Hess’s Law. Enthalpy is a State Variable ‘State variable’ just means something that doesn’t change depending on the path you take to get from A to B.

Slides:

Advertisements

Similar presentations

Mr. Chapman Chemistry 30. Enthalpy Change Can Be Tough Sometimes it is impossible or completely impractical to measure the enthalpy change of a reaction.

Advertisements

Hess’s Law. Several reactions in chemistry occur in a series of steps, rather than just one step. For example, the following reaction explains the combustion.

Standard Enthalpy (Ch_6.6) The heat change that results when 1 mole of a compound is formed from its elements at a pressure of 1 Atm.

Thermochemistry Chapter 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Title Heats of Combustion, Heats of Formation, Heats of Hydrogenation and Bond Dissociation Energies.

Using Standard Molar Enthalpies of Formation SCH4U0.

Advanced Chemistry Ms. Grobsky Enthalpies of Reactions and Hess’ Law.

1 Chapter 6 EnergyThermodynamics. 2 Energy is... n The ability to do work. n Conserved. n made of heat and work. n a state function. n independent of.

Chemistry Chapter 7 – Hess’s Law Teacher: H. Michael Hayes.

Hess’s Law Review  Q - What is the first Law of Thermodynamics?

Energy as Heat A sample can transfer energy to another sample. One of the simplest ways energy is transferred is as heat. Heat is the energy transferred.

Department of Chemistry and Biochemistry CHM Reeves The Nature of Energy The First Law of Thermodynamics Enthalpy Enthalpies of Reaction Calorimetry.

Topic: Thermodynamics Do Now: packet p.1. Every physical or chemical change is accompanied by energy change  Energy released = _________________  Energy.

Chapter 5 - Thermochemistry Lindblom AP Chemistry.

Section 7.5—Hess’s Law How can we find the enthalpy of a reaction using step-wise reactions?

Reactions in Chemistry The Basics of Chemical Equations.

Chemistry and Heat That’s hot stuff.. Energy is the capacity to do work. Energy is measured in Joules 1 Joule of energy can raise 1 N of weight exactly.

1 Chapter 6 EnergyThermodynamics. 2 Energy is... n The ability to do work. n Conserved. n made of heat and work. n a state function. n independent of.

More Heat Calculations What have we done?. We can figure out heat values and then put them into kJ / mole.

Thermochemistry ENERGY CHANGES.. Energy is the capacity to do work Thermal energy is the energy associated with the random motion of atoms and molecules.

A Comparison of  H and  E 2Na (s) + 2H 2 O (l) 2NaOH (aq) + H 2 (g)  H = kJ/mol  E =  H - P  V At 25 0 C, 1 mole H 2 = 24.5 L at 1 atm P 

Hess’s Law SECTION 5.3. Hess’s Law  The enthalpy change of a physical or chemical process depends only on the initial and final conditions of the process.

Thermochemistry Chapter 8.

Heat of reaction equations Using simultaneous equations.

Today, we will burn a gummy worm containing 2.1 g of sucrose (C 12 H 22 O 11 ). When one mole of sucrose burns in oxygen gas, it produces carbon dioxide.

Topic: Enthalpy and Table I Do Now: packet p.1. Chemistry is the study of matter and energy. So far we have learned about the types of matter and how.

By: Brooke Mason. The formula of cellular respiration is C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + Energy C C C C N C You know that it is a compound because.

Standard Enthalpy of Formation Chapter 5.5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Energy & Rates.

Chapter 17 Thermochemistry 17.4 Calculating Heats of Reaction

Thermochemistry Chapter 6. Thermochemistry is the study of heat change in chemical reactions.

It is impossible to measure enthalpy directly

2 AlBr Cl 2 2 AlCl Br 2 Energy 2 AlBr Cl 2 2 AlCl Br 2  H rxn = Heat content of products – heat content reactants  H rxn < 0.

A simpler problem: How much heat is given off when 1.6 g of CH 4 are burned in an excess of oxygen if  H comb = -802 kJ/mol? Step 1: Write the reaction.

