8. Thermochemistry

Endothermic & Exothermic Reactions

Endothermic vs. Exothermic

At one time, a common means of forming small quantities of
oxygen gas in the laboratory was to heat KClO3:
2 KClO31s2¡2 KCl1s2 + 3 O21g2 H = -89.4 kJ
(c) The decomposition of KClO3
proceeds spontaneously when it is heated. Do you think
that the reverse reaction, the formation of KClO3 from KCl
and O2, is likely to be feasible under ordinary conditions?
Explain your answer.

Several processes are given in the table and labeled as endo-
thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process
(c) Water freezes into ice in the freezer.

(a) Is the dissociation of fluorine molecules into atomic
fluorine, F2(g)Δ2 F(g) an exothermic or endothermic
process?

Without referring to tables, predict which of the following
has the higher enthalpy in each case: (a) 1 mol I21s2 or
1 mol I21g2 at the same temperature

Several processes are given in the table and labeled as endo-
thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process
(d) Rubbing alcohol evaporates from your skin.

Several processes are given in the table and labeled as endo-
thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process
(b) Methane, the main component of natural gas, is burned to produce a flame on a stovetop.

Imagine a container placed in a tub of water, as depicted in
the accompanying diagram. (a) If the contents of the container
are the system and heat is able to flow through the
container walls, what qualitative changes will occur in the
temperatures of the system and in its surroundings? From the
system’s perspective, is the process exothermic or endothermic?

Determine whether each process is exothermic or endothermic
and indicate the sign of ΔH.
a. natural gas burning on a stove
b. isopropyl alcohol evaporating from skin
c. water condensing from steam
Indicate the sign of ΔH for the following processes.

Consider the decomposition of liquid benzene, C6H61l2, to
gaseous acetylene, C2H21g2:
C6H61l2 ¡ 3 C2H21g2 H = +630 kJ
(c) Which is more likely to be thermodynamically favored,
the forward reaction or the reverse reaction?

Without referring to tables, predict which of the following
has the higher enthalpy in each case: (b) 2 mol of iodine
atoms or 1 mol of I2

Exothermic vs Endothermic Reactions

Imagine a reaction that results in a change in both volume and temperature: <img src="https://lightcat-files.s3.amazonaws.com/problem_images/b527caace238e9f3-1669729469787.jpg" alt="" /> (b) Has there been an enthalpy change? If so, what is the sign of ∆H? Is the reaction exothermic or endothermic?

(b) Is this process endothermic or exothermic?

Endothermic and Exothermic Reactions

Determine whether each process is exothermic or endothermic
and indicate the sign of ΔH.
a. natural gas burning on a stove
b. isopropyl alcohol evaporating from skin
c. water condensing from steam
Determine whether each of the following is exothermic or endothermic.

 Correct, solid water (ice) is a lower energy state then liquid water. 

 Sublimation requires an input of energy. 

 Evaporation requires an input of energy. 

 All of the above are endothermic processes. 

 Only one of these processes releases energy. 

 An exothermic process releases heat into the surroundings. 

 Which of the following is the result of an exothermic process? 

 The mass of octane must be converted to moles. 

 This answer has units of kJ/g of octane. 

 The mass of octane must be converted to moles and then divided into the energy released in the calorimeter. 

 The mass of octane must be converted to moles and the heat capacity yields units of J unless converted to kJ. 

qcal = &#x2212; qrxn

A 0.1250-g sample of octane (C8H18) is burned in a bomb calorimeter with a heat capacity of 5.1 &#215; 102 J/&#x2103;. The temperature inside the calorimeter increases by 11.8&#x2103;. What is the &#916;E (in kJ/mol octane) for the combustion of octane?

Exothermic Energy Diagram: Activation Energy, Transition States and Enthalpy Change - TUTOR HOTLINE

The contents of the closed box in each of the following illustrations
represent a system, and the arrows show the
changes to the system during some process. The lengths
of the arrows represent the relative magnitudes of q and w.
(a) Which of these processes is endothermic? 
<img src="https://lightcat-files.s3.amazonaws.com/problem_images/d7f72530dd83bf5f-1660670401859.jpg" alt="" />

A piece of dry ice (solid CO2) is placed inside a balloon, and the balloon is tied shut. Over time, the carbon dioxide sub- limes, causing the balloon to increase in volume. Give the sign of the enthalpy change and the sign of work for the sublima-
tion of CO2.

What Are Endothermic & Exothermic Reactions | Chemistry | FuseSchool

Endothermic & Exothermic Reactions Example 2

In the chemical process called electron transfer, an electron is
transferred from one atom or molecule to another. (We will
talk about electron transfer extensively in Chapter 20.) A simple
electron transfer reaction is
A1g2 + A1g2¡A+1g2 + A-1g2 
For a representative nonmetal such as chlorine, is this process exothermic?

Endothermic Reactions are energy absorbing, bond breaking reactions.&nbsp;

Without referring to tables, predict which of the following
has the higher enthalpy in each case: (c) 1 mol I21g2 and 1 mol H21g2 at
25 °C or 2 mol HI1g2 at 25 °C

The thermite reaction,
Fe2O3 + Al S Al2O3 + Fe
produces so much heat that the Fe product melts. This reaction
is used industrially to weld metal parts under water,
where a torch cannot be employed. It is also a favorite chemical
demonstration in the lecture hall (on a small scale).
(d) If you performed the reverse reaction— aluminum oxide
plus iron makes iron oxide plus aluminum—would that
reaction have heat as a reactant or a product?

