Friday, February 15, 2013

Grade 7 study guide for chemistry quiz on Thursday

In our experiment today, we placed a few grams of Ca into H2O(l) in the following reaction: Ca(s) + H2O(l)  = Ca(OH)2(aq)  + H2(g). Note the phases of matter. Solid Ca reacted with liquid H2O to form the very slightly soluble PRECIPITATE Ca(OH)2. A precipitate is a (usually) insoluble or poorly soluble solid that settles out of a liquid. Our precipitate PRODUCT was a white substance, which, as expected, did not resemble either of the REACTANTS. This is the case in the vast majority of reactions: REACTANTS DO NOT RESEMBLE PRODUCTS. By the way, note that the above reaction is not BALANCED. There are 4H on the product side, only 2H on the reactant side. We can't "create" H, nor can we destroy H. So, let's balance this reaction, as follows: Ca + 2H2O = Ca(OH)2 + H2. The 2 in front of the reactant H2O is a COEFFICIENT. The entire particle, in this case H2O, is multiplied by the coefficient. One molecule of H2O has a mass of 18amu, 2H2O has a mass of 36amu. When balancing a chemical equation, you CANNOT change any SUBSCRIPTS (the 2 in H2O, for example).
Water, a dipolar solvent in its liquid phase. So how do water molecules bond to each other? By WEAK INTERMOLECULAR FORCES, called van der Waals forces. The H end of the dipole (slightly + charged) is WEAKLY attracted to the slightly - charged O end of the water molecule. These weak bonds are easily broken. H2O(l) evaporates readily, very little energy is required for this to happen. H2O(l) will change to H2O(g) at a wide range of temperatures. The H2O molecules at the SURFACE will escape into the atmosphere as H2O(g), but we sped up this process by adding heat. At 100C H2O(l) becomes H2O(g) rapidly. Water is not nearly as volatile as acetone,CH3COCH3, though. In our experiment regarding volatility, the acetone evaporated much more rapidly than the water. VOLATILITY- THE ABILITY OF A LIQUID TO PHASE CHANGE TO A GAS.
Enough for now, let's add a bit more tomorrow and put it on our little quiz on Thursday, Valentine's day! And we love chemistry! how appr

Mr. Noonan