John Dalton, a British school teacher, published his theory about atoms in the year 1808. His findings were based on experiments and also from laws of chemical combination.
Main assumptions or postulates of Dalton
* All matter consists of indivisible particles called atoms.
* Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements.
* Atoms cannot be created or destroyed.
* Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.
* Atoms of same element can combine in more than one ratio to form two or more compounds.
* Atoms are the smallest unit of matter that can take part in a chemical reaction.
Drawbacks of Dalton’s atomic theory of matter
* The indivisibility of an atom was proved wrong, for, an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle, which takes part in chemical reactions.
* According to Dalton, the atoms of same element are similar in all respects. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.
* Dalton also said atoms of different elements are different in all respects. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar.
* According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11.
* The theory completely fails to explain the existence of allotropes. The difference in properties of charcoal, graphite, diamond went unexplained in spite of being made up of same kind of atoms.
Merits of Dalton’s atomic theory
* It has enabled us to explain the laws of chemical combination.
* Dalton was the first person to recognize a workable distinction between the ultimate particle of an element (atom) and that of a compound (molecule).
Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0). In the lab, exothermic reactions produce heat or may even be explosive.
There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).
Examples of Endothermic and Exothermic Processes
Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:
sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)
An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced:
Na(s) + 0.5Cl2(s) = NaCl(s)
Learn what catalysts are and how they affect the activation energy and reaction rate of a chemical reaction.
A catalyst is a chemical substance that affects the rate of a chemical reaction by altering the activation energy required for the reaction to proceed. This is called catalysis. A catalyst is not consumed by the reaction and it may participate in multiple reactions at a time. The only difference between a catalyzed reaction and an uncatalyzed reaction is that the activation energy is different. There is no effect on the energy of the reactants or the products. The ΔH for the reactions is the same.
Usually when someone refers to a catalyst, they mean a positive catalyst, which is a catalyst which speeds up the rate of a chemical reaction by lowering its activation energy. There are also negative catalysts or inhibitors, which slow the rate of a chemical reaction or make it less likely to occur.
A promoter is a substance that increases the activity of catalyst. A catalytic poison is a substance that inactivates a catalyst.
Catalysts permit an alternate mechanism for the reactants to become products, with a lower activation energy and different transition state. A catalyst may allow a reaction to proceed at a lower temperature or increase the reaction rate or selectivity. Catalysts often react with reactants to form intermediates that eventually yield the same reaction products and regenerate the catalyst. Note that the catalyst may be consumed during one of the intermediate steps, but it will be created again before the reaction is completed.
Learn what Le Chatelier’s Principle is and how it can be applied to predict the effect of a change in conditions on chemical equilibrium.
Le Chatelier’s Principle is the name given to the principle in which a change in a chemical system prompts an opposing reaction. In chemistry, this principle was discovered independently by Henry Louis Le Chatelier and Karl Ferdinand Braun, so it is sometimes called the Le Chatelier-Braun Principle. The principle can be stated as follows:
If the temperature, concentration, volume, or partial pressure of a chemical system at equilibrium changes, then the equilibrium shifts to compensate for the change.
The more general form of the principle applies to other disciplines. Homeostasis and Lenz’s law are examples. Le Chatelier’s Principle is known by the same name when applied to economic equilibrium.
Le Chatelier’s Principle is used to predict how a change in pressure, volume, concentration, or temperature will affect chemical equilibrium. Knowing the impact on equilibrium allows chemists to manipulate the chemical reaction. For example, a chemist might apply Le Chatelier’s Principle to maximize yield from a reaction.
Enzymes are complex biochemical catalysts, speeding up a particular reaction to produce an ordered, stable reaction system in which the products of any reaction are made when they are needed. A specific enzyme controls each reaction in a series of metabolic reactions. Enzymes also control cell metabolism by regulating how and when reactions occur.
They are made up of globular proteins that have complex tertiary or quaternary structure. Enzyme shape is maintained by hydrogen bonds and ionic forces and their function can be affected by changes in temperature and pH.
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