Name: reaction heat, hess’s law, and calorimetry
Uploaded: 2020-03-26T07:22:25.000Z
Duration: 3 min 37 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Physical Science

Chemical Reactions

3.2.9-12.D

Enthalpy of Reaction

Thermodynamics is the study of heat and its relation to energy and work. One of the most important thermodynamic properties is enthalpy, which indicates the heat energy transferred by a process at constant pressure. Enthalpy is used to describe chemical reactions, where the enthalpy change, &#916;H, tells us how much heat is absorbed or released during a chemical reaction.
Enthalpy is a state function, meaning that the change is independent of the path and only takes into account the initial and final state. Thus, the &#916;H of a reaction is calculated by subtracting the sum of the enthalpies of the reactants from the sum of the enthalpies of the products.
If &#916;H is positive, the reaction is endothermic and absorbs heat from the surroundings. If &#916;H is negative, the reaction is exothermic and releases heat to the surroundings.
We can measure the enthalpy change of a reaction using an isolated system like a calorimeter. A simplified calorimeter, like the one we have here, consists of an insulated sample cell with a stirrer and a thermometer. As the reaction proceeds inside of the insulated cell, the temperature changes and we are able to measure &#916;T.
We can assume that no energy is transferred into or out of the sample cell because it is insulated. So, we can then assume that the temperature change is due to the heat change in the reaction.
So, how do we use &#916;T to calculate &#916;H? At constant pressure, &#916;H is equal to the heat flow, Q. Q is equal to the mass of the substance times its specific heat capacity and the change in temperature. The specific heat capacity, cs, is a measure of how much heat is required to increase the temperature of 1 g of a material by 1 &#176;C.
A material with a higher specific heat capacity requires more energy to raise its temperature and vice versa. Specific heat capacity values for common substances can be found in the literature. Thus, we can use the specific heat capacity, along with the mass of the substance and the temperature change, to calculate &#916;H for the reaction.
In your experiment, we will measure the change in enthalpy for the formation of magnesium oxide from magnesium and oxygen. Using our simple calorimeter requires us to measure the heat of reaction indirectly with a series of separate reaction steps. So, we will run each reaction separately and measure &#916;T and, by extension, &#916;H for each reaction. Then, according to Hess&#39;s Law, the enthalpy of the overall reaction is equal to the sum of each of the enthalpies of reaction for each reaction step.
In this lab, you will construct a simple calorimeter using polystyrene cups and then experimentally determine the enthalpy of formation of magnesium oxide.

