Oxidation Number Calculation

One of the most necessary procedures in Chemistry is learning how to calculate oxidation numbers. Oxidation states are straightforward to work out and to use, but it is quite difficult to define what they actually are in any quick way. How do we calculate oxidation numbers?

First off, an oxidation number is the the degree of oxidation of an atom, ion, or molecule; for simple atoms or ions the oxidation number is equal to the ionic charge.

For example, the oxidation number of hydrogen is +1 and of oxygen is -2.

It helps to use a periodic table to determine oxidation numbers.

In between +2 and +3 we do not assign any numbers because there tends to be more than one oxidation number assigned to those elements.

Although the above method is dependable, oxidation states change. Elements can be oxidized or reduced.

Oxidation involves an increase in oxidation state or the decrease in number of electrons

Reduction involves a decrease in oxidation state or the increase in number of electrons

Recognising this simple pattern is the single most important thing about the concept of oxidation states. A few rules to follow when looking for the oxidation number include:

• The oxidation state of an uncombined element is zero. That’s obviously so, because it hasn’t been either oxidised or reduced yet! This applies whatever the structure of the element – whether it is, for example, Xe or Cl₂ or S₈, or whether it has a giant structure like carbon or silicon.
• The sum of the oxidation states of all the atoms or ions in a neutral compound is zero.
• The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.
• The more electronegative element in a substance is given a negative oxidation state. The less electronegative one is given a positive oxidation state. Remember that fluorine is the most electronegative element with oxygen second.

• Some elements almost always have the same oxidation states in their compounds but some, like Hydrogen although usually +1, can be different.

A few examples that you can work out are below.

What is the oxidation state of chromium in Cr²⁺?

For a simple ion like this, the oxidation state is the charge on the ion – in other words: +2 (Don’t forget the + sign.)

What is the oxidation state of chromium in CrCl₃?

This is a neutral compound so the sum of the oxidation states is zero. Chlorine has an oxidation state of -1. If the oxidation state of chromium is n:

                                                n + 3(-1) = 0

                                                n = +3 (Again, don’t forget the + sign!)

I hope this helps you as much as it has helped me.

-Nnana Amakiri

 

 

Energy and Temperature Dependence

How does energy and temperature affect reaction rates? I grew interest from this topic when I sought to gain a deeper and more clear understanding of how energy and temperature affect the rate of a reaction.

Consider the reaction from a chemical reference online:

                                                                 H₂ + Cl₂ -> 2HCl

         On a molecular level, bonds must be broken (H-H and Cl-Cl) before the reaction can proceed too far into products. This means that as the reactant molecules come together, the collision must have enough energy to initiate the bond breakage for the reaction to occur. Not all collisions will have this amount of energy. The collisions that do not have sufficient energy to react end up as elastic scattering events. Dictionary.com shows that Elastic suggests that it is able to maintain its shape or speed after a collision.

Only collisions with enough energy react to form products. The energy of the system changes as the reactants approach each other. The critical amount of energy to make the reaction proceed is called the Activation Energy.

The picture below comes from our chemistry textbook and details the amount of energy needed in a reaction.

From Wikipedia we are told that each reaction rate k has a temperature dependency which is usually given by the Arrhenius equation:

Ea is depicted as our activation energy or the amount of energy needed for the reaction to occur. R represents the gas constant. T represents temperature.

What is important to take from this is that when you conduct a reaction at a higher temperature, you deliver more energy into the system and increase the reaction rate by causing more collisions between particles, as explained by collision theory. However, the main reason that temperature increases the rate of reaction is that more of the colliding particles will have the necessary activation energy (Ea) resulting in more successful collisions (when bonds are formed between reactants).

Ex:

Coal burns in a fireplace in the presence of oxygen, but it does not when it is stored at room temperature. The reaction is spontaneous at low and high temperatures but at room temperature its rate is so slow that it is negligible. The increase in temperature, as created by a match, allows the reaction to start and then it heats itself, because it is exothermic. That is valid for many other fuels, such as methane, butane, and hydrogen.

Energy and Temperature can greatly influence the presence or the rate of a reaction

Nnana Amakiri