Review Sheet 6 Math and Chemistry

Transcription

1 Review Sheet 6 Math and Chemistry The following are some points of interest in Math and Chemistry. Use this sheet when answering these questions. Molecular Mass- to find the molecular mass, you must add up the masses of each atom in the molecule Ex. Find the molecular mass of H2SO4 Atomic Element # of atoms Mass Product H 2 1amu 2amu S 1 32amu 32amu O 4 16amu 64amu 98amu Mole/Mass/Volume Interconversions Stoichiometry Branch of chemistry concerned with mass and volume relationships of reacting substances Remember, the coefficients in a balanced equation can be thought of in 2 ways: it is the # of molecules of the chemical in the reaction or it is the # of moles of the chemical involved in the reaction. In these problems, it is best to think of them as moles.

2 Example 1 Mass Mass Problem How many grams of oxygen are required to combine with 8g of hydrogen gas to form water? 1) 8g X 2 H2 + O2 2 H2O Put grams above 2mol 1mol and moles below 2) 4mol X 2 H2 + O2 2 H2O Convert grams to 2mol 1mol moles 2) 4mol X 2 H2 + O2 2 H2O Convert grams to 2mol 1mol moles 3) Set up a ratio with these numbers and solve for X: 4mol = X 2mol 1mol 2mol = X So you need 2mol of O2 or 64 g. Example 2 Mass Volume Problem Find the mass of aluminum required to produce 1.32 L of hydrogen gas at STP from the following reaction: 2Al + 3 H2SO4 Al2(SO4) H2 1) Write the Liters above and the moles below each substance in the equation X 1.32L 2Al + 3 H2SO4 Al2(SO4) H2 2mol 3mol 2) Now, you don t need to convert Liters to moles. Simply set up the ratio as it is: X = 1.32L 2mol 3mol X = 0.88L You have 0.88 L of Aluminum 3)Now convert these Liters to moles (0.88L)/ (22.4L/mol) = 0.04mol 4)Lastly they want mass so convert to mass: (0.04mol) x (27g/mol) = 1.08 g of aluminum

3 Example 3 Volume Volume Problem Real easy. No need to convert anything. If 0.38L of hydrogen gas reacts with chlorine gas, what volume of HCl gas will be produced? H2 + Cl2 2 HCl 1) Put the Liters above and the moles below the substances in the equation. 0.38L X H2 + Cl2 2 HCl 1mol 2mol 2) Set up the ratio with these numbers: 0.38L = X 1mol 2mol X = 0.76 L You are done! Example 4 Mole Mole Problem Once again, no conversions are necessary. How many moles of HCl are needed to react with 2.3 moles of Zn in the following reaction? 2 HCl + Zn ZnCl2 + H2 1)Put the number of moles you have above, and the number of moles in the equation below. X 2.3mol 2 HCl + Zn ZnCl2 + H2 2mol 1mol 2) Set up the ratio: X = 2.3mol 2mol 1mol X = 4.6 mol

4 Percentage Composition is the ratio of the mass of element in a compound to the mass of the whole compound Example 5- Percentage Composition of FeS Atomic Element # atoms Mass Product Fe S g/mol For 1 mol: 56 g/mol Fe has a % composition = 88 g/mol = 63.6 % 32 g/mol S has a % composition = 88 g/mol = 36.4 % Empirical Formula From Percentage Composition If you have an unknown chemical, and you decompose it into its constituent elements, you can find its % composition. Once you have this you can determine the empirical formula of the unknown chemical and thus identify what it is. Example 6 - A chemist analyzes an unknown compound and finds that it is 70.9% K and 29.1% S in composition. What is the empirical formula of this compound? Step 1 convert the given percentages to grams 70.9 g of K and 29.1 g of S Step 2 divide these masses by the element s atomic mass in grams K: 70.9 g = 1.82 mol 39 g/mol - S: 29.1 g = 0.91 mol 32 g/mol Step 3 divide the larger of these numbers by the smaller. This gives you the ratio of the elements. 1.82mol/0.91mol = 2 => This means you have 2 K s for each S Therefore the empirical formula is K2S Density of Gases Density = mass/volume or Density = GFM/ (22.4L/mol) Because for any gas you know that the Gram Formula Mass is the Mass of 1 mole of the gas and that the volume of 1 mole is 22.4 L/mol

