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UNIT 3 –Quantities in Chemical Reactions THE MOLE! In chemistry as in other aspects of life it is sometimes more convenient to count in groups of items rather than count items individually. Quantity Amount In chemistry chemical reactions are __________________________________________. To count the # of atoms in a chemical reaction individually is inconvenient so chemists group atoms into groups. Chemists count in ____________________________________________ ________________________________________________________________________ This value is known as the __________________________________________________ The mole is defined as: ________________________________________________________________________ ________________________________________________________________________ How big is the Avogadro constant? Try the Thought lab on pg. 175. - Canada covers 9976140 km2 - Distance to moon is 384,403 km - Length of $5 is 152.4 mm - The worlds oceans hold 1.37x109 km3 water. - One teaspoon is equal to 5ml - Average size apple has a mass of 0.15 kg - Mass of the Earth is 5.9742 × 1024 kg - Current population of Earth is 6,654,181,537 Converting Moles to Number of Particles N= n= NA = Problem 1. A sample contains 2.25 moles of carbon dioxide, CO2. a. How many molecules are in the sample? b. How many atoms are in the sample? Problem 2. How many moles of nitrogen dioxide are there in a sample containing 4.35 x 1024 molecules? Molar Mass When we measure the mass of a substance we express its mass in grams. You have already learned that one atom of ______________ has a mass of exactly _____ and that ______________ atoms has a mass of exactly _______. This means: __________________________________________________________________ _________________________________________________________________. This is known as ________________________________________________________. Molar mass can be applied to any substance and is equivalent to the _______________________________________________________________________. Molar mass of Al Molar mass of HNO3 or MHNO Using Molar Mass The following equation will allow you to solve problems involving ________________________________________________________________________ Problem 1. A flask contains 0.950 mol of carbon monoxide, CO. What is the mass of CO in this sample? Problem 2. How many moles of acetic acid CH3COOH are in a 15.2g sample? Problem 3. What is the mass of 6.35 x 1024 molecules of copper (II) nitrate. Number of Particles Moles Mass of substance Chapter 6 – Chemical Proportions in Compounds In the late 1700’s Joseph Proust discovered that samples of copper (II) carbonate always contained the same proportion of Cu, C and O. This led to the ___________________________________________________________ – the elements in a chemical compound are always present _______________________________________________________________________ The mass of an element in a compound, expressed as a % of the total mass is known as an elements ________________________________. Ex H2O The law of definite proportions _____________________ that the elements in compounds are always the same. CO CO2 If we state all of the mass percents for a compound we call it _______________________________________________________________________ A compound with a mass of 48.72g is found to contain 32.69g of Zn and 16.03g of S. What is the % composition? Calculating % composition from a formula Pyrite (aka _________________________) is a compound with the chemical formula _______________. If you wanted to extract the iron and know how much to expect, you first need to know the _____________________________. Calculate the % composition for ________________. ______________________________ C9H8O is the molecule responsible for the ____________________________________________________________________. Find the % composition of this compound. Empirical Formula of a Compound The _____________________________________ of a compound shows the ____________________________________________ of the elements in the compound. Ex H2O2 The ____________________________________________ of a compound shows the ___________________________________________ of each element in the compound. The relationship between the 2 types of formulas is: Determining Empirical Formulas To determine an empirical formula you need to use the concepts of ________________________________________________________________________ Ex1. Calculate the empirical formula for a compound that is 85.6% C and 14.4% H. Ex2. The percentage composition of a fuel is 81.7% carbon and 18.3% hydrogen. Find the empirical formula. Molecular Formula of a Compound Being able to identify unknown substances as many uses. Think forensic science. Since many substances have the same empirical formulas, we need to be able to determine the _____________________________________ for a substance. To do this we use _________________________. Chemists use a ___________________________________ to determine the molar mass of a substance. Then: Ex. 1 The empirical formula for ribose (a sugar) is CH2O. The molar mass of ribose is 150g/mol (by experiment). What is the molecular formula of ribose? Finding Formulas by Experiment There are many ways to determine formulas by experiment. The Carbon-Hydrogen Combustion Analyzer This is a device used to ____________________________________________________. How it works Stream of ______________________________________ containing compounds. CH compound will react completely to produce __________________________ All the H2O is absorbed by ___________. Final mass – initial mass = mass of H2O. All ____________________________. Use ______________________ of H in H2O to determine mass of H All the CO2 is absorbed by __________________. All C ends up here. Use _____________________________ of CO2 to determine mass of C. If original compound contained a third element, then: Ex1. A 1.000g sample of a pure compound containing only C and H was combusted. 