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

Fill in the Blanks

A 15.75 g piece of iron absorbs 1086.75 joules of heat energy, and its temperature changes from 25 °C to 175 °C. Calculate the specific heat capacity Round to 2 decimal places

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

Fill in the Blanks

A block of brass is heated from 22 °C to 78 °C. The mass of the block is 15 kg. Brass has a specific heat capacity of 376 J/kg°C. How much thermal energy was added to the block of brass?

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

Fill in the Blanks

Calculate the enthalpy of formation of methane

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

​

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CO + ​

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O₂ → ​

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CO₂

Drag these tiles and drop them in the correct blank above

2

1

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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

Fill in the Blanks

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

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