external image beakers5.gifChemical Reactions


When a chemical reaction occurs, there is always a change in the properties and energy content of the substances involved. There is a change in both the physical and chemical properties as substances are changed into new substances.




At least one of these should be obvious when a chemical reaction takes place:
Color change - new substances might reflect different colors of light.


Heat change - the container may get hotter or colder.

Gas produced - bubbles or smoke may be observed.

Precipitate formed - an insoluble solid might form in a liquid.


In order for a chemical reaction to take place, the reactants must have the ability to form chemical bonds. These bonds are formed by the valence electrons of the reactant atoms. If substances are brought together whose valence electrons do not match together to complete the Octet Rule, there will be no chemical reaction.
Parts of a chemical reaction:

external image tridot.gif Reactants - * Substances that interact with each other.
  • On the left side of the arrow in a chemical equation.
external image tridot.gif Products - * New substances formed during a reaction.
  • On the right side of the arrow in at a chemical equation.
chemical reaction
chemical reaction

Law of conservation of mass:

During a chemical reaction, atoms are neither created nor destroyed. This means that the number of atoms remains constant throughout the reaction. Since the number of atoms doesn't change, the mass must remain constant as well.




Exothermic reaction - a reaction that releases energy. external image tridot.gifThe reactants have more energy than the products.
external image heatplus.gif
Endothermic reaction - a reaction that absorbs energy.external image tridot.gifThe products have more energy than the reactants.
external image heatmins.gif



Reaction rate is the rate at which products form or reactants are used up.

Reaction rate is affected by these five things:
  • Concentration: an increase in concentration means there are more particles. More particles means there will be more collisions. This should increase the reaction rate.
  • Surface area: when one of the reactants is a solid, the reaction can only take place at the surface of the solid. Breaking the solid into smaller pieces will increase the surface area exposed to the other reactant. This should increases the reaction rate.
  • Temperature: since temperature is a measure of the motion of particles, increasing the temperature will cause the particles to move faster. When particles move faster, more collisions occur and the collisions are more violent. This should increase the reaction rate.
  • Catalysts: catalysts are substances that change the rate of a chemical reaction without being changed in the reaction. Catalysts are most often used to speed up a chemical reaction. They do this by changing the steps needed between the reactants and the formation of the products. If the number of steps can be reduced, the reaction rate should increase.
  • Inhibitors: inhibitors



There are 4 general types of reactions:

  • synthesis
  • decomposition
  • single displacement
  • double displacement


external image tridot.gif
Synthesis: two simple substances combine to form a more complex one.


SYNTHESIS REACTION
In a synthesis reaction two or more simple substances combine to form a more complex substance. Two or more reactants yielding one product is another way to identify a synthesis reaction.
For example, simple hydrogen gas combined with simple oxygen gas can produce a more complex substance-----water!

The chemical equation for this synthesis reaction looks like:
synthesisimage
synthesisimage

reactant + reactant -------> product

To visualize a synthesis reaction look at the following cartoon:
birdandwormimage
birdandwormimage

In the cartoon, the skinny bird (reactant) and the worm (reactant) combine to make one product, a fat bird.
Examples of synthesis reactions:
  • 2Na + Cl2 external image arrow.jpg 2NaCl
  • 4Fe + 3O2 external image arrow.jpg 2Fe2O3

external image tridot.gif
Decomposition: a complex substance breaks down into two simpler substances.

DECOMPOSITION REACTION
In a decomposition reaction a more complex substance breaks down into its more simple parts. One reactant yields 2 or more products. Basically, synthesis and decomposition reactions are opposites.
For example, water can be broken down into hydrogen gas and oxygen gas.
The chemical equation for this decomposition reaction looks like:
decomposeimage
decomposeimage

reactant -------> product + product

To visualize a decomposition reaction look at the following cartoon:
eggandturtleimage
eggandturtleimage

In this cartoon the egg (the reactant), which contained the turtle at one time, now has opened and the turtle (product) and egg shell (product) are now two separate substances.

Examples of decomposition reactions:
  • 2H2O external image arrow.jpg 2H2 + O2
  • H2CO3 external image arrow.jpg H2O + CO2



external image tridot.gif
Single replacement: a single element replaces an element in a compound.

SINGLE REPLACEMENT REACTION
In a single replacement reaction a single uncombined element replaces another in a compound. Two reactants yield two products. For example when zinc combines with hydrochloric acid, the zinc replaces hydrogen.

The chemical equation for this single replacement reaction looks like:
singlereplaceimage
singlereplaceimage

reactant + reactant ---------> product + product

To visualize a single replacement reaction look at the following cartoon:
dancerimage
dancerimage

Notice, the guy in the orange shirt steals the date of the other guy. So, a part of one of the reactants trades places and is in a different place among the products

Examples of single displacement reactions:
  • Zn + 2HCl external image arrow.jpg ZnCl2 + H2
  • 2Na + 2H2O external image arrow.jpg 2NaOH + H2


external image tridot.gif
Double replacement: two different atoms in two different compounds exchange places.

DOUBLE REPLACEMENT REACTION
In a double replacement reaction parts of two compounds switch places to form two new compounds. Two reactants yield two products. For example when silver nitrate combines with sodium chloride, two new compounds--silver chloride and sodium nitrate are formed because the sodium and silver switched places.
The chemical equation for this double replacement reaction looks like:
doublereplaceimage
doublereplaceimage

reactant + reactant ---------> product + product

To visualize a double replacement reaction look at the following cartoon:
tradinghatsimage
tradinghatsimage


Notice how the first guy exchanged hats with the second guy, so they are both wearing each other's hat.

Examples of double displacement reactions:
  • MgCO3 + 2HCl external image arrow.jpg MgCl2 + H2CO3
  • 2KCl + Pb(NO3)2 external image arrow.jpg 2KNO3 + PbCl2