The basis for calculating enthalpies of reaction is known as Hess’s law: the overall enthalpy change in a reaction is equal to the sum of enthalpy changes.

Thermochem Hess’s Law and Enthalpy of Formation Sections 5.6 and 5.7.

General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 6.2 Types of Reactions Chapter 6 Chemical Reactions and Quantities © 2013 Pearson.

3 Enthalpy. Units SI unit = joule 1KJ = 1000J = cal 1st law of Thermodynamics The total energy of the universe is constant i.e energy cannot be.

PRACTICE PROBLEMS Sample Problem Calculate the enthalpy change for the reaction in which hydrogen gas, H 2 (g), is combined with fluorine gas, F 2(g),

Why are chemical reactions important??? Chemical reactions allow living things (cells, animals, people, insects) to grow, develop, reproduce, and adapt.

IIIIII Chapter 16 Hess’s Law. HESS’S LAW n If a series of reactions are added together, the enthalpy change for the net reaction will be the sum of the.

Stoichiometry Chemistry 6.0. The Mathematics of Chemical Reactions: STOICHIOMETRY I. Balanced Chemical Equations A. Provide qualitative and quantitative.

Hess’s Law “In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes.

Hess’ Law: Adding Reaction Enthalpies Some chemical reactions can not be carried out easily in order to determine the amount of heat or energy that can.

Heat and Energy of Ractions Chapter 10 Energy World of Chemistry Zumdahl Last revision Fall 2009.

Energy Thermodynamics.  The ability to do work.  Conserved.  made of heat and work.  a state function.  independent of the path, or how you get from.

CHEMISTRY 122 Calculating the Heats of Reaction. Hess's Law  It is sometimes difficult to measure the change in enthalpy in a reaction because the reaction.

HESS’S LAW what is it ? how is it used ? AS Chemistry.

Enthalpy and Hess’s Law

Energy Thermodynamics

Calorimetry is a very effective way of determining enthalpy changes

Heat and Enthalpy.

Hess’s Law & Standard Enthalpies of Formation

Hess’s Law and Standard Enthalpies of Formation

5.2 Hess’s Law The enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual.

Enthalpy of Reactions -We can describe the energy absorbed as heat at constant pressure by the change in enthalpy (ΔH) -the enthalpy of a reaction is the.

Chapter 10 Energy.

Standard Enthalpy of Formation

5.2 Hess’s Law The enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual.

Thermochemistry Part 2 – enthalpy.

Hess’s Law and Standard Enthalpies of Formation Unit 10 Lesson 4

Hess’s Law and Standard Enthalpies of Formation Unit 10 Lesson 4

Hess’s Law and Standard Enthalpies of Formation

Hess’s Law Germain Henri Hess.

In Summary from Yesterday:

LO: I understand what is meant by Hess’s law.

Changes in Enthalpy During Chemical Reactions

Presentation transcript:

Hess’s Law

Enthalpy is a State Variable ‘State variable’ just means something that doesn’t change depending on the path you take to get from A to B. If you start at the bottom of K2, your overall change in altitude is the same no matter which route you take up.

For Chemistry Purposes This means that if you don't know the  H for a reaction, you can figure it out by using the ones for any other reactions that start and end at the same point: A A B C If you put 100 kJ in to go from A to C and get 40 kJ out from C to B then it must be 60 kJ in from A to B

Example Reaction Let's burn some random compound: 2C 7 H 10 O O 2 → 14CO H 2 O  H = ???? Since we picked this compound arbitrarily, we probably have no idea how much energy we'll get from burning it. BUT! If we can find a set of reactions that, when combined, start with 2C 7 H 10 O 2 and 17 oxygens, and end with 14 carbon dioxides and 10 waters, their individual  H values will add up to our desired value

This Is Not Easy Finding the right reactions can be...problematic. There are some shortcuts, though: If you're interested, or planning on taking AP chem, look up “heat of formation” or “enthalpy of formation”

Summary Enthalpy is a state function (path doesn't matter) You can use a series of reactions to get  H The details can be messy, but there are shortcuts