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Chapter 5: Problem 101

The following questions may use concepts from this and previous chapters. Without doing calculations, decide whether each of the following is exo-or endothermic. (a) the combustion of natural gas (b) the decomposition of glucose, \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6},\) to carbon and water

Short Answer

Expert verified
(a) Exothermic; (b) Endothermic.

Step by step solution

01

Understanding Exothermic vs. Endothermic

Exothermic reactions are those that release energy, usually in the form of heat, to the surroundings, making the environment warmer. Endothermic reactions, on the other hand, absorb energy from the surroundings, causing the environment to become colder.
02

Examining Combustion of Natural Gas

Combustion reactions, such as the burning of natural gas, involve the reaction of a substance with oxygen to release energy. The energy released manifests as heat and light, making combustion reactions exothermic.
03

Evaluating the Decomposition of Glucose

The decomposition of glucose into simpler substances like carbon and water involves breaking down chemical bonds. This process requires input of energy from the surroundings to overcome bond energies, making it an endothermic reaction.

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Key Concepts

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

Exothermic Reactions
Exothermic reactions are chemical reactions that release energy into the surroundings. This energy is often released as heat, but can also appear as light or sound.

When a reaction is exothermic, you may notice the temperature of the surroundings increase. This is because the reaction is giving off energy that heats up the nearby environment. A simple way to remember this is to think of a campfire: when the wood burns, it releases heat that warms you up.

  • Key Examples:
    • Combustion of fuels (like natural gas and wood)
    • Respiration in living organisms
  • Characteristics of Exothermic Reactions:
    • Energy is released to the surroundings
    • The temperature of the surroundings increases
    • The enthalpy change ( \( \Delta H \)) is negative
Understanding these characteristics helps in identifying and predicting exothermic reactions in everyday chemical processes.
Endothermic Reactions
Endothermic reactions are the opposite of exothermic ones. In an endothermic reaction, energy is absorbed from the surroundings. This absorption often results in a noticeable decrease in temperature of the surroundings, making the environment feel colder.

Imagine the endothermic process as taking a cool sip of a drink that absorbs heat from your hand, leaving your hand feeling cold.

  • Examples of Endothermic Reactions:
    • Photosynthesis - plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen
    • Melting Ice - when ice melts, it takes in heat from its surroundings
  • Characteristics of Endothermic Reactions:
    • Energy is absorbed from the surroundings
    • The temperature of the surroundings decreases
    • The enthalpy change ( \( \Delta H \)) is positive
Knowing these traits can help in identifying when a reaction needs external energy to proceed.
Combustion Reactions
Combustion reactions are a specific type of exothermic reaction that involve a substance reacting with oxygen to produce energy. The energy that is released is usually in the form of heat and light.

These reactions are essential in everyday life, as they provide the energy necessary for heating homes, fueling vehicles, and even cooking food.

  • Key Characteristics of Combustion Reactions:
    • Require oxygen as a reactant
    • Produce carbon dioxide and water as common products
    • Release significant amounts of energy (both heat and light)
  • Examples of Combustion Reactions:
    • Burning of candles
    • Combustion of gasoline in car engines
Understanding combustion is crucial for both practical applications and theoretical chemistry, as it highlights how energy is efficiently harnessed from chemical reactions.

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Most popular questions from this chapter

The enthalpy changes of the following reactions can be measured: \(\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{g})+3 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{CO}_{2}(\mathrm{g})+2 \mathrm{H}_{2} \mathrm{O}(\ell)\) $$ \Delta_{i} H^{\circ}=-1411.1 \mathrm{kJ} / \mathrm{mol}-\mathrm{rxn} $$ \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\ell)+3 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{CO}_{2}(\mathrm{g})+3 \mathrm{H}_{2} \mathrm{O}(\ell)\) \(\Delta, H^{\circ}=-1367.5 \mathrm{kJ} / \mathrm{mol}-\mathrm{rxn}\) (a) Use these values and Hess's law to determine the enthalpy change for the reaction \(\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\ell)\) (b) Draw an energy level diagram that shows the relationship between the energy quantities involved in this problem.

Suppose you burned 0.300 g of \(\mathrm{C}(\mathrm{s})\) in an excess of \(\mathbf{O}_{2}(g)\) in a constant-volume calorimeter to give \(\mathrm{CO}_{2}(\mathrm{g})\) $$ \mathrm{C}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) $$ The temperature of the calorimeter, which contained 775 g of water, increased from \(25.00^{\circ} \mathrm{C}\) to \(27.38^{\circ} \mathrm{C} .\) The heat capacity of the bomb is \(893 \mathrm{J} / \mathrm{K} .\) Calculate \(\Delta U\) per mole of carbon.

After absorbing \(1.850 \mathrm{kJ}\) of energy as heat, the temperature of a 0.500 -kg block of copper is \(37^{\circ} \mathrm{C} .\) What was its initial temperature?

You want to heat the air in your house with natural gas \(\left(\mathrm{CH}_{4}\right) .\) Assume your house has \(275 \mathrm{m}^{2}\) (about \(2800 \mathrm{ft}^{2}\) ) of floor area and that the ceilings are \(2.50 \mathrm{m}\) from the floors. The air in the house has a molar heat capacity of \(29.1 \mathrm{J} / \mathrm{mol} \cdot \mathrm{K}\). (The number of moles of air in the house can be found by assuming that the average molar mass of air is \(28.9 \mathrm{g} / \mathrm{mol}\) and that the density of air at these temperatures is \(1.22 \mathrm{g} / \mathrm{L} .\) ) What mass of methane do you have to burn to heat the air from \(15.0^{\circ} \mathrm{C}\) to \(22.0^{\circ} \mathrm{C} ?\)

Identify whether the following processes are exothermic or endothermic. Is the sign on \(q_{\text {sys }}\) positive or negative? (a) combustion of methane (b) melting of ice (c) raising the temperature of water from \(25^{\circ} \mathrm{C}\) to \(100^{\circ} \mathrm{C}\) (d) heating \(\mathrm{CaCO}_{3}(\mathrm{s})\) to form \(\mathrm{CaO}(\mathrm{s})\) and \(\mathrm{CO}_{2}(\mathrm{g})\)
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