CHM116 Lecture 2- Student Slides

7/31/2019 CHM116 Lecture 2- Student Slides

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CHM 116 Spring 2012

Todays Lecture

Section 3.4, 3.5, 4.1, 4.2, 4.4

Next Lecture

Read Sec. 16.1, 16.2, 16.3In Silberberg


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CHM 116 textbook: Chemistry, 6eby Martin Silberberg

Any of the covers below is the correct book

Nationaledition

Fall 2011Custom edition

Spring 2012Custom edition
http://www.amazon.com/gp/product/images/0077468449/ref=dp_image_0?ie=UTF8&n=283155&s=books


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Lecture 2: Learning Objectives

Be able to convert between g and moles, for

pure substances and substances in solution(molarity). (Review from 115).

Understand the impact of a limiting reactanton yield

Know the difference between theoretical and

actual yield (% yield)

Know how to determine empirical or

molecular formulas from elemental analysis


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STOICHIOMETRY

- the study of the

quantitativeaspects ofchemicalreactions.


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Massreactant

StoichiometricFactor(bridge)

from equation

Molesreactant

Molesproduct

Massproduct

General Plan for

Stoichiometry Calculations

Molarityreactant

Molarityproduct

g

M

mol/L


6/18

Hydrocarbons as Fuel: Combustion Reactions

Combustion involves reaction with oxygen;hydrocarbons are converted to CO2 and H2O.

Unbalanced equation:

C8H18 + O2 CO2 + H2O

Balanced equation:

2 C8H18 (l) + 25 O2 (g) 16 CO2 (g) + 18 H2O(l)

How many moles of carbon dioxide areproduced from 1 mole of octane?


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Calculating amounts of reactants and productsPROBLEM: If 454 g of NH4NO3 decomposes, what

mass in grams of H2

O is formed?

What is the theoretical yield of this product?

First: make sure you have abalanced equation !

__NH4NO3 __N2O + __ H2O


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PROBLEM: If 454 g of NH4NO3 decomposes, what

mass in grams of H2O is formed?


Mass

reactant

Molesreactant

Molesproduct

Mass

product

NH4NO3 N2O + 2 H2O

1. Mass NH4NO3 to moles NH4NO3

2. Moles NH4NO3 to moles H2O

3. Moles H2O to mass H2O


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STEP 1




NH4NO3 N2O + 2 H2O


2. Moles NH4NO3 to moles H2O3. Moles H2O to mass H2O

molar mass (or molecular weight, units g/mol)

454 g * 1 mole/80.04 g =5.68 moleNH4NO3


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NH4NO3 N2O + 2 H2O


2. Moles NH4NO3 to moles H2O3. Moles H2O to mass H2O

STEP 2

1

5.68 mole NH4NO3


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NH4NO3 N2O + 2 H2O


2. Moles NH4NO3 to moles H2O

3. Moles H2O to mass H2O

STEP 3

Called the THEORETICAL YIELD

11.4 mole H2O * 18.02 g/1 mole= 204 grams


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In a given reaction, there may not

be enough of one reagent to use up

the other reagent completely.

The reagent in short supply LIMITS

the quantity of product that can beformed.

Limiting Reactants


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Limiting Reagents


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Sample Problem 2 Calculating Amounts of Reactant and Product inReactions Involving a LimitingReactant

PROBLEM: A fuel mixture used in the early days of rocketry iscomposed of two liquids, hydrazine (N2H4) and dinitrogentetraoxide (N2O4), which ignite on contact to form nitrogen

gas and water vapor. How many grams of nitrogen gas

form when 100g of N2H4 and 200g of N2O4 are mixed?

1. Start with a balanced chemical equation

2. Find the number of moles of one of the products that could be

produced from each reactant if it reacted completely

3. The reactant that gives the smallest amount of product (inmoles) is the limiting reactant

4. If requested, convert the smallest amount of product (in moles)

to grams

SOLUTION: N2H4(l) + N2O4(l) N2(g) + H2O(l)2 43


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Percent Yield

How to calculate the percent yield of a reaction


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Sample Problem 3 Calculating Percent YieldPROBLEM: Silicon carbide (SiC) is an important ceramic material that is

made by allowing sand(silicon dioxide, SiO2) to react with

powdered carbon at high temperature. Carbon monoxide is also

formed. When 100.0 kg of sand is processed, 51.4 kg of SiC is

recovered. What is the percent yield of SiC from this process?SOLUTION:

1. Write and balance equation

2. Calculate theoretical yield

3. Calculate % yield

___SiO2(s) + 3(s) ___ 1 SiC(s) + __2_CO(g)1


17/18

Introduction to Lab next week:

Electrolytes & Nonelectrolytes

(& Acid-Base Reactions)

Definition of Electrolyte: compound whose aqueoussolution conducts electricity[all soluble ionic compounds and compounds that react

with water to give ions (molecular acids and bases) areelectrolytes]

BatteryNaCl(s) Na+

(aq) + Cl-(aq) yes

Ethanol(l)CH3CH2OH ethanol(aq) no

Sucrose(s) sucrose(aq) no

HCl(g) + H2O(l) H3O+

(aq) + Cl-(aq) yes


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Strong and Weak Electrolytes

Using acids and bases as examples

1) An acid is a substance that, when dissolved in water,increases the concentration of hydrogen ions, H+, in solution

2) An acid is a proton, H+, donor

A base is a proton, H+, acceptor

HCl(g) + H2O H3O+

(aq) + Cl-(aq)

Hydrochloric acid is a strong electrolyte (and a strong acid), itdissociates completely in H2O (~100% product at equilibrium)

CH3COOH(aq) + H2O H3O+

(aq) + CH3COO-(aq)

Acetic acid is a weak electrolyte (and a weak acid), it dissociates only

slightly in H2O (

CHM116 Lecture 2- Student Slides

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