Observing Chemical Change

Consider the metal objects that are part of the everyday world. A mailbox, for example, stands outside day in and day out, without seeming to change. Under what conditions does metal exhibit chemical change  

Always wear eye goggles, gloves, and an apron when experimenting with chemicals. Use caution when handling an open flame. 

Place a piece of zinc metal in a large test tube. 

Add approximately 10 ml of 3M hydrochloric acid (HCI) to the test tube. Record your observations. 

When the zinc and HCI have reacted for approximately 1 min, bring a lighted, glowing wood splint to the mouth of the test tube. CAUTION Be sure the test tube is facing away from your face when the splint is brought near. Again record your observations. 

---

Explanation of the observed chemical changes at the particulate and symbolic levels. (An example is illustrated below for a strong acid-strong alkali neutralisation reaction). 

Deducing ionic equations from observed chemical changes, not by mechanically cancelling out spectator ions in chemical equations. 

The three exchange reactions that do lead to net observable chemical change are ... 

One of the most interesting characteristics of matter, and one that drives the study and exploration of chemistry, is the fact that matter changes. By examining a dramatic chemical reaction, such as the reaction of the element copper and the compound silver nitrate in a water solution, you can readily observe chemical change. Drawing on one of the fundamental laboratory techniques introduced in this chapter, you can separate the products. Then, you will use a flame test to confirm the identity of the products. 

It follows that irradiating any substance (including a polymer) will lead to the formation of ions and free radicals, and these are responsible for most of the observed chemical changes. If the irradiated substance is a solid, these reactive intermediates often remain trapped for a considerable time after irradiation and cause further chemical transformations, the so-called aftereffects. ... 

Work by Warburg and Bodenstein (1912-1925) clarified earlier confusions between photon absorption and observed chemical change. Molecules which absorb photons become physically excited , and this must be distinguished from becoming chemically active. Excited molecules may lose their energy in nonchemical ways, or alternatively may trigger off thermal reactions of large chemical yield. The socalled law , therefore, rarely holds in its strict sense, but rather provides essential information about the primary photochemical act. 

Attempts to sensitize the rearrangement of [79a] to [80a] with xanthone (ET = 74 kcal/mole) under conditions where the sensitizer absorbed essentially all of the incident radiation resulted in no observable chemical change (35). To determine if the cyclobutanone was receiving triplet energy from the sensitizer, the direct and sensitized photochemistry of syn- and anti-2-sec-butylidenecyclobutanones [81] and [82] were investigated (35). Under direct irradiation (313 nm), the isomeric acetals [83] and [84] were produced quantitatively ( = 0.1-0.2) yet under sensitized conditions (e.g., with xanthone, acetophenone, benzophenone, or triphenylene),... 

The first step in most photochemical reactions involves a transition of high probability (an optically allowed transition) from the ground state to an electronically excited state of a reactant species the excited state either initiates the observed chemical change or spontaneously dissociates into fragments which initiate those chemical changes. 

FT-Raman Spectroscopy. In order to get information about the observed chemical changes of the surface, FT-Raman spectroscopy was used to examine the surface after laser irradiation. Due to the high penetration depth of the Nd YAG laser beam of the spectrometer we were well aware that possible information would mainly consist of bulk properties. But nevertheless the spectra are valuable because there is also a large possibility of decomposition, especially of the triazene chromophore in the bulk. Raman spectroscopy was used, as compared to IR spectroscopy, because of the less complex band structure of the polymer and the larger Raman cross section of the N=N-N chromophore. Measurements with normal Raman spectroscopy were not possible, due to a high fluorescence background with the use of visible excitation light. 

In irradiated dilute aqueous solutions at concentrations <0.1 mol/1, practically all the energy absorbed is deposited in the water molecules. Hence, the observed chemical changes are the result of the reactions between the solutes and the products of the water radiolysis. With increasing solute concentrations, the direct radiolysis of the solute gradually becomes important and the solute may also interfere with the spur reactions. The use of high... 

It not infrequently happens that a chain reaction and a molecular reaction take place concurrently and make contributions of comparable magnitude to the total observed chemical change. In the thermal decomposition of acetaldehyde vapour, for example, there are probably two major mechanisms, a direct molecular rearrangement CH3CHO = C0-[-CH4, and a chain process similar to (3) above. The activation energy, Ui, for the formation of radicals is very much higher than that for the rearrangement, Ii, and in consequence the number of molecules which initiate chains is smaller in about the ratio than the number which suffer simple... 

Most of the chemical dosimeters consist of a bulk component, often a liquid in which practically the total energy imparted by the ionizing radiation is deposited, and a solute that reacts with the radiation-induced species formed by the reaction of the bulk component to produce the observed chemical change. 

To observe chemical changes in the rubbed surface, Lee and Paek measured the polarity of a rubbed surface by using water contact angle measurements, and discussed a correlation between the pretilt angle and the polarity of the rubbed surface [44,62]. The measurement method is illustrated in Fig. 2.2. 

The second reason for caution is the concentration-sensitivity of the technique. Within a given core level, detection of a particular functional group is not possible below 0.5%. In most practical situations, if the area of the component peak is less than 1%, then it will not be detected. For example, in monitoring the oxidation of a polyolefin, the Cls envelope will not exhibit discernable carbon-oxygen functionality until at least 1 in 100 of the carbon atoms present in the surface has been oxidised. When comparing this level of oxidation with that monitored in the bulk of the polymer, it is found that observable chemical changes in the surface correspond to extensive bulk oxidation. The moral of the above description is that if a species is not detected by XPS, it does not mean that it is not present. 

There are two senses to this abstraction. First, the element is abstract because it is the result of an active effort to detach significant relevant features from the particular local context in which the elements exhibit their chemical action. No two reactions are identical in every detail, but the salient features of a series of similar reactions can be abstracted by the senses of experienced chemists with or without the aid of their instruments. But there is a second and less concrete form of abstraction involved in the construction of the explanatory structure of the periodic system. The idea that macroscopic chemical properties depend on an invisible causal factor (subatomic structure whether it is indicated by atomic number or atomic weight) that can be inferred based on a theoretical construction represents another significant form of abstraction. In this respect, the chemical element is close to a mathematical abstraction. Thus, it becomes an instrument that allows one to construct a series that is assumed to lie behind and to be more fundamental than all individual, local, observable chemical changes. This abstraction allows the chemist to describe the order behind the apparently chaotic multiplicity of the phenomenal world. 

---