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Important content:
Important content:
Exchange of energy
Activation energy
Evidences of chemical reactions
Conservation mass
Chemical reaction types
Chemical equations
Stoichiometry
Mol: molar mass and molecular mass
In memoriam: Antonie Lavoisier
In memoriam: Antonie Lavoisier
La République n'a pas besoin de savants ni de chimistes (...)
La République n'a pas besoin de savants ni de chimistes (...)
The Republic has no need of scientists or chemists (...)
The Republic has no need of scientists or chemists (...)
Antoine-Laurent de Lavoisier was a French nobleman and chemist who was central to the 18th-century chemical revolution and who had a large influence on both the history of chemistry and the history of biology. Some of his main contributions are:
Oxygen theory of combustion
Pioneer of stoichiometry
Chemical nomenclature
Mass conservation law
Lavoisier was convicted and guillotined on 8 May 1794 in Paris, at the age of 50: He discovers the conservation of the mass in chemical reactions... but he was not able to conserve his head!
Keep in mind this meticulous experimenter and father of modern chemistry.
CHEMICAL REACTIONS
CHEMICAL REACTIONS
The meeting of two personalities is like the contact of two chemical substances; if there is any reaction, both are transformed.
Chemical reactions are the proccess by which some substances transform into others, changing their chemical composition and their chemical bonds.
As we can see in the images below in reaction there are reactants and products.
Note that reactants, on the left, are separed by an arrow from products, which are on the right. The arrow indicates the direction of the reaction.
When a chemical reaction happen, something new is created from the starting materials.
Everyday examples can occur in our kitchen! When a cake is baked, a chemical reaction occurs. The combination of different ingredients, along with the heat from the oven, cause chemical reactions. Through these chemical reactions a cake is formed!
Reactions depend on the exchange of energy!
Reactions depend on the exchange of energy!
All substances contain chemical energy. This depends, as we have seen, on the stability of their chemical bonds. The more stable the bonds, the less energy the substance have.
When the products are more stable that the reactants the reaction release energy to the surroundings. On the contrary, when the products are less stable than the reactants, the reactions absorbs energy form the surroundings.
Often, reactants need an activation energy to be transformed into products. Different reactions have different activation energy.
Evidences
Evidences
We could detect a reaction because the physical and chemical properties of the reactants are different from the products.
Be aware: changes in color, smell, temperature or physicial properties (e.g. electrical conductivity or fusion point) indicate that a chemical reaction is taking place!
Conservation of mass
Conservation of mass
In any change, the amount of matter is conserved. The mass of the substances before the reaction must be always the same as the mass of the substances after the reaction. We can conclude that in all chemical reactions there is a conservation of mass. It is easy to detect in those reactions that occur in a closed system, as they can not exchange matter with its surroundings, as it happens when considering open system.
Likewise, the number of atoms of each element must be the same both before and after the reaction. Note that atoms do not disappear but they rearrange. New bonds may be formed or, on the contrary, broken down.
How fast can A reaction BE?
How fast can A reaction BE?
The reaction rate or rate of reaction is the speed at which reactants are converted into products.
For example, the oxidative rusting of iron under Earth's atmosphere is a slow reaction that can take many years, but the combustion of cellulose in a fire is a reaction that takes place in fractions of a second. For most reactions, the rate decreases as the reaction proceeds.
The rate of reaction depends on the frequency of collisions within reactants. It depends on:
Temperature
Surface area of reactants
Concentration of reactants
This concept is tyically expressed in moles per second (mol/s) or also as the change in molarity per second (M/s)
This rate of reaction can increase with the action of catalysts, substances that lower the activiation energy doing the collisions between reactants more effective.
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