Hess’s Law.

Slides:

Advertisements

Similar presentations

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

Advertisements

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.

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.

 Certain reactions cannot be measured by calorimetry ◦ Ex: slow reactions, complex reactions, hazardous chemicals…  We can substitute in other reactions.

Learning Goals: Will be able to apply Hess’s Law to determine the enthalpy change of chemical equations. Will be able to write target equations from word.

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.

16.1(b) Hess’s Law 1 2 POINT > Recall enthalpies of reaction, formation and combustion POINT > Define Hess’s Law POINT > Use Hess’s law to determine.

Example 17.1 Predicting the Sign of Entropy Change

Thermochemistry Heat and Chemical Change

Hess’s Law 5.3 Energetics.

Hess’ Law energy changes are state functions. The amount of energy depends only on the states of the reactants and products, but not on the intermediate.

Section 4: Calculating Enthalpy Change

Calculating Enthalpy Change

Reaction Mechanism The reaction mechanism is the series of elementary steps by which a chemical reaction occurs. The sum of the elementary steps must give.

Industrial Chemistry Hess’s law.

Calculating Heats of Reaction

Obj Notes 19-3.

How much heat is released when 4

Hess’s Law Start Finish A State Function: Path independent.

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.

Representing Energy Changes

|---> ∆Hvap ---> ---> ---> -->|

Hess’s Law & Standard Enthalpies of Formation

Hess’s Law and Standard Enthalpies of Formation

Hess’s Law H is well known for many reactions, and it is inconvenient to measure H for every reaction in which we are interested. However, we can estimate.

Bill Vining 232 PSCI Chemistry 112 Sections 1-7 Bill Vining 232 PSCI

Unit 5: Thermochemistry

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?

Unit 5: Thermochemistry

10.4 Using Equilibrium Constants

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.

Stoichiometry Calculations involving Enthalpy

Heat in Chemical Reactions and Processes

Le Chatelier’s Principle

Hess' Law Learning Goals:

List of enthalpies for several kinds of reactions.

15.3 Thermochemical Equations

Equilibrium Practice.

Hess’s Law 17.4.

Hess’s Law 17.4.

Not all chemical energy changes can be studied conveniently using simple calorimetry. Methods used to study these reactions are based on the principle.

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

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

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

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

TOPIC 5 ENERGETICS/THERMOCHEMISTRY

Section 11.4 Calculating Heat Changes

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 Hess’s law allows you to determine the energy of chemical reaction without directly measuring it. The enthalpy change of a chemical process.

THERMOCHEMISTRY Thermodynamics

Hess’s Law Start Finish A State Function: Path independent.

Hess’s Law Start Finish A State Function: Path independent.

Chapter 5 Thermochemistry Part B

Either way, you get to the finish.

Hess’s Law Start Finish A State Function: Path independent.

__C3H8(g) + __O2(g) ⇌ __CO2(g) + __H2O(g)

Hess’s Law and Standard Enthalpies of Formation

Hess’s Law Start Finish A State Function: Path independent.

Intro to chemistry Therm o

Warm-up: Hess’s Law and Calorimetry

Mg(s) + ½ O2(g)  MgO(s) H = ? kJ/mol

5.4.6 Hess's Law of Heat Summation [1840]

Changes in Enthalpy During Chemical Reactions

Presentation transcript:

Hess’s Law

Hess’s law states: If a reaction can be written as a series of steps that add up to the overall reaction, the ΔH for the overall reaction will be the sum of the ΔH’s for each of the steps. If we know ΔH for some reactions, we can manipulate them (reverse the order, multiply or divide by a factor) to add up to the desired overall equation.

Why do we need it? A science reference will only include ΔH for some types of reactions (like formation). We can use the reactions included in the reference to find ΔH for reactions not included in the reference. The ΔH for some reactions are very difficult to measure experimentally.

Why do we need it? From the AP course description! Students should understand these Hess’s law ideas: When a reaction is reversed, the sign of the enthalpy of the reaction is changed; when two (or more) reactions are summed to obtain an overall reaction, the enthalpies of reaction are summed to obtain the net enthalpy of reaction.

Use these hints for doing problems with Hess’s Law. The reactions that add up to the overall reaction will not necessarily be given in the correct direction or with the correct number of moles. Use these hints for doing problems with Hess’s Law. 1. Use the overall reaction as a guide. 2. If you reverse the reaction, change the sign of H. 3. If you multiply the reaction by a constant, do the same to H.

calculate H for the reaction Examples 1. Given the following data: S(s) + 3/2 O2(g)  SO3(g) H = -395.2 kJ/molrxn 2 SO2(g) + O2(g)  2 SO3(g) H = -198.2 kJ/molrxn calculate H for the reaction S(s) + O2(g)  SO2(g)

calculate H for the reaction Examples 2. Given the following data: 2 O3(g)  3 O2(g) H = -427 kJ/molrxn O2(g)  2 O(g) H = +495 kJ/molrxn NO(g) + O3(g)  NO2(g) + O2(g) H = -199 kJ/molrxn calculate H for the reaction NO(g) + O(g)  NO2(g)

Examples 3. Given the following data: C2H2(g) + 2H2(g) → C2H6(g) ΔH = -94.5 kJ/molrxn H2O(g) → H2(g) + ½ O2 (g) ΔH = 71.2 kJ/molrxn C2H6(g) + 7/2 O2(g) → 2CO2(g) + 3H2O(g) ΔH = -283 kJ/molrxn calculate H for the reaction 2CO2(g) + H2O(g) → C 2H2(g) + 5/2 O2(g)

Examples 4. Given the following data: P4(s) + 6 Cl2(g)  4 PCl3(g) H = -1225.6 kJ/molrxn P4(s) + 5 O2(g)  P4O10(s) H = -2967.3 kJ/molrxn PCl3(g) + Cl2(g)  PCl5(g) H = -84.2 kJ/molrxn PCl3(g) + ½ O2(g)  Cl3PO(g) H = -285.7 kJ/molrxn calculate H for the reaction P4O10(s) + 6 PCl5(g)  10 Cl3PO(g)