Hess’s Law.

Slides:

Advertisements

Similar presentations

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.

Advertisements

Thermochemistry Exothermic reactions release heat to the surroundings. Fe 2 O Al  2 Fe + Al 2 O kJ Potassium Permanganate Reaction Demo.

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)

Lecture 3: Hess’ Law Reading: Zumdahl 9.5 Outline –Definition of Hess’ Law –Using Hess’ Law (examples)

Enthalpy EQ: How do you predict the sign of delta H?

Using Standard Molar Enthalpies of Formation SCH4U0.

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

Standard Enthalpy Changes =  H o P = 1 bar (0.997 atm) T = 298K, unless otherwise specified n = 1 mole for key compound.

Chapter 6B Notes Thermochemistry West Valley High School AP Chemistry Mr. Mata.

{ Exothermic & Endothermic REACTIONS Energy in Reactions.

Enthalpy and Hess’s Law. From the homework, you may have realized that  H can have a negative number. It relates to the fact that energy as heat has.

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

Chapter : Chemical Reactions That Involve Heat Suggested Reading: Pages

Reaction Rate How Fast Does the Reaction Go?. Collision Theory l In order to react molecules and atoms must touch each other. l They must hit each other.

Bomb Calorimetry (Constant Volume Calorimetry)

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.

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.

Describing Chemical Reactions The process by which the atoms of one or more substances are rearranged to form different substances is called a chemical.

Do Now 2NaHCO kJ  Na 2 CO 3 + H 2 O + CO 2 Is this an endothermic or exothermic reaction? Calculate the amount of heat transferred when 36 g of.

It is impossible to measure enthalpy directly

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.

Energy: Standard Enthalpy of Formation and Reaction – Direct & Hess’s Law Enthalpy Enthalpy (H): heat flow for a chemical reaction. q constant P.

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.

Hess’ Law. Many reactions can occur by many alternative routes. Hess' Law states: The enthalpy change for a reaction depends only on the energy of the.

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

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 1 Chapter 9 Chemical Quantities in Reactions 9.5 Energy in Chemical Reactions Cold packs use an.

I. Reaction Energy Section 17-1 and 17-2 Ch. 17 – Reaction Energy and Reaction Kinetics.

Topic 5.1 and 5.2 Hess’s Law and Bond Enthalpies.

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.

REPRESENTING ENTHALPY CHANGES. 1. ΔH Notation 2. Thermochemical Equation 3. Potential Energy Diagram.

Thermodynamics. Every physical or chemical change is accompanied by energy change Thermodynamics = branch of chemistry that studies energy changes –Specifically:

Physical vs. Chemical Change How are they different?

5.4 Bond enthalpies Define the term average bond enthalpy Explain, in terms of average and enthalpies, why some reactions are exothermic and.

Topic 5.3 and 5.4 Hess’s Law and Bond Enthalpies.

Ch. 16 Reaction Energy. Thermochemistry  Thermochemistry: the study of the transfers of energy as heat that accompany chemical reactions and physical.

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.

Ch 5: Hess ’ s Law. Hess ’ s Law states.. “ the enthalpy change for a chem rxn is the same whether the rxn takes place in one step or several steps ”

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.

Hess’ Law Practice Problems 1)-79.6 kJ 2)-155 kJ 3)17.4 kJ 4)-3.70 kJ 5)-142 kJ 6) 88.1 kJ 7) kJ 8) kJ or kJ 9) -256 kJ, exo 10)

Unit: Thermochemistry Chapter 16 in text

Thermochemistry Heat and Chemical Change

Section 4: Calculating Enthalpy Change

Chapter 17: Thermochemistry

How much heat is released when 4

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.

Chemical Reactions Physical Science.

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.

Unit 5: Thermochemistry

Hess’s Law Germain Henri Hess.

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

List of enthalpies for several kinds of reactions.

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

Balancing Equations and The Law of Conservation of Mass

Endothermic & Exothermic Reactions

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

Hess’s Law Germain Henri Hess.

