(i) Low dissociation energies

All the halogens have very low dissociation energies. As a result, they can readily dissociate into atoms and react with other substances. As shown below, the dissociation energies of halogens are quite low in comparison to common molecules such as H2, O2 and N2.

(ii) High electron affinity

Halogens have very high electron affinity values and therefore, have very strong tendency to gain an electron. Thus halogens are very reactive elements due to their low dissociation energies and high electron affinity values. As clear from the values of bond dissociation energies, fluorine has the lowest bond dissociation energy. This is due to weak F-F bond because of the repulsion between the non-bonding electrons in the small molecule. Therefore, it is most reactive among the halogens.

Some of the important chemical reactions of halogens are discussed ahead.

Free Radicals Defined:

Free radicals are a byproduct of normal cell function. When cells create energy, they also produce unstable oxygen molecules. These molecules, called free radicals, have a free electron. This electron makes the molecule highly unstable. The free radical bonds to other molecules in the body – causing proteins and other essential molecules to not function as they should. Luckily, antioxidants can minimize free radical damage.

Antioxidants – the Free Radical Sponge:

Antioxidants are substances found in plants that soak up free radicals like sponges. If your body has plenty of antioxidants available, it can minimize the damage caused by free radicals. Get your antioxidants from eating plants. There is some evidence that we can only get the full antioxidant benefits from eating real plants and other foods. Supplements appear not to be as effective.

How Free Radicals Cause Aging:

This theory asserts that many of the changes that occur as our bodies age are caused by free radicals. Damage to DNA, protein cross-linking and other changes have been attributed to free radicals. Over time, this damage accumulates and causes us to experience aging.

The Evidence:

There is some evidence. Studies have shown that increasing the amount of antioxidants in the diets of mice and other animals can slow the effects of aging. This theory does not fully explain all the changes that occur during aging. It is likely that free radicals are only one part in the aging equation.

There’s been a smattering of conversation about light production around my house this weekend between sailing the Laser, setting off fireworks and observing fireflies. One of my teen guests wondered how the fire in fire flies was different from the fire in fireworks. All light is not created in quite the same way….though there are some fundamental similarities.

There are really two fires in fireworks, the thermal explosives that send them skyward, and the “rockets red glare” — the glittering burst of color in the sky. The heat from the thermal explosion (usually blackpowder or a similar substance) is what trigger the colors.

If you ever done a flame test, putting a solid substance or a concentrated solution on a wire loop and placing it in a flame to see what color is produced, you’ve done the same chemistry. The extreme heat excites electrons in an atom or molecule, and as they fall back down to their lowest energy, or ground state, emitting a photon (a bit of light) that just exactly matches the difference in energy between the excited state and the ground state. An orange flame meant you had sodium on the wire, while a violet flame suggested potassium. More properly this technique is called atomic emission spectroscopy.

For atoms the picture you usually see in a high school text of this process is of a ladder, where electrons are shown moving from rung to rung. The larger the distance between the two rungs (or states) the higher the energy of the photon emitted. If the distance corresponds to photons in the visible region, you see a color, otherwise you have to use something fancier to figure out the energy of the photons being released.

Different atoms have different spacings between states and so the colors they emit when heated to high temperatures are likewise different. There are in fact many states, and so many types of photons can be emitted, but few are in the visible region.

If you click here, you can see a simulation of the photons you’d expect to see when an excited sodium atom returns to the ground state. Are you surprised that sodium can be used for yellow-orange in fireworks? Some urban legends suggest that lead (or radioactive barium) are used in fireworks, but if you look at the line spectrum of lead you can see why it can’t be true — there is no rung to rung jump in lead that corresponds to a visible photon. So a lead firework would be invisible! (Lead used to be used to make the fireworks “crackle”…)

(And it’s true that barium salts are used in fireworks, but they are not radioactive. There are no naturally occurring radioactive isotopes of barium.)

source

Hydrazine is a hypergolic propellant – one that ignites as soon as it comes into contact with an oxidant (something that will react with it to effectively strip away some electrons from the reactant and force the molecule to bond differently, the changes in the bonds between atoms are what release the energy). Hypergolic is apparently a term coined by the German rocket program from hyper (very) + ergon (Greek for work) + ol (from oleum, the Latin for oil). Hydrazine is that, a liquid (if not particularly oily one) that can be used to push satellites around in orbit – to do work.

Hydrazine is a solid in the satellite’s tanks, and once thawed can be catalytically and rapidly decomposed. Almost any metal will do, though iridium is the usual choice. The reactions produce lots of very hot gases, which you can direct through a thruster:

3 N2H4 → 4 NH3 + N2
N2H4 → N2 + 2 H2
NH3 + N2H4 → 3 N2 + 8 H2

A little thermochemistry can quickly tell you just how much energy you might produce from 1000 pounds of hydrazine. The overall reaction is:

5 N2H4 → 5 N2 + 10 H2

which releases 50,000 Joules of energy per mole of hydrazine. A mole of hydrazine weighs about 32 grams, so you get enough energy to make a cold cup of coffee hot from just over an ounce of hydrazine (do NOT try this at home!). If all the hydrazine in that satellite went up at once, it would release about 8 billion Joules (enough to keep the average US citizen in energy for more than a week).