chemistry

Oxidation / Reduction Oxidation Numbers: Assigning oxidation numbers to atoms in chemical compounds - Examples   In a previous post entitled “ Oxidation – Reduction (Redox) Reactions – Balancing Redox Reactions ” the general steps for assigning oxidation numbers to atoms in a molecule or ion were presented. Assignment of oxidation numbers is a basic step for balancing redox reactions . A few solved examples are given below:   Example #1 Calculate the oxidation number of the S atom in the following compounds: α) H 2 S    b) SO 4 -2   α) Step 1 : Find the most electronegative atom The most electronegative atom in H 2 S is the S atom   Step 2 : The oxidation number of the most electronegative atom is equal to the charge of its ion Since the charge of sulfur’s ion is unknown let’s suppose that is equal to x   Step 3 : Oxidation numbers that are not known have to be calculated. The oxidatio...

Oxidation / Reduction reactions - Balancing redox reactions Redox Reactions – Balancing Redox Reactions In many important chemical reactions, electrons are transferred from atom to atom. Such reactions are called either oxidation – reduction reactions or redox reactions . Many important chemical reactions involve oxidation and reduction. In fact, most reactions used for energy production are redox reactions. In living species the oxidation of sugars, fats, and proteins provides the energy necessary for life. The electric current from the batteries that power computers and cell phones is generated by oxidation-reduction reactions.  One of the most familiar redox reactions is corrosion of a metal. Corrosion is the conversion of a metal into a metal compound by a reaction between the metal and some substance in its environment. When a metal corrodes, each metal atom loses electrons and a cation is formed, which can combine with an anion to form an ionic comp...

Statistical Treatment of Analytical Data - One-Sample t-test Home > Statistics – Frequency Distributions, Normal Distribution, z-scores > Statistics – Frequency Distributions, Normal Distribution, z-scores Statistical Treatment of Analytical Data - One-Sample t-test in Chemical Analysis A one-sample t-test is used to compare two means provided that data are normally distributed (plot of the frequencies of data is a histogram of normal distribution ). A t-test is a parametric test and relies on distributional assumptions. It is a useful tool in analytical work when two means have to be compared. A situation like this is presented in the following example. A new analytical instrument is tested in a chemical laboratory by determining the mass m (in mg) of Cu contained in a certain mass (i.e. 1 g) of a certified reference material (CRM). The analysis certificate of the CRM states that the average mass of Cu (in mg) is 4.54 per...

Solved Examples - Entropy changes and thermodynamic equilibrium Entropy Changes ΔS and Thermodynamic Equilibrium – Solved Examples   In a previous post entitled “ Entropy, Free Energy and Thermodynamic Equilibrium ” the Boltzmann definition of entropy was given and how entropy changes ΔS are associated with chemical processes was discussed. Below, some examples are given regarding entropy changes ΔS and chemical reactions. Example #1 Choose the reaction expected to have the greatest increase in entropy: a)  N 2 (g) + O 2 (g) ———› 2 NO(g) b)  H 2 O (l)    ———›   H 2 O (g) c)  2 XeO 3 (s)  ———›   2 Xe(g) + 3 O 2 (g) d) C(s) + O 2 (g)   ———›  CO 2 (g)   Answer: The reaction with the greatest increase in the moles of gas will have the greatest increase in entropy . Answer (c) is correct. In general, when a reaction involves gaseous ...

Entropy, free energy and thermodynamic equilibrium Entropy, Free Energy and Thermodynamic Equilibrium Chemical reactions are performed by mixing the reactants and regulating external conditions such as temperature and pressure . Two basic questions though arise: Is it possible for the reaction to occur at the selected conditions? If the reaction proceeds, what determines the ratio of products and reactants at equilibrium?   Both questions are answered by chemical thermodynamics : Thermodynamics can tell us whether a proposed reaction is spontaneous (possible) under particular conditions even before the actual experiment. Thermodynamics can also predict the ratio of products and reactants at equilibrium provided that the reaction is spontaneous . Note: Thermodynamics cannot answer though how fast a reaction will proceed. The field of Chemical Kinetics studies reaction rates. After many years of observation scientists conclude...

Entropy, free energy and thermodynamic equilibrium Entropy, Free Energy and Thermodynamic Equilibrium Chemical reactions are performed by mixing the reactants and regulating external conditions such as temperature and pressure . Two basic questions though arise: Is it possible for the reaction to occur at the selected conditions? If the reaction proceeds, what determines the ratio of products and reactants at equilibrium?   Both questions are answered by chemical thermodynamics : Thermodynamics can tell us whether a proposed reaction is spontaneous (possible) under particular conditions even before the actual experiment. Thermodynamics can also predict the ratio of products and reactants at equilibrium provided that the reaction is spontaneous . Note: Thermodynamics cannot answer though how fast a reaction will proceed.   After many years of observation scientists concluded that the characteristic common to all spontaneous...

Phase changes - energy changes - heating curves Phase Changes - Energy Changes - Heating Curves   Many important properties of liquids and solids relate to the ease with which they change from one state to another. Water for example, when heated it evaporates that is changes from liquid to the gas state. In general, each state of matter (solid, liquid, gas)  can change into either of the the other two states. Figure I.1 shows these transformations which are called phase changes or changes of state . What happens when a solid is heated? Typically, it melts to form a liquid. If the heating continues, the liquid at some point boils and forms the vapor phase (gas). This process can be represented by a heating curve : a plot of temperature versus time for a process where energy is added at a constant rate. The heating curve of water is shown in Fig. I.2. There are five separate zones on the graph (heating curve) of Fig. I.2:   Zone 1 (Ice) : As energy ...