Unit 7 Part 2- Enthalpy & Hess's Law

​Lesson 8.2: Enthalpy

Multiple Choice

Heat always travels in which direction

• From hotter to colder

• From colder to hotter

• In random directions

• The direction with the shortest distance

Multiple Choice

The unstable cluster of atoms that exist during the transition between reactants and products is known as what?

• The Activated Complex

• The Middling Point

• The Transitional Cluster

• The In between Point

Enthalpy

• ​Internal Energy: The total energy contained in a system

  • a change in the internal energy is (heat absorbed + any work on the system​)-( heat released + work done by the system)

• Enthalpy (ΔH):​ the amount of heat in a system at constant pressure

  • The energy absorbed as heat during a chemical reaction is shown with the symbol ΔH

    • ΔH= H_products - H_reactants

Types of Reactions​

• ​Enthalpy of reaction: the quantity of energy transferred as heat during a chemical reaction

  • The difference between the stored energy of the reactants and the products

  • Energy can be written into a chemical equation that includes the state of matter

Exothermic Reactions

• Exothermic Reactions: Reactions where energy is released as a product of the reaction

  • Heat flows out of a system, so ΔH is a negative number

  • Heat/energy is a product of the reaction

  • EXO=exit

  • 2H_{2 (g)} + O_{2 (g)}→2H₂O _(g) + 483.6 kJ

Endothermic Reactions

• Endothermic Reactions: Reactions where energy is absorbed as a reactant

  • Heat/energy is a reactant

  • Heat flows into a system, so ΔH is a positive number

    • EN=In

  • 2H₂O _(g) + 483.6 kJ →​2H_{2 (g)} + O_{2 (g)}

Multiple Choice

Define 'Enthalpy'

• Energy stored in the movement of molecules in a substance

• The opposite of temperature

• The temperature of a molecule

• Energy stored in the chemical bonds in a substance

Dropdown

An endothermic reaction ​

Select answer

heat, so it feels​

Select answer

Multiple Choice

How do you calculate Enthalpy of a reaction?

• ΔH = ΔH_products - ΔH_reactants

• ΔT = q / mC

• ΔG = ΔH -TΔS

• E = mc²

Dropdown

An exothermic reaction ​

Select answer

heat, so it feels ​

Select answer

Bond Enthalpy

• ​Bond Enthalpy: The amount of energy needed to break the bonds of one mole of a substance

  • Energy is absorbed from the surroundings when a bond is broken

  • Energy is released to the surroundings when bond is formed

Total Bond Enthalpy

• During a chemical reaction, bonds are broken and rearranged

  • You can use the average bond enthalpies to estimate the total change in energy for a reaction, or enthalpy of reaction​

Activation Energy

• Activation Energy: the minimum energy that colliding particles must have in order to react

  • Without enough energy, the reaction will not take place and products will not form​

Match

Match the following Vocabulary Words to their definitions

• Exothermic Reactions

• Endothermic Reactions

• Enthalpy (ΔH)

• ​Bond Enthalpy

• ​Internal Energy

Reactions where energy is released as a product of the reaction

Reactions where energy is absorbed as a reactants

the amount of heat in a system at constant pressure

The amount of energy needed to break the bonds of one mole of a substance

The total energy contained in a system

• Reactions where energy is released as a product of the reaction

• Reactions where energy is absorbed as a reactants

• the amount of heat in a system at constant pressure

• The amount of energy needed to break the bonds of one mole of a substance

• The total energy contained in a system

Standard Enthalpy

• Standard Enthalpy: The heat exchange during a reaction when reactants and products are in their standard states

  • Standard State conditions:

    • at 25 °C

    • 1 atm pressure

    • the most stable form of the substance

Standard Enthalpy of Formation

• Standard Enthalpy of formation ΔH_f: The enthalpy change during a reaction that forms a compound from elements that are in standard state

  • ΔH_f for all elements = 0

  • Compounds are found on a chart

Hess's Law

• Russian Chemist Germain Hess

• Hess's Law: the overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process

  • that the change in enthalpy in a chemical reaction is independent of the path the reaction takes​

  • you can add all the enthalpies of formation of products and reactants to find the enthalpy of the reaction

  • ΔH_reaction= ΣΔH_products- ΣΔH_reactants

    • Make sure to keep in mind the coefficients in the balanced equation!

Hess's Law Sample Problem1

Multiple Choice

Is the formation of Methane endothermic or exothermic

• endothermic

• exothermic

Hess's Law Sample Problem 2

• What is the standard enthalpy of reaction for the reaction of CO_(g)​ with O_2(g) to form CO_2(g)?

  • Step 1: write the balanced equation of the reaction

Drag and Drop

​

Placeholder

CO + ​

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O₂ → ​

Placeholder

CO₂

Drag these tiles and drop them in the correct blank above

2

1

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8

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10

Hess's Law Sample Problem 2

• What is the standard enthalpy of reaction for the reaction of CO_(g)​ with O_2(g) to form CO_2(g)?

  • Step 1: write the balanced equation of the reaction

    • 2 CO_(g) +O_2(g) →2 CO_2(g)

  • Step 2: Look up the enthalpies of formation for all the reactants and products

    • 2 CO =2 × ?

    • O₂ =?

    • 2 CO₂= 2×?

Hess's Law Sample Problem 2

• What is the standard enthalpy of reaction for the reaction of CO_(g)​ with O_2(g) to form CO_2(g)?

  • Step 1: write the balanced equation of the reaction

    • 2 CO_(g) +O_2(g) →2 CO_2(g)

  • Step 2: Look up the enthalpies of formation for all the reactants and products

    • 2 CO =2 ×-110.5= -221

    • O₂ = 0.0

    • 2 CO₂= 2× -393.5=787

  • Step 3: Plug your numbers into Hess's law Equation

Multiple Choice

Is the reaction of CO_(g)​ with O_2(g) to form CO_2(g) endothermic or exothermic

• endothermic

• exothermic

Multiple Choice

A reaction is endothermic if ΔH is

• Positive

• Negative

• Zero

Multiple Choice

A reaction is exothermic if ΔH is

• Positive

• Negative

• Zero