Hess’s Law. Some enthalpy changes can not be measured directly in the lab. Hess’s Law is used to calculate these enthalpy changes Use the information.

Slides:

Advertisements

Similar presentations

When do you use this? You use Keq to find the concentration of a reversible reaction at equilibrium. You use Q to find the concentration of a reversible.

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.

Hess’s law calculations. 2C(s) + 3H 2 (g) + ½O 2 (g) → C 2 H 5 OH(l) C(s) + O 2 (g) → CO 2 (g) ∆H = –393 kJ mol –1 H 2 (g) + ½O 2 (g) → H 2 O(l) ∆H =

Hess’s Law EQ: Why is Hess’s Law a useful tool in solving for ∆Hrxn?

Lecture 4: Hess’s Law Reading: Zumdahl 9.5 Outline: Definition of Hess’ Law Using Hess’ Law (examples)

CHEMISTRY 161 Chapter 6

© 2006 Brooks/Cole - Thomson Some Thermodynamic Terms Notice that the energy change in moving from the top to the bottom is independent of pathway but.

Using Standard Molar Enthalpies of Formation SCH4U0.

STOICHIOMETRY CALCULATIONS COACH COX. MOLE TO MOLE CONVERSIONS Converting from moles of one substance in a chemical reaction to moles of another substance.

Chapter 6 Thermochemistry

Heat Transfer and Specific Heat Heat Transfer and Specific Heat Energy Changes in Chemical Reactions Energy Changes in Chemical Reactions Calculating ∆H.

Chapters 4, and 5.  Solution:  Electrolyte:  Nonelectrolyte:

Enthalpy Change and Chemical Reactions

Industrial Chemistry Hess’s law. Index Hess’s Law and its experimental verification Hess’s Law calculations, 4 examples. Hess’s Law.

Slide 1 of Indirect Determination of  H: Hess’s Law  H is an extensive property. – Enthalpy change is directly proportional to the amount of substance.

Hess’s Law and Standard Enthalpies of Formation

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

Chapters 4, and 5.  Solution:  Electrolyte:  Nonelectrolyte:

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.

enthalpy of formation (DHf): the enthalpy change

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.

Hess’s Law Conservation of Energy

Energy: Standard Enthalpy of Formation and Reaction – Direct & Hess’s Law Standard Enthalpy of Formation and Reaction.

Chapter 17. Thermochemisty Thermochemistry –Is the study of energy changes that occur during chemical reactions and changes in state Two types of energy.

1 Hess suggested that the sum of the enthalpies (ΔH) of the steps of a reaction will equal the enthalpy of the overall reaction. Hess’s Law.

Heat of Formation If you understood Hess’ Law, you will have NO problem here.

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.

Balancing Equations H 2 + Cl 2 → HCl. Balancing Equations KI + Br 2 → KBr + I 2.

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.

CHEMICAL REACTIONS. CHEMICAL CHANGES When bonds break and new bonds form. Chemical changes are also called chemical REACTIONS.

8.4 Thermochemical Equations Pages Thermochemical Equations A thermochemical equation is a balanced chemical equation that includes the physical.

Looking at your calorimetry results…

© 2009, Prentice-Hall, Inc. Enthalpies of Formation An enthalpy of formation,  H f, is defined as the enthalpy change for the reaction in which a compound.

Entry Task Monday, May 23 How many grams of HCl will you need to produce 514 kJ of energy using the following equation? CaCO 3 (s) + 2 HCl (aq) → CaCl.

Chapter 5 – Thermochemistry. 2 Energy - capacity to do work. Heat is the transfer of thermal energy between two bodies that are at different temperatures.

1.4 Energetics Practical 1.6 – Finding an enthalpy change that cannot be measured directly e. recall Hess’s Law and apply it to calculating enthalpy.

Hess’s Law 5.3 Energetics.

Section 4: Calculating Enthalpy Change

Industrial Chemistry Hess’s law.

Enthalpy of Formation DHrxn has been tabulated for many different reactions. Often tabulated according to the type of chemical reaction or process DHvap.

Chapter 17: Thermochemistry

Calculating ΔHfθ from combustion data

As always… Lon-Capa assignments Lecture videos Textbook Read

Warm-up (11/9) WS Day 3 out on desk, please

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 place in one.

Hess’s Law & Standard Enthalpies of Formation

5.1 Enthalpy of Formation IB Chemistry.

Hess’s Law and Standard Enthalpies of Formation

Unit 5: Thermochemistry

Hess’s Law Germain Henri Hess.

Hess’s Law Determine the enthalpy change of a reaction that is

Hess’s Law Determine the enthalpy change of a reaction that is

5.3 Hess’s Law How do we determine the enthalpy change of a reaction that is the sum of two or three reactions with known enthalpy changes?

Clicker #1 Which of the following shows the correctly balanced equation for this reaction: H2O2 → H2O + O2 A) 3 H2O2 → 3 H2O + O2 B) H2O2 → 2 H2O + O2.

