Basic Chemistry 3

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Chemistry

Author: oLahav

Basic Chemistry, Part 3

Good news! You should already know from reading lesson 1 and lesson 2 all about substances, specifically atoms, elements, and how they combine to form ionic or molecular compounds.

Now, if chemicals were nice and quiet and didn't do anything we would have a lot less to know. But we would also have a lot less brains, bodies, and in general everything else in the world either, so it's a good thing chemicals do stuff. What they do is- they react.

Chemical reactions are processes through which one or more chemical substances form one or more different chemical substances. This is very different from physical changes, which include change of state (like water melting), change of shape (cutting things, separating molecules from one another), etc. In physical changes, the molecules and substances don't change, only their locations, relative energy, or other generic properties.

There are 5 indicators for chemical changes- a change in color, the release of energy in forms of heat or light, change of odour (smell), the production of new gases or solids from mixing 2 liquids, and a difficulty in reversing the process.

There are 4 types of chemical reactions. The first is synthesis, which involves 2 substances (or more) forming 1 new big one. The general formula for this is $A + B \to AB$ . A and B can be elements or compounds. Some examples are $Mg+O_2 \to MgO$ , and $CO_2+H_2O \to H_2CO_3$ .

Second type is known as decomposition. As the name suggest, here we start with 1 big reactant and end up with 2 or more smaller products. The general formula is $AB \to A+B$ . Examples of this would be $CaCO_3 \to CaO+CO_2$ and $H_2O_2 \to H_2O+O_2$ .

Now we come to the more interesting types. Single displacement reactions have this general formula: $AX+B \to A+BX$ . See how the X switched? That's single displacement, 1 element stealing part of a compound. This only works if A and B are different metals (or hydrogen), in which case X represents an anion, or if A and B are halogens, in which case X represents a cation. For this reaction to occur, B has to be more reactive than A. A few examples of single displacement include $Ca+H_2O \to Ca(OH)_2 + H_2$ and $Cl_2+NaBr \to NaCl + Br_2$ .

How do we know whether B is more reactive than A? We check the Reactivity series. This list of elements has them ordered from most to least reactive. You'll notice that the alkali metals are at the top, followed by the alkaline earth metals group (group 2), and then the transition metals. This is because, as we've said before, the metals with the least electrons to lose will be more reactive than those with more electrons to lose.

Finally, we have double displacement reactions. These are simply 2 multi-elemental compounds switching partners. Like this: $AX+BY \to AY+BX$ . These reactions often result in one product being a gas or solid precipitate. Examples are $AgNO_3+NaCl \to AgCl + NaNO_3$ as well as $HBr+KOH \to H_2O+KBr$ .

This is all nice and funky, but we're missing one crucial thing- balance. According to the Law of Conservation of Mass, matter can neither be created nor destroyed. So when we combine nitrogen, N_2 and hydrogen, H_2 to form ammonia, NH_3 ... where does the extra nitrogen go to? And where does the third hydrogen come from?

In real life, it doesn't work just like that. We have lots of nitrogen and hydrogen molecules around bumping into each other and reacting, so the atoms aren't lost or created at all. We can show it like so: $N_2+3H_2 \to 2NH_3$ . Now it all adds up! So one nitrogen molecule reacts with 3 hydrogens to form 2 ammonia molecules. Cool. As you can see, it's important to be able to Balance equations.