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 Lesson # 4: Hess’s Law.

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

Hess’s Law and Standard Enthalpies of Formation

Hess’s Law Germain Henri Hess.

Presentation transcript:

Hess’s Law

After today you will be able to… Add, multiply, divide, or reverse chemical equations Calculate the enthalpy changes for an overall reaction using Hess’s Law

Enthalpy Enthalpy: The total energy or total heat of a system. Its symbol is H, and ∆H shows the change in energy of a reaction.

Hess’s Law Hess’s Law: If you add two or more chemical equations to get an overall equation, then you can also add the heat changes (ΔHs) to get the overall heat change.

How to add chemical equations: If two identical substances are on opposite sides of the arrow, they will cancel. If two identical substances are on same side of the arrow, add the coefficients together. Keep substances on the same side of the arrow in the final equation.

Helpful hint: Align arrows underneath each other! Example: Add the following equations. C + O2  CO2 CO2  CO + ½ O2 Helpful hint: Align arrows underneath each other! ½ C + ½O2  CO If the same chemical appears on both sides of the equation, nothing changes, that’s why it cancels.

Example: Add the following equations Example: Add the following equations. 2 Cu + O2  2 CuO 4 Cu + O2  2 Cu2O 6Cu + 2O2  2CuO + 2Cu2O In this example, Cu and O2 are both reactants, and they are not recreated on the products side in the same form. So instead of cancelling, these chemicals are added together.

How to reverse chemical equations: To reverse an equation, the products and reactants change sides. The sign of the ΔH also reverses. If the forward reaction is exothermic, the reverse reaction is endothermic (ex: - to +).

Example: Reverse this reaction. 2 P + 3 Cl2  2 PCl3 H = -574 kJ Switch sides! Reverse the sign! 2 PCl3  2 P + 3 Cl2 H = +574 kJ The forward reaction was exothermic, so the reverse reaction is endothermic. The value of H is still the same.

How to multiply/divide a chemical equation: Multiply or divide all coefficients by the same number. Multiply or divide the H by the same number. If more or less chemicals are reacting, H will change by the same factor.

Example: Multiply this equation by 2. Ca + 2 C  CaC2 H = -62.8kJ [ ] [ ] x2 x2 2 Ca + 4 C  2 CaC2 H = -125.6 kJ If there are twice as many reactants and products, twice as much heat is going to be released (because this particular reaction is exothermic).

These rearrangements can be combined… Example: Reverse and multiply by 3. 2Fe + 3/2 O2  Fe2O3 H = -824.2kJ [ [ ] ] x3 x3 Reverse the sign! Switch sides! 3 Fe2O3  6 Fe + 9/2 O2 H = +2472.6 kJ The reaction is reversed, so it goes from exothermic to endothermic. The reaction is also increased by a factor of three times as many chemicals, so the heat involved is increased by a factor of three.

Now it’s time to apply Hess’s law to some examples!

Example: Find the H for C2H4 + H2  C2H6 using the following information: 2 C + 2 H2  C2H4 H = +52kJ 2 C + 3 H2  C2H6 H = -85kJ Helpful hint: Pick one substance that appears only in the overall equation! (Reverse) (Same) C2H4  2 C + 2 H2 ΔH= -52kJ 2 C + 3 H2  C2H6 ΔH= -85kJ C2H4 + H2  C2H6 ΔH= -137kJ

3 H2 + 3 Cl2  6HCl ΔH= -552kJ 2 NH3  N2 + 3 H2 ΔH= +92kJ Example: Find the H for 2 NH3 + 3 Cl2  6 HCl + N2 using the following information: ½ H2 + ½ Cl2  HCl H = -92kJ ½ N2 + 3/2 H2  NH3 H = -46kJ (x 6) (Rev, x2) 3 H2 + 3 Cl2  6HCl ΔH= -552kJ 2 NH3  N2 + 3 H2 ΔH= +92kJ 3 Cl2 + 2 NH3  6HCl + N2 ΔH= -460kJ

Questions?