=2 atoms

Metal + Acid -> Salt + Hydrogen

Systems and surroundings

A system refers to that part of universe in which observations are made.

The remaining part of the universe other than system is surroundings.

Types of a system:

  • Open system

A system that exchanges energy or mass between the system boundary and its surroundings is an open system.

The amount of mass and energy after a change will be different.

Example: Leaving fresh limewater for a while

  • Closed system

A closed system is a system that exchanges only energy.

No mass transfer occurs between the system and its surroundings.

Therefore the amount of mass after a change remains the same but the energy changes.

Examples: Solubility of sugar in water and melting ice.

  • Isolated system

A system that can’t transfer both energy and mass outside the system boundary is an isolated system.

Example: thermally insulated bottles

Law of conservation of mass

“Mass in any closed or isolated system can neither be created nor destroyed by chemical reactions and physical transformations” – Antoine Laurant Lavoisier

Closed system

     A    +        B ->     C    +    D

Mass A + Mass B = Mass C + Mass D

2Mg       +        O2 -> 2MgO

Mass Mg + Mass O2 -> Mass MgO

Exothermic Change

When there is high energy in a system after a change, it means there is a release of heat, which raises the temperature of its surroundings; this change is called exothermic change.

Examples: solubility of calcium chloride, combustion of fuel, breathing, condensation of water vapour

Endothermic Change

When a temperature is lower after a change, it means there is an absorption of heat from the surroundings.

This lowers its temperature.

This change is called endothermic change.

Examples: evaporation of sweat, the solubility of potassium nitrate, melting of ice, etc.

Combustion

Combustion is a chemical reaction that occurs when a fuel, which mostly consists of elements, carbon (C) and/or hydrogen (H)reacts with oxygen.

This compound of Carbon and Hydrogen is known as a hydrocarbon.

Incomplete Combustion

Incomplete combustion occurs when there is limited oxygen supply and/or heat energy.

Carbon soot and water are formed.

Fuel         +     Oxygen -> Carbon monoxide + Soot + Water Vapour

                      Deficient                                Carbon

C3H8 (g)         +     3O2 (g)  ->    2CO (g)       +   C (s)    +    4H2O   (g)

Propane          Oxygen     Carbon monoxide    Carbon     Water Vapour

Harmful Effects

How to prevent it?

  • taking the public transformation to reduce fuel combustion.
  • vehicle inspection on a regular period.

Experimental Skills

Using and organizing techniques, apparatus, and materials

Balance

 * The unit of mass is the kilogram (kg).

 * 1 kg = 1000 grams

 • A beam balance is accurate to the size of the smallest mass that tilts the balance beam.

 • A digital top-pan balance is accurate to the size of the smallest mass that can be measured on the scale setting you are using, probably 0.01 g.

 • When using an electronic balance you should wait until the reading is steady before taking it.

Measuring Cylinder

 • The volume of a liquid can be obtained by pouring it into a measuring cylinder.

 • Measuring cylinders are often marked in milliliters (ml) where 1 milliliter = 1 cm3

 • The accuracy of the reading will be 1 cm3

 • Note that 1 litre= 1000cm3 = 1 dm3

 1000 millilitres = 1000 cm3

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