0.6919g of water and 3.338g of carbon dioxide were produced. a. Calculate the masses of C and H in the sample. b. Find the empirical formula of the compound. Hydrated Ionic Compounds Many ______________________ crystallize from an aqueous solution, with H2O molecules ___________________________________________________________. These are called ____________________________. _______________________ have a specific # of _______________________ chemically bonded to each formula unit. Chemical Name Formula Compounds that have no H2O molecules are called ______________________________ Just remember: when doing calculations you must ________________________________________________________________________ Ex 1. A 50.0g sample of a hydrate of barium hydroxide Ba(OH)2xH2O contains 27.2g of Ba(OH)2. a. Calculate the percent by mass of water in Ba(OH)2xH2O b. Find the value of x in Ba(OH)2. Chapter 7 Quantities in Chemical Reactions Stoichiometry Stoichiometry is the study of the ________________________________________________________________________ It all begins with ___________________________________________, which are essential for making calculations related to chemical reactions. For example, the __________________________________ (used to make ammonia gas) is based on the following balanced equation: What does this equation tell you? 1. s 2. s 3. If we multiply each by 12 we could say: 1 dozen molecules of nitrogen gas reacts with 3 dozen molecules of hydrogen gas to produce 2 dozen molecules of ammonia gas. 4. The relationship between moles in a balanced chemical equation are known as MOLE RATIOS We use mole ratios to solve problems. 1. From the equation above, how much ammonia gas would be produced from 3.5 mol of hydrogen gas? 2. When carbon and oxygen combine 2 reactions are possible. The product can be CO2 or CO. a. Write the balanced chemical equations. b. How much oxygen is needed to react with 0.75mol carbon to produce CO2? c. How much oxygen is needed to react with 0.75mol carbon to produce CO? Mole Ratios can also be used to find the mass ratios for chemical equations. N2(g) + 3H2(g) 2NH3(g) Balanced Equation # of Particles Amount Mass = n x M Total mass Solving Stoichiometric Problems Follow this process: 1. 2. 3. 4. Ex 1. Mass to Mass Astronauts must be able to remove the CO2 from their spacecraft. If not hypoxia (lack of O2) would occur. CO2 is removed using the following chemical reaction: CO2(g) + 2LiOH(s) Li2CO3(g) + H2O(g) . If an astronaut produces 1.50x103 g of CO2 a day, what mass of LiOH would be needed per day? Ex 2. On Feb, 20, 2008 the US Navy shot down a spy satellite in space from a war ship in the Pacific ocean. They claimed the satellite contained hydrazine (rocket fuel), that could harm people if the satellite fell back to Earth. When hydrazine is combined with dinitrogen tetroxide it produces nitrogen gas and water. If 100g of hydrazine is reacted, how much nitrogen gas would be produced? Ex 3. During WWI, chlorine gas was used as a weapon. When chlorine gas enters the lungs it reacts with water and forms corrosive hydrochloric acid and oxygen gas. How many molecules of chlorine react to produce 5.0g of HCl? The Limiting Reactant Chemical reaction equations give the ideal _______________________________________________________________________. However, the reactants for a reaction in an experiment are not necessarily a _______________________________________________________________________. In a chemical reaction, one of the reactants may ___________________ when the reaction is finished. These are called ___________________. The reagent that is ______________________________ or reacted is called the ______________________, because its quantity limit the amount of products formed. Think of when you put gas in your car: 2C8H18 + 25O2 16CO2 + 18H2O Ideally, ____________ of oxygen react with _________________ of octane. But in reality __________________________________________so ___________ would be the ________________________________________. When you run out of gas ____________________________ In stoichiometric problems, _________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Ex 1. Glucose reacts with oxygen gas to produce carbon dioxide and water during cellular respiration. If 1.25 g of glucose reacts with 7.51g of oxygen gas, find the limiting reactant. Now that you know the limiting reactant above, how much carbon dioxide would be produced? Ex 3. Percentage Yield Chemists use _____________________ to predict the ________________________ that can be __________________ from a chemical reaction. This amount is called the __________________________________________. This amount is _____________________________. The ____________________ obtained in an experiment is called the ______________________________. The ____________________ is usually less then the _______________________ and for a variety of reasons. For example: a. b. c. d. _________________________________________________ compares the ______________________________________________________________ (actual) to the ____________________________________________ (theoretical). Ex1. When 75g of nitrogen gas reacts with sufficient hydrogen gas, the theoretical yield of ammonia is 9.10g. If 1.72 g of ammonia is obtained what is the percentage yield? Ex 2. When Calcium carbonate is decomposed to calcium oxide and carbon dioxide there is a 92.4% yield of calcium oxide. How many grams of calcium oxide can you expect if 12.4g of calcium carbonate is heated. Many times substances may not be pure. The substance might contain _______________________. If gold is 99% pure then it must contain 1% other substances. ________________________________________ describes what proportion _________________ of a sample is composed of a specific compound or element. Ex 1. Your have a 13.9 g sample of “fools gold”. You heat the sample in oxygen and produce 8.02g of iron (III) oxide. What % purity of iron pyrite was in the original sample? 4FeS2 + 11O2 2Fe2O3 + 8SO2