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Chapter 14: Problem 70

Why must we use a calorimeter to measure the heat evolved during a reaction? Why can't we simply use a thermometer?

Short Answer

Expert verified
A calorimeter is required because it accounts for both temperature change and material properties, providing a complete measure of heat evolution, unlike a thermometer which only measures temperature.

Step by step solution

01

Understanding Heat Evolution

In a chemical reaction, heat can either be absorbed or released, known as endothermic or exothermic reactions, respectively. The amount of heat change is essential for understanding the reaction's thermodynamics.
02

The Role of a Thermometer

A thermometer is used to measure temperature, not heat. It provides a measurement of the average kinetic energy of particles but doesn't account for the total heat emitted or absorbed by the system.
03

Understanding Heat Capacity

Heat (q) is calculated using the formula: \[ q = mc\Delta T \]where \( m \) is the mass, \( c \) is the specific heat capacity, and \( \Delta T \) is the change in temperature. The meter has input on only \( \Delta T \) - the change in temperature - and not on the material or the mass involved.
04

Purpose of a Calorimeter

A calorimeter measures the total heat exchange during chemical reactions by isolating the system from external temperature changes. It accounts for the mass of reactants and specific heat, allowing an accurate calculation of heat evolved or absorbed.
05

Why Calorimeter over Thermometer

A calorimeter is necessary to measure heat accurately because it considers both the temperature change and other factors like mass and specific heat capacity, unlike a thermometer which only measures temperature.

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Key Concepts

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

Thermodynamics
In the fascinating world of thermodynamics, we study the relationship between heat, work, and energy in chemical processes. Thermodynamics helps us understand how energy is transferred in reactions. It explains why some reactions release energy while others absorb it. When we talk about reactions in a calorimetry context, we are really diving into the thermodynamic principles.
  • Thermodynamics focuses on energy changes. Specifically, it helps identify whether a reaction is endothermic (absorbing heat) or exothermic (releasing heat).
  • Understanding thermodynamics is crucial for predicting how a reaction will behave and determining the energy requirements or results.
The study of thermodynamics provides an essential framework for analyzing reactions in calorimetry, assisting us to see beyond just temperature changes.
Heat Capacity
Heat capacity is a critical concept in understanding how substances interact with heat. It tells us how much heat is needed to change the temperature of a substance by a certain amount. This is crucial in calorimetry where we need to measure heat evolved or absorbed accurately.
  • Specific heat capacity is denoted by \( c \) and varies between different substances. It's a measure of how a material's temperature changes when it absorbs or releases heat.
  • The formula \( q = mc\Delta T \) helps calculate the total heat (q) exchanged during a process, considering the mass (m) and the temperature change (\Delta T).
A thermometer only measures the temperature change, \( \Delta T \), but doesn't take into account the mass of the substances or their specific heat capacity. This is why calorimeters are essential for accurate measurements in thermodynamics as they provide a more complete picture.
Exothermic and Endothermic Reactions
Understanding the difference between exothermic and endothermic reactions is essential in calorimetry and general chemistry.
  • Exothermic reactions release heat into the environment, causing an increase in the temperature of the surroundings. These processes usually feel warm or hot to the touch.
  • Endothermic reactions absorb heat, resulting in a decrease in temperature of the surroundings. These reactions can feel cool to the touch as they draw in heat from their surroundings.
Calorimeters help us accurately measure the heat exchanged during these reactions, ensuring we can correctly classify and understand the thermodynamics at play. By using calorimeters, we can observe these processes more accurately than by just using a thermometer, which only measures temperature change without context regarding mass or specific heat capacities.

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Most popular questions from this chapter

How much work (in joules) must be done on a system to decrease its volume from \(10.0\) liters to 2.0 liters by exerting a constant pressure of \(4.0\) atm?

The French chemists Pierre L. Dulong and Alexis T. Petit noted in 1819 that the molar heat capacity of many solids at ordinary temperatures is proportional to the number of atoms per formula unit of the solid. They quantified their observations in what is known as Dulong and Petit's rule that says that the molar heat capacity of a solid can be expressed as $$ C_{\mathrm{p}} \approx N \times 25 \mathrm{~J} \cdot \mathrm{K}^{-1} \cdot \mathrm{mol}^{-1} $$ where \(N\) is the number of atoms per formula unit. The observed heat capacity per gram of a compound containing thallium and chlorine is \(0.208 \mathrm{~J} \cdot \mathrm{K}^{-1} \cdot \mathrm{g}^{-1}\). Use Dulong and Petit's rule to determine the formula of the compound.

How much work (in joules) does a system do if its volume increases from 10 liters to 25 liters against a constant pressure of \(3.5 \mathrm{~atm} ?\)

A \(100.0\) -mL sample of \(0.200\) -M aqueous hydrochloric acid is added to \(100.0 \mathrm{~mL}\) of \(0.200-\mathrm{M}\) aqueous ammonia in a calorimeter with a total heat capacity of \(480 \mathrm{~J} \cdot \mathrm{K}^{-1} .\) The temperature increase is \(2.34 \mathrm{~K}\). Calculate the value of \(\Delta H_{\mathrm{rxn}}^{\circ}\) for the equation $$ \mathrm{HCl}(a q)+\mathrm{NH}_{3}(a q) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(a q) $$ which describes the reaction that occurs when the two solutions are mixed.

Liquid sodium is being considered as an engine coolant. How many grams of liquid sodium are needed to absorb \(1.00 \mathrm{MJ}\) of energy in the form of heat if the temperature of the sodium is not to increase by more than \(10^{\circ} \mathrm{C}\) ? Take \(C_{\mathrm{p}}=30.8 \mathrm{~J} \cdot \mathrm{K}^{-1} \cdot \mathrm{mol}^{-1}\) for \(\mathrm{Na}(l)\) at \(500 \mathrm{~K}\).
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