enthalpy of reaction

<h4>Enthalpy of Reaction</h4>
Thermodynamics is the study of heat energy and other types of energy, such as work. The laws of thermodynamics are used across all known science fields and have applications ranging from biology to physics.
<h4>Three Laws of Thermodynamics</h4>
There are three principle laws of thermodynamics that describe the interactions occurring within the universe, regardless of scale.
The first is the law of conservation of energy, which describes that the total energy of an isolated system is constant. Energy can be transferred from one form of energy to another, but it can never be created or destroyed. For example, thermal energy can be transformed into work.
The second law of thermodynamics is the law of increased entropy. Entropy is a thermodynamic property that is related to the possible microscopic configurations of the system. Entropy describes the disorder of the thermodynamic system. The second law states that the sum of entropies in an isolated thermodynamic system must always increase because systems tend toward more disorder rather than less. Therefore, thermodynamic systems can never decrease in entropy, which would suggest a change from disordered to ordered, because that would violate this fundamental law of entropy.
Finally, the third law of thermodynamics is the law of absolute zero. This law states that the entropy of a system approaches a constant value as the system approaches the temperature of absolute zero. Absolute zero is a theoretical temperature where the motion of all matter stops, and it is defined as 0 degrees Kelvin.
<h4>Enthalpy</h4>
In physics and chemistry, there are two types of functions: state functions and path functions. Path functions depend on the transition a system undergoes from the initial state to the final state. The most common path functions are heat (Q) and work (W).
State functions are independent of the path and rely on the current equilibrium state of the system. State functions include pressure, temperature, volume, enthalpy, and entropy.
Enthalpy, H, is a thermodynamic property that describes the heat lost or gained in a system. The change in enthalpy, &#916;H, is equal to the sum of the change in internal energy, &#916;U, plus the product of the constant pressure, P, and the change in volume, &#916;V.
&#916;H = &#916;U + P&#916;V
During a chemical reaction, energy is either gained or released. Since enthalpy is a state function, the change in enthalpy during a chemical reaction only depends on the difference between the final and initial enthalpies.
&#916;H = Hfinal - Hinitial
The initial enthalpy represents the enthalpy of the reactants, whereas the final enthalpy represents the enthalpy of the products. Thus, the enthalpy change of a reaction can be described by the following equation:
&#916;Hrxn = Hproducts - Hreactants
When the enthalpy of the products is greater than the enthalpy of the reactants, &#916;H is positive, indicating that the reaction absorbs heat and is endothermic. When the enthalpy of the reactants is greater than the enthalpy of the products, &#916;H is negative, and the reaction releases heat and is exothermic. If the &#916;H values are known for a reaction, the reverse reaction is the negative value of that &#916;H. An exothermic forward reaction would become an endothermic reverse reaction, for example.
Some reactions occur in multiple steps, and each have their own enthalpy of reaction. According to Hess&#8217;s Law, we can determine the overall enthalpy of reaction by adding the enthalpy of reaction for each step.
For example, the formation of magnesium oxide from solid magnesium and oxygen gas can be divided into three individual reactions.
Mg(s) + &#189;O2(g) &#8594; MgO(s) &#916;Hrxn
Mg(s) + 2H+(aq) &#8594; Mg2+(aq) + H2(g) &#916;Hrxn1
Mg2+(aq) + H2O(l) &#8594; MgO(s) + 2H+(aq) &#916;Hrxn2
&#189;O2(g) + H2(g) &#8594; H2O(l) &#916;Hrxn3
According to Hess&#8217;s law, the enthalpy of reaction for the overall reaction (&#916;Hrxn) is equal to the sum of the individual enthalpies of reactions of each step.
&#916;Hrxn = &#916;Hrxn1 + &#916;Hrxn2 + &#916;Hrxn3
<h4>Calorimeter</h4>
A calorimeter is a device that measures the heat released or absorbed by a physical process or chemical reaction. A constant-pressure calorimeter consists of an insulated reaction chamber that is isolated from the surroundings. This minimizes the impact of any heat or work lost to the ambient environment. The calorimeter has a stirrer to mix the solution and a thermometer to measure temperature changes.
To measure heat using a calorimeter, reactants are placed inside of the reaction chamber and mixed. As the reaction occurs, the temperature changes are recorded as &#916;T. Since the calorimeter is insulated and isolated from the surroundings, any temperature change is due to heat gained or lost during the chemical reaction.
The calorimeter can be used to determine the enthalpy of a reaction by determining the thermodynamic value of heat, Q, using the change in temperature. When Q is positive, heat is absorbed by the system, whereas a negative Q indicates heat released by the system.
Heat is related to the change in temperature during a reaction, &#916;T, by the mass of the substance, m, and its specific heat capacity, cs. The specific heat capacity represents the amount of energy needed, in the form of heat, to raise the temperature of one unit of mass of a pure substance by one unit and is written in units of J/kg&#183;K.
Q = mcs&#916;T
The subscript p indicates that the reaction is performed under constant pressure.
Q = mcp&#916;T
Since each component of the reaction and calorimeter absorbs or loses heat, all components must be taken into account when calculating the thermodynamic heat of the reaction, Qrxn. Therefore, the total heat of the reaction is equal to the heat gained or lost by the solution plus the heat gained or lost by the calorimeter.
Qrxn = - (Qsoln + Qcalorimeter)
Qrxn = - (msolncsoln&#916;T + Ccalorimeter&#916;T)
The thermodynamic heat of the reaction, Qrxn, measured in the calorimeter is equal to the heat of the reaction, &#916;Hrxn.
<h4>References</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chemistry and Chemical Reactivity. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chemistry: The Molecular Nature of Matter and Change. Boston, MA: McGraw Hill.</li>
</ol>

Enthalpy of Reaction
Thermodynamics is the study of heat energy and other types of energy, such as work. The laws of thermodynamics are used across all ...

