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Chapter 6: Problem 25

Draw an enthalpy diagram for a general endothermic reaction; label the axis, reactants, products, and \(\Delta H\) with its sign.

Short Answer

Expert verified
The enthalpy diagram for an endothermic reaction shows reactants at a lower enthalpy level and products at a higher enthalpy level with \(\Delta H\) being positive.

Step by step solution

01

- Draw Axes

Begin by drawing a vertical axis and a horizontal axis. The vertical axis represents the enthalpy (H), and the horizontal axis represents the reaction progress.
02

- Mark Reactants and Products

Label a point near the beginning of the horizontal axis as 'Reactants' and a point further along the horizontal axis as 'Products'.
03

- Draw Enthalpy Levels

Draw a horizontal line near the bottom of the vertical axis to represent the enthalpy of the reactants. Next, draw a higher horizontal line to represent the enthalpy of the products.
04

- Indicate the Reaction Path

Connect the reactants line to the products line with a smooth curve, indicating the pathway of the reaction. Since it's endothermic, the curve should go upwards.
05

- Label \(\Delta H\)

Draw a vertical arrow from the reactants' enthalpy level to the products' enthalpy level, pointing upwards. Label this arrow as \(\Delta H\). Since the reaction is endothermic, add a positive sign (\(\Delta H > 0\)).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Endothermic Reaction
An endothermic reaction is a process in which the system absorbs energy from its surroundings in the form of heat. As a result, the temperature of the surroundings decreases.
This type of reaction is common in various chemical and physical processes. To visualize an endothermic reaction in an enthalpy diagram, the enthalpy of the products is higher than that of the reactants.
Key characteristics of an endothermic reaction include:
  • Absorption of heat
  • A rise in enthalpy
  • A positive \(\Delta H\) (change in enthalpy)
Examples of endothermic reactions include photosynthesis, melting of ice, and the evaporation of water. Understanding these reactions is crucial in fields like chemistry and thermodynamics.
Reaction Progress
Reaction progress represents the extent to which a reaction has occurred. In an enthalpy diagram, this is shown on the horizontal axis. As the reaction progresses from reactants to products, you can observe the changes in enthalpy.
The horizontal axis, therefore, is crucial for showing how the reactants transform into products over time.
Typically, a reaction progresses through a specific pathway which includes:
  • Starting Point - Reactants: Initial chemicals involved in the reaction.
  • Reaction Pathway: The transition process as reactants convert to products.
  • End Point - Products: Substances formed as a result of the reaction.
This framework helps in understanding how energy is taken in or released during the reaction process.
Enthalpy Change
Enthalpy change, denoted as \(\Delta H\), is the difference in enthalpy between products and reactants in a chemical reaction. In an enthalpy diagram of an endothermic reaction, this is shown as a vertical arrow pointing upwards.
This indicates that energy is absorbed and the system gains enthalpy.
This value is crucial for understanding the energy dynamics of a reaction, encapsulating whether it is endothermic or exothermic.
The main points to remember about enthalpy change include:
  • Positive \(\Delta H\) for endothermic reactions (energy absorbed).
  • Negative \(\Delta H\) for exothermic reactions (energy released).
  • \

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Most popular questions from this chapter

Kerosene, a common space-heater fuel, is a mixture of hydrocarbons whose "average" formula is \(\mathrm{C}_{12} \mathrm{H}_{26}\) (a) Write a balanced equation, using the simplest whole-number coefficients, for the complete combustion of kerosene to gases. (b) If \(\Delta H_{\mathrm{rn}}^{\circ}=-1.50 \times 10^{4} \mathrm{~kJ}\) for the combustion equation as written in part (a), determine \(\Delta H_{\mathrm{f}}^{\circ}\) of kerosene. (c) Calculate the heat released by combustion of 0.50 gal of kerosene \((d\) of kerosene \(=0.749 \mathrm{~g} / \mathrm{mL})\) (d) How many gallons of kerosene must be burned for a kerosene furnace to produce \(1250 .\) Btu \((1 \mathrm{Btu}=1.055 \mathrm{~kJ}) ?\)

At a given set of conditions, \(241.8 \mathrm{~kJ}\) of heat is released when \(1 \mathrm{~mol}\) of \(\mathrm{H}_{2} \mathrm{O}(g)\) forms from its elements. Under the same conditions, \(285.8 \mathrm{~kJ}\) is released when \(1 \mathrm{~mol}\) of \(\mathrm{H}_{2} \mathrm{O}(l)\) forms from its elements. Find \(\Delta H\) for the vaporization of water at these conditions.

Most ethylene \(\left(\mathrm{C}_{2} \mathrm{H}_{4}\right),\) the starting material for producing polyethylene, comes from petroleum processing. It also occurs naturally as a fruit-ripening hormone and as a component of natural gas. (a) The heat transferred during combustion of \(\mathrm{C}_{2} \mathrm{H}_{4}\) is \(-1411 \mathrm{~kJ} / \mathrm{mol} .\) Write a balanced thermochemical equation. (b) How many grams of \(\mathrm{C}_{2} \mathrm{H}_{4}\) must burn to give \(70.0 \mathrm{~kJ}\) of heat?

Diamond and graphite are two crystalline forms of carbon. At 1 atm and \(25^{\circ} \mathrm{C},\) diamond changes to graphite so slowly that the enthalpy change of the process must be obtained indirectly. Using equations from the list below, determine \(\Delta H\) for C(diamond) \(\longrightarrow\) C (graphite) (1) C(diamond) \(+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) \quad \Delta H=-395.4 \mathrm{~kJ}\) (2) \(2 \mathrm{CO}_{2}(g) \longrightarrow 2 \mathrm{CO}(g)+\mathrm{O}_{2}(g)\) \(\Delta H=566.0 \mathrm{~kJ}\) (3) C(graphite) \(+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) \quad \Delta H=-393.5 \mathrm{~kJ}\) (4) \(2 \mathrm{CO}(g) \longrightarrow \mathrm{C}\) ( graphite) \(+\mathrm{CO}_{2}(g) \quad \Delta H=-172.5 \mathrm{~kJ}\)

At constant temperature, a sample of helium gas expands from \(922 \mathrm{~mL}\) to \(1.14 \mathrm{~L}\) against a pressure of \(2.33 \mathrm{~atm} .\) Find \(w\) (in \(\mathrm{J}\) ) done by the gas \((101.3 \mathrm{~J}=1 \mathrm{~atm} \cdot \mathrm{L})\)
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