5 Regents Review Sheet 6 Solutions These are some important points to remember about Solutions. Use this sheet when you do the Review Problems. Points of Interest Solutions - A solution is a homogeneous ( same throughout ) mixture of two or more substances in a single physical state Solute substance that is dissolved Solvent substance that does the dissolving (something that is insoluble will not dissolve) 1. Solid solutions not all solutions are liquids; some are solids Alloys are solutions where one metal has been dissolved in another (when both were melted) Examples of these are gold and sterling silver; both have some copper mixed into them to strengthen the metal 2. Gaseous solutions all mixtures of gases, including air 3. Liquid solutions can dissolve solids, liquids or gases into liquids Liquids when one liquid is dissolved in another they are said to be miscible Water and ethanol When two liquids won t dissolve together, they are said to be immiscible Oil and water Aqueous Solutions - Solutions in which water is the solvent When an ionic substance is dissolved in water it forms ions this is called an electrolyte, since the solution can then conduct electricity very well Example: NaCl in water Concentration of Solutions

6 The amount of a solute in a given amount of a solvent (or how much stuff is dissolved in the solvent) 1.) Molarity (M) = moles of solute liters of solution 2.) Parts per Million - Another common measure of concentration is parts per million PPM = grams of solute grams of solution x 1,000,000 3.) Percent Solution-Another way that the concentration of solutions are often expressed is in Percent solution Percent solution = grams solute 100 ml solvent x 100 Saturated Solutions -When no more solute can be dissolved in a solution A saturated solution contains as much solute as can possibly be dissolved under the existing conditions of temperature and pressure Unsaturated Solutions - If there is less solute than can be dissolved (for example, you could actually dissolve more in if you wanted), the solution is said to be unsaturated Supersaturated Solutions - You can sometimes make a solution that holds more solute than is normally present in a saturated solution You can make these by cooling a saturated solution. As they cool, most saturated solutions will precipitate solute out of solution. For some solutions this does not occur and a supersaturated solution will form. Know the Solubility Curve (Table G) If the amount added is below the curve, it is unsaturated, on the curve is saturated, above the curve can be either saturated or unsaturated depending on whether it fully dissolve or not.

7 Solubility Factors The things that determine how something will dissolve into something else 1.) Nature of the solute and solvent Like dissolves like polar solutes dissolve in polar solvents very well Example: CH3OH and H2O mix easily because both are polar Non-polar solutes dissolve in non-polar solvents Oil dissolves in turpentine because both are non-polar Non-polar substances don t dissolve well in polar solvents Oil in water CH4 doesn t dissolve in water Like dissolves like 2.) Temperature gases in liquids are more soluble at lower temperature Solids are more soluble in higher temperatures 3.) Pressure increasing the pressure increases the solubility of gases in liquids (think of the pressure as pushing the gas into the liquid) Pressure has no effect on the solubility of liquids or solids in a liquid solvent Colligative Properties Property that depends on the concentration of solute, but not on the nature of the solute (how much you have makes a difference, not what it is) 1.) Vapor Pressure decreases as a non-volatile solute is added to a liquid 2.) Boiling Point related to 1.) above As a non-volatile solute is added, the boiling point of the solution increases 3.) Freezing Point Depression a dissolved solute will normally lower the freezing point of the solvent