Stoichiometry Calculations involving Enthalpy

15.2 Heat Calorimetry.

Objectives - understand that chemical reactions involve the making and breaking of bonds and the concept of bond enthalpy - be able to determine bond.

AP Chem Get Heat HW stamped off Today: Enthalpy Cont., Hess’ Law

Hess’s Law Determine the enthalpy change of a reaction that is

Section 11.4 Calculating Heat Changes

Hess’s Law Germain Henri Hess.

1.2.5 Hess’s Law- the equation

Thermochemistry Lesson # 4: Hess’s Law.

1.2.5 Hess’s Law- the equation

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

Presentation transcript:

Hess’s Law

Some enthalpy changes can not be measured directly in the lab. Hess’s Law is used to calculate these enthalpy changes Use the information provided to calculate ∆H f CH 4

C (gr) + 2H 2(g) CH 4(g) CO 2(g) + 2H 2 O (g) + 2O 2(g)  H c C x  H c H 2 2 x -286 = -572  H c CH ∆H f CH 4

∆H f CH 4 = -76kJmol -1

C (gr) + 2H 2(g) CH 4(g) CO 2(g) + 2H 2 O (g)  H c C x  H c H 2 2 x -286 = -572  H c CH ∆H f CH 4 ∆H f CH 4 =  H c C + 2 x  H c H 2 -  H c CH 4 ∆H f CH 4 = (2 x -286) - (-890) = -76kJmol -1

Hess’s Law Hess’s Law states In a reaction the total enthalpy change is independent of the route

Hess’s Law Diagrams Write unknown equation across the top Look at info provided & complete triangle Balance equations (ignore O 2 ) Think about definitions of ∆H f & ∆H c Make sure arrows point in the correct directions Do you need to multiply any values? Write figures on arrows if this helps

Hess’s Law Diagrams Draw overall direction arrows Write out in full ∆H = expression without n o. In same direction as arrow + Against direction of arrow - Put numbers into ∆H = expression, keep ( ) Be very careful with signs Calculate value – check over !!!

Hess’s Law Diagrams Draw a Hess’s Law diagram to calculate the  H r of the coffee can reaction CaO (s) + H 2 O (l) → Ca(OH) 2 (s)  H f CaO (S) = -635  H f H 2 O (l) = -286  H f Ca(OH) 2(S) = -986

CaO (s) + H 2 O (l) Ca(OH) 2(s) Ca (s) + O 2(g) + H 2(g)  H f CaO -635  H f H 2 O -286  H f Ca(OH) ∆H r ∆H r = -  H f CaO (s) -  H f H 2 O (l) +  H f Ca(OH) 2 (s) ∆H f CH 4 = -(-635) – (-286) + (-986) = -65kJmol -1

Hess’s Law Hess’s Law states In a reaction the total enthalpy change is independent of the route Draw a Hess’s Law diagram to calculate the  H r of the decomposition of NaHCO 3(s) 2NaHCO 3(s) → Na 2 CO 3(s) + CO 2(g) + H 2 O (l)

2NaHCO 3(s) Na 2 CO 3(s) + H 2 O (l) + CO 2(g) 2Na (s) + 3O 2(g) + H 2(g) + 2C (gr) 2 x  H f NaHCO 3 2 x -948 ∆H r ∆H r = - 2 x  H f NaHCO 3(s) +  H f Na 2 CO 3 +  H f H 2 O (l) +  H f CO 2 (g) ∆H r = -(2 x -948) + (-1131) + (-286) + (-394) = +85kJmol -1  H f CO  H f Na 2 CO  H f H 2 O -286

Hess’s Law Draw a Hess’s Law diagram to calculate the following :  H r of the formation of propane C 3 H 8(g) 3C (gr) + 4H 2(g) → C 3 H 8(g)  H r of photosynthesis 6CO 2(g) + 6H 2 O → C 6 H 12 O 6(aq) + 6O 2(l)

3C (gr) + 4H 2(g) C 3 H 8(g) 3CO 2(g) + 4H 2 O (g) + 3O 2(g) 3 x  H c C 3 x -394 = x  H c H 2 4 x -286 =  H c C 3 H ∆H f C 3 H 8 ∆H f C 3 H 8 = 3x  H c C + 4 x  H c H 2 -  H c C 3 H 8 ∆H f C 3 H 8 = (3 x -394) + (4 x -286) - (-2220) = -106kJmol -1

6CO 2(g) + 6H 2 O (l) C 6 H 12 O 6 (s) + 6O 2 (g) 6C (gr) + 9O 2(g) + 6H 2(g) 6 x  H f CO 2 6 x -394 ∆H r ∆H r = - 6 x  H f CO 2(g) – 6 x  H f H 2 O (l) +  H f C 6 H 12 O 6 ∆H r = -(6 x -394) - (6 x -286) + (-1273) = +2807kJmol -1  H f C 6 H 12 O x  H f H 2 O 6 x -286