Tutor profile: Stephanie L.
Subject: Study Skills
I did poorly on my first chemistry exam, what can I do to improve my score for the second exam?
In answering this question, I would first ask what you did to study for the first exam. From that point, we would discuss what helped and what did not help. After working with students for many years, I will then suggest several different things that have worked for students in the past and talk with you about whether you think they would be a good fit for you or not.
Rank the following in order of increasing melting point: NaCl, RbI and CaO.
The first step is to identify whether the species listed are ionic compounds or molecules. In this case, all of the species listed are ionic compounds. When melting these pure substances, the interaction that needs to be overcome is the electrostatic interaction between the cations and anions. This type of interaction can be qualitatively assessed by comparing the "lattice energy" of each species (lattice energy = the change in energy associated with the separation of an ionic solid into separate ions in the gas phase). Lattice energy and melting point have the same trend: Compounds with a higher lattice energy have a higher melting point. When melting an ionic compound, the electrostatic interaction between the charged particles is weakened so that the substance transitions from a solid to a liquid. Lattice energy can be qualitatively assessed using Coulomb's law for the electrostatic potential energy between two charged particles: Eel is proportional to Q1Q2/d. When considering lattice energy of ionic compounds, we will always be looking at an energetically favorable (attractive) interaction between a cation and anion. This means that energy will need to be ADDED to break up the interaction; hence, lattice energy is ALWAYS positive. As such, we look at |Q1Q2|/d for lattice energy. 1. Compare the CHARGES of the ions in each ionic compound. The HIGHER the charges, the HIGHER the lattice energy. Mathematically, the higher the charges, the greater the |Q1Q2| product. NaCl is composed of Na+ and Cl-, so the charges are +1 and -1 RbI is composed of Rb+ and I-, so the charges are +1 and -1 CaO is composed of Ca2+ and O2-, so the charges are +2 and -2 Based on this, we can do a rough ranking of lattice energy: NaCl, RbI < CaO The same is true for the melting point: NaCl, RbI < CaO 2. For ionic compounds with the same charges (|Q1Q2|), compare the ionic radii. The SMALLER the ionic radii, the HIGHER the lattice energy. (This comes back to the fact that the electrostatic potential energy is inversely related to the distance - in this case, we are evaluating the internuclear distance by looking at the radii of the two ions.) For these types of problems, you can look at the atomic size (without thinking about what happens to the size when electrons are lost or gained). To determine the difference in lattice energy and melting point for NaCl and RbI, we must look at their size by looking at their placement in the periodic table. Na and Cl are both in period 3 Rb and I are both in period 5 This means that Na and Cl are smaller than Rb and I. We can then say that NaCl will have a higher lattice energy (and melting point) than RbI. Final ranking for lattice energy: RbI < NaCl < CaO Final ranking for melting point: RbI < NaCl < CaO
Subject: Basic Chemistry
How do you identify an ionic compound?
While everyone knows that a metal + non-metal form an ionic compound, there are many other types of ionic compounds. An ionic compound is a compound that contains a cation (ion with a positive charge) and an anion (ion with a negative charge). In order to identify ionic compounds, it is also important to recognize polyatomic cations (the main one being NH4+) and polyatomic anions (NO3-, OH-, CO32-, etc). Ionic compounds exist in four different forms: metal cation + non-metal anion, metal cation + polyatomic anion, polyatomic cation + non-metal anion, and polyatomic cation + polyatomic anion. Ionic compounds can contain ONLY non-metals; the key to recognizing ionic compounds is being able to look at a compound and recognize if it can be broken up into a cation and anion. Examples: - NH4NO3 is an ionic compound that contains only non-metals but can be broken up into NH4+ and NO3- - H2O is NOT an ionic compound because it contains only non-metals and no polyatomic ion(s) - KOH is an ionic compound that contains a metal cation, K+, and a polyatomic anion, OH-.
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