8 Regents Review Sheet 8 Kinetics and Equilibrium Here are some of the most important concepts in Kinetics and Equilibrium. Use this sheet when doing the Review Problems for this topic. Equilibrium exists when two opposing processes occur at the same rate Chemical Equilibrium Example: If you put NO2 into a container, the following reaction occurs: 2 NO2 N2O4 browncolorless gas gas The reason for this is because the reaction never reaches completion. The reactants are never totally converted into products. This is a reversible reaction, which means that while the products are being formed from the reactants, the reactants are being reforme from the products (the reaction goes in the forward and reverse directions at the same time) When the forward and reverse reactions reach a point where the rates are the same, they are said to be in chemical equilibrium. At this point the concentration of the products and reactants remain constant over time. (You are making as much NO2 as N2O4 ) Concentration is symbolized by placing square brackets around a substance s symbol [NO2] = concentration of NO2 The Law of Mass Action expresses the relative concentration of reactants and products at equilibrium in terms of an equilibrium constant called Keq. Consider this fake reaction: am + bn cp + dq The equilibrium expression is : Keq = [P]c [Q]d [M]a [N]b It measures the extent to which a reaction goes to completion

9 If it is a very large number it means you made a lot of product, so the reaction proceeded to completion (or close to it) If it is a small number, you didn t make much product, so it did not proceed to completion. The reaction barely got going. LeChatelier s Principle - If a change in conditions is imposed on a system at equilibrium, the equilibrium position will shift in a direction that reduces this change (makes it go back to the way it was before the change 1). Change in Concentration 2 NO2 (g) N2O4 (g) If the above reaction is at equilibrium and I add some more NO2 (g), this extra NO2 (g) will be used up to produce N2O4 (g). The reaction will then return to equilibrium. 2). Change in Pressure- a reaction at a certain volume will try to counteract an increase in pressure by producing fewer molecules (since they take up less space) 2 NO2 (g) N2O4 (g) 2 NO2 (g) = 2moles = 44.8L N2O4 (g) = 1 mole = 22.4L So if I increase the pressure, more N2O4 (g) will be made since it takes up less volume 3). Change in Temperature depends on whether the reaction is endothermic or exothermic You do this the same way that you do the concentration problems H2 + I2 2 HI + heat Here if I increase the heat it will force the reaction to go to the left and make more reactants Haber Process - N2 (g) + 3H2 (g) 2NH2 (g) kJ In order to make more ammonia in the above reaction: You can: 1. Remove the product continuously 2. Increase the pressure

10 3. Increase the temperature this sounds wrong, but it isn t. Think about it in terms of the motions of particles Ksp Tells us the same thing that Keq does: Small Ksp = not much substance dissolves Large Ksp = a lot of substance dissolves AgCl (s) Ag+ (aq) + Cl- (aq) Ksp = [Ag+ ] [Cl- ] [AgCl] since it is solid, ignore Ksp = [Ag+ ] [Cl- ] Kinetics Chemical kinetics - the area of chemistry dealing with the speed at which reactions occur Rate = change / time Reaction rate rate at which the reactants disappear and the products appear You can also think of it as the change in concentration of reactants and products in a certain amount of time Collision Theory molecules must collide in order to react Usually involves a collision between 2 molecules (occasionally 3 molecules, but the chances of 3 things colliding in the correct way at the same time are pretty slim) Effective collisions leads to product Needs sufficient energy and needs to be aligned correctly during the collision

11 Energy Diagram - Endothermic Energy Diagram - Exothermic

12 Energy Diagram Addition of Catalyst Activation energy energy needed to start a reaction Activated complex at this point the chemicals are neither product nor reactant; they are in an intermediate state between both (like a mixture of each) Factors Affecting Reaction Rate 1.) Nature of reactants what the reactants are have a lot to do with how fast the reaction takes place 2.) rate. Why? Temperature the higher the temperature, the faster the 3.) Concentration the higher the concentration, the faster the rate. Why? 4.) Surface Area the higher the surface area, the faster the rate. How do you increase surface area? Why does this increase the rate?

13 5.) Catalysts these molecules orient the reactants so that they react better (basically hold the reactants in the correct position so the reaction takes place faster) Spontaneous reaction one that will proceed on its own without outside help For a reaction to be spontaneous the Enthalpy must be decreasing ( = a negative number) Entropy a measure of the disorder of a system; Disorder is greatest in gases and least in solids Summary A reaction is spontaneous if the Enthalpy decreases (is negative; see Table I in the reference tables) and the system has an increase in disorder (positive Entropy; this is easy to figure, since Entropy increases from solid to liquid to gas)