Without doing any calculations, predict the sign of H for
each of the following reactions:
(a) NaCl1s2¡Na+ 1g2 + Cl-1g2

An important reaction for the conversion of natural gas to
other useful hydrocarbons is the conversion of methane to
ethane.
2 CH41g2¡C2H61g2 + H21g2
In practice, this reaction is carried out in the presence of oxygen,
which converts the hydrogen produced into water.
2 CH41g2 + 12
O21g2¡C2H61g2 + H2O1g2
Use Table 8.3 to estimate H for these two reactions. Why is
the conversion of methane to ethane more favorable when
oxygen is used?

Why is the conversion of methane to ethane more favorable when oxygen is used?

Exothermic vs Endothermic Chemical  Reactions

 Correct, both the heat and work terms have negative signs. 

 This is just the pressure-volume work done by the system. 

 This is the pressure-volume work but is it done by the system not on the system. 

 This is just the pressure-volume work done by the system in L-atm. 

 The pressure-volume work must be converted from L-atm to J. 

&#916;E = q + w, where w is pressure-volume work.

If a cylinder equipped with a piston expands against an external pressure of 1.25 atm, increasing the volume of a cylinder by 0.500 L and releasing 25.0 J of heat, what is &#916;E for the process?

Exothermic Reactions are energy releasing, bond forming reactions.

Consider the vaporization of liquid water to steam at a pressure of 1 atm. (a) Is this process endothermic or exothermic?

Several processes are given in the table and labeled as endo-
thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process
(a) Ammonium nitrate dissolves in water, and the temperature of the solution decreases.

Which of the following reactions in the stratosphere cause an increase in temperature there? (a) O(g) + O2(g) → O3+(g) (b) O3*(g) + M(g) → O3(g) + M*(g) (c) O2(g) + hv → 2 O(g) (d) O(g) + N2(g) → NO(g) + N(g) (e) All of the above

Tricky Question: Exothermic or Endothermic?

Consider the decomposition of liquid benzene, C6H61l2, to 
gaseous acetylene, C2H21g2: 
C6H61l2 ¡ 3 C2H21g2 H = +630 kJ 
(d) If C6H61g2 were consumed instead of C6H61l2, would you 
expect the magnitude of H to increase, decrease, or 
stay the same? Explain.

Endothermic & Exothermic Reactions Example 1

GCSE Chemistry Exothermic and endothermic reactions (OCR 9-1)

 snow melts on a mountaintop (snow is the system) 

 water vapor condenses on a cold surface (water vapor is the system) 

 a rock tumbles from a mountain (the rock is the system) 

 In both of these examples, the system undergoes a heat loss. 

 An increase in the heat of a system is denoted by a positive sign.  When a system has work done on it, work is positive. 

 Which of the following exchanges of energy has a positive sign for heat (relative to the system)? 

For the following processes, calculate the change in internal
energy of the system and determine whether the process is
endothermic or exothermic: (a) A balloon is cooled by removing
0.655 kJ of heat. It shrinks on cooling, and the atmosphere
does 382 J of work on the balloon.

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1. Intro to General Chemistry

2. Atoms & Elements

3. Chemical Reactions

4. BONUS: Lab Techniques and Procedures

5. BONUS: Mathematical Operations and Functions

6. Chemical Quantities & Aqueous Reactions

7. Gases

9. Quantum Mechanics

10. Periodic Properties of the Elements

11. Bonding & Molecular Structure

12. Molecular Shapes & Valence Bond Theory

13. Liquids, Solids & Intermolecular Forces

14. Solutions

15. Chemical Kinetics

16. Chemical Equilibrium

17. Acid and Base Equilibrium

18. Aqueous Equilibrium

19. Chemical Thermodynamics

20. Electrochemistry

21. Nuclear Chemistry

22. Organic Chemistry

23. Chemistry of the Nonmetals

24. Transition Metals and Coordination Compounds

[BOOK CHEM] Clutch Version

Hess's Law

Hess's Law involves the use of partial reactions in the determination of the overall enthalpy of reaction.

Nature of Energy

Thermochemistry is the study of matter and energy associated with chemical reactions or physical changes.&nbsp;

Endothermic and Exothermic Reactions involve the absorbing and releasing of thermal energy respectively.&nbsp;&nbsp;

Heat Capacity

Heat Capacity is the amount of heat required to change the temperature of a substance.&nbsp;

Thermochemical Equations

Thermochemical Equations&nbsp;involve a balanced chemical equation with a given enthalpy value.

Internal Energy

The Internal Energy of a system is the total energy from all forms of kinetic and potential energy.&nbsp;

Kinetic & Potential Energy

Mechanical Energy is the energy associated with an object's velocity (Kinetic Energy) and its position (Potential Energy).&nbsp;

Formation Equations

Formation Equations involve the standard states of elements combining to form 1 mole of product.

Enthalpy of Formation

The Enthalpy of Formation for an element is a key component in determining the enthalpy of reaction.&nbsp;

Thermal Equilibrium

Thermal Equilibrium involves two substances that are in physical contact reaching the same final temperature over time.&nbsp;

Constant-Pressure Calorimetry

Constant-Pressure Calorimetry uses a coffee cup calorimeter to determine the heat transfers occurring in a liquid solution.

Constant-Volume Calorimetry

Constant-Volume Calorimetry uses a bomb calorimeter and a combustion reaction to determine its enthalpy of reaction.&nbsp;

First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created nor destroyed, but instead is transferred.&nbsp;