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Chemical reactions

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José Vicente Pacheco

on 21 April 2014

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Transcript of Chemical reactions

What happens in
a chemical reaction?
Physics or Chemistry?
Let's remember:
In a
physical change,
substances don't change into others. In other words, there are the same substances after and before the process.
Changes of state and processes of mixture are physical.
In a
chemical change
, also called chemical reactions, new substances are produced. We can recognize these processes because the new substances have different properties than initial substances.
Some evidences can show us that a chemical reaction has happened:
A gas appears
.
A solid deposit appears.
There is a colour change.
...
Substances become other substances
Chemical reactions' laws
THE MOLE
The mole is the unit of amount of substance in the SI.
1 mole = 6.022 · 10^23 particles
(atoms, molecules, etc)
6.022 · 10^23 is the Avogadro's number
1 mole He = 6.022 · 10^23 atoms of He
1 mole H2O = 6.022 · 10^23 molecules of water
Chemical equations
The best way to represent a chemical reaction. They have:
An arrow.
Some chemical formulas:
The reactants' formulas in the left side.
The products' ones in the right side.
We can also find symbols that show the physical state of the substances: (s), (l), (g) or (ac)
CHEMICAL REACTIONS
Do a quiz about physical and chemical changes:
http://vital.cs.ohiou.edu/steamwebsite/downloads/ChangeLab.swf
Dalton's atomic theory stated that
atoms don't change
in chemical reactions:
Atoms aren't created, destroyed or modified in chemical reactions.
A chemical reaction is a
rearrangement of the atoms
in order to form new molecules. This happens in three steps:
Molecules collide to each other.
As a result of this, bonds are broken.
New bonds are formed.
In a chemical reaction, some substances
(reactants)
transform in others
(products)
Law of conservation
of mass
A. Lavoisier, 1774:
The mass of a system doesn't change during a chemical reaction.
The total mass of reactants equals the total mass of products.
This happens because there is the same number of atoms before and after the reaction.
Law of definite proportions
J. L. Proust, 1806:
When two elements react to form a compound, they always do it in the same proportion.
The proportion is given by the atomic masses of both elements.
8 g of O react with 1 g of H and produce 9 g of water.
Proportion O:H = 8 g/1 g = 8
20 g of O react with 2.5 g of H and produce 22.5 g of water.
Proportion O:H = 20 g/2.5 g = 8
If the elements are not in the right proportion, one of them will partially remain
The mass in Chemistry
Atoms and molecules are really tiny. Measuring their masses in grams doesn't make sense:
Unit of atomic mass (u):
one twelfth of the mass of an atom of carbon-12.
Atomic mass of an element:
is the average mass of all its isotopes.
Molecular mass of a compound:
the add of the masses of all the atoms in its formula.
H2SO4 -> M = 2 · 1 u + 32 u + 4 · 16 u = 98 u
CaCl2 -> M = 40 u + 2 · 35.5 u = 111 u
[Ionic compounds don't have molecules, but we also use this concept with them]
Mass of a mole
The mass of a mole (in grams) equals the mass of a molecule -or an atom- (in u)
Samples:
Mass of one molecule of NH3:
14 u + 3 · 1 u = 17 u
Mass of a mole of NH3 = 17 g
Mass of an atom of He = 4 u
Mass of a mole of He = 4 g
Volume of a gas' mole
The volume of any gas doesn't depend on which substance it is.
It only depends on its number of particles and the conditions of temperature and pressure
At 0 ºC (273 K) and 1 atm of pressure (stp conditions), 1 mole of any gas occupies 22.4 liters
1 mol CO2 -> 22.4 liters
1 mol O2 -> 22.4 liters
1 mol NH3 -> 22.4 liters
Calculations with moles
Notice that:
to change any quantity to moles you must divide it.
to change the number of moles to any other unit you must multiply it.
This is impossible, atoms can't disappear, be changed or created.
Chemical equations must be balanced.
How to balance chemical equations
First, we count the number of atoms of a specific element in both sides of the arrow.
If it is different, we write a number before the formulas we need in order to get the same number of atoms of that element.
We continue with the other elements.
We finish when all element are balanced.
The numbers before the formulas express the number of molecules of each substance that are involved in the reaction.
See an example explained step by step in this link: http://www.wikihow.com/Balance-Chemical-Equations
Practice balancing chemical equations in this other link: http://berenato.net/tutorials/balancing6_I.swf
Chemical equations and proportions
A balanced chemical equation gives us information about in what proportions the substances are involved.
Proportion of molecules:
1 molecule of nitrogen
and
3 molecules of hydrogen
react and form
2 molecules of ammonia
.
Proportion of moles:
1 mole of nitrogen
and
3 moles of hydrogen
react and form
2 moles of ammonia
.
Proportion of mass:
28 grams of nitrogen
and
3 · 2 grams
of hydrogen react and form
2 · 17 grams of ammonia
.
Proportion of volume (in gasses):
22.4 liters of nitrogen
and
3 · 22.4 liters
of hydrogen react and form
2 · 22.4 liters of ammonia
.
Equivalently:
1 liter of nitrogen
and
3 liters
of hydrogen react and form
2 liters of ammonia
.
The word "mole" can be translated into Spanish as three different ways:
mol (the unit used in Chemistry)
topo (the animal)
verruga o lunar (e. g. in your skin)
TYPES OF CHEMICAL REACTIONS
Synthesis reactions
A compound is formed from several simpler substances.
A + B -> C
2 or more reactants becomes an only product.
Decomposition reactions
An only substance becomes several simpler ones:
A -> B + C + ...
One reactant is broken apart in some products.
An element occupies the place of another in a compound.
AB + C -> AC + B
Double displacement reactions
Two element of two different compounds switch their places.
AB + CD -> CB + AD
Combustion reactions
A hydrocarbon burns with oxygen and produces carbon dioxide and water.
hydrocarbon + O2 -> CO2 + H2O
We use them to get energy: thermal power stations and cars' engines are samples of this.
Displacement reactions
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