Rates of Reaction

•March 31, 2009 • Leave a Comment

Rates Of Reaction

· A chemical reaction involves a collision between particles.

· The particles collide and make new substances

· The particles which react are called the reactants

· The substances which are made are called the products

How do we make the reaction go faster?

There are four things that we can change to make the reaction go faster.

They are

Temperature

Surface area

Concentration

Using a catalyst

Temperature

When we increase the temperature we give the particles energy

This makes them move faster

This means they collide with other particles more often

So the reaction goes faster.

Surface area

If we make the pieces of the reactants smaller we increase the number of particles on the surface which can react.

This makes the reaction faster


The particles on the surface can react

The particles on the surface can react

When cut into smaller pieces the particles on the inside can react

When cut into smaller pieces the particles on the inside can react

Concentration

If we make one reactant more concentrated (like making a drink of orange squash more concentrated)

There are more particles in the same volume to react

So the reaction goes faster.

a) There are less red particles in the same volume so there is less chance of a collision

b) There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

A catalyst is a chemical which is added to a reaction.

It makes the reaction go faster.

The catalyst does not get used up in the reaction.

It gives the reaction the energy to get started

Models of the Atom: a Historical Perspective

•March 31, 2009 • Leave a Comment

1.  Democritus a fifth century B.C. Greek philosopher proposed that all matter was composed of indivisible particles called atoms (Greek for uncuttable).

2.  Billiard Ball Model  (1803)- John Dalton viewed the atom as a small solid sphere.  He really got the “ball” rolling for modern chemistry!

  • Each element was composed of the same kind of atoms.
  • Each element was composed of different kinds of atoms.
  • Compounds are composed of atoms in specific ratios.
  • Chemical reactions are rearrangements of atoms (mass is conserved).

3.  Plumb Pudding Model (1897)-  Joseph John Thomson proposed that the atom was a sphere of positive electricity (which was diffuse) with negative particles imbedded throughout after discovering the electron, a discovery for which he was awarded the Nobel Prize in physics in 1906.

Thomson atom

Thomson atom

4.  Solar System Model-Ernest Rutherford discovered that the atom is mostly empty space with a dense positively charged nucleus surrounded by negative electrons.  Rutherford received the Nobel Prize in chemistry in 1908 for his contributions into the structure of the atom.  In 1913 Neils Bohr proposed that electrons traveled in circular orbits and that only certain orbits were allowed.  This model of the atom helped explain the emission spectrum of the hydrogen atom.  He received the Nobel Prize in physics in 1922 for his theory.

5.  Electron Cloud Model (1920’s)- an atom consists of a dense nucleus composed of protons and neutrons surrounded by electrons that exist in different clouds at the various energy levels.  Erwin Schrodinger and Werner Heisenburg developed probability functions to determine the regions or clouds in which electrons would most likely be found.


 
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