Education

JoVE Lab Manual

Chemistry

reaction heat, hess’s law, and calorimetry

Reaction heat is explained through enthalpy, Hess’s law, and calorimetry. These ideas show how heat moves during chemical reactions and how scientists measure it.

Thermodynamics is the study of heat energy and other forms of energy, such as work. Its laws are used across science, from biology to physics. The first law says energy is conserved in an isolated system. Energy can change form, such as thermal energy becoming work, but it cannot be created or destroyed.

The second law of thermodynamics is the law of increased entropy. Entropy is a thermodynamic property linked to the possible microscopic arrangements of a system. It describes disorder. In an isolated system, total entropy must increase because systems tend toward more disorder. The third law is the law of absolute zero. It states that entropy approaches a constant value as temperature approaches absolute zero, which is defined as 0 K.

In chemistry and physics, state functions and path functions describe different properties. Path functions depend on the route a system takes from start to finish. Heat and work are common path functions. State functions depend only on the current equilibrium state. Pressure, temperature, volume, enthalpy, and entropy are state functions.

Enthalpy, H, describes the heat lost or gained in a system. The change in enthalpy is written as ΔH = ΔU + PΔV, where ΔU is the change in internal energy, P is constant pressure, and ΔV is the change in volume. For a reaction, ΔH also equals the final enthalpy minus the initial enthalpy. That is why the reaction enthalpy can be written as ΔHrxn = Hproducts - Hreactants.

A positive ΔH means the products have more enthalpy than the reactants. The reaction absorbs heat and is endothermic. A negative ΔH means the reactants have more enthalpy than the products. The reaction releases heat and is exothermic. If the reaction is reversed, the sign of ΔH also reverses. An exothermic forward reaction becomes endothermic in reverse.

Some reactions happen in steps. Each step can have its own enthalpy change. Hess’s law says the total enthalpy of reaction equals the sum of the enthalpy changes for each step. The formation of magnesium oxide from solid magnesium and oxygen gas can be written as three separate reactions. Adding ΔHrxn1, ΔHrxn2, and ΔHrxn3 gives the overall ΔHrxn.

A calorimeter measures the heat absorbed or released by a physical process or chemical reaction. A constant-pressure calorimeter has an insulated reaction chamber, a stirrer, and a thermometer. The insulation helps reduce heat and work lost to the surroundings. Reactants are mixed in the chamber, and the temperature change, ΔT, is recorded.

The heat change in a reaction can be found from temperature data using Q = mcsΔT. Here, m is mass and cs is specific heat capacity, which is the energy needed to raise the temperature of one unit mass of a pure substance by one unit. Under constant pressure, the equation is written as Q = mcpΔT. Because the solution and the calorimeter both absorb or lose heat, the reaction heat is Qrxn = -(Qsoln + Qcalorimeter). In expanded form, this is Qrxn = -(msoln csoln ΔT + Ccalorimeter ΔT). The heat measured in the calorimeter is equal to ΔHrxn.

Reaction Heat, Hess’s Law, and Calorimetry

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chemistry

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Learn how reaction heat is linked to enthalpy, Hess’s law, and calorimetry. Explore endothermic and exothermic reactions, plus temperature-based heat measurements.

Reaction Heat, Hess’s Law, and Calorimetry | JoVE K12

Georgia Standards of Excellence

SC3.

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Measure calorimeter heat capacity with hot and cold water, then use magnesium and magnesium oxide reactions in HCl to estimate reaction enthalpy and MgO formation.

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3.2.9-12.C

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Bond energy is the energy required to break a bond homolytically. These values are usually expressed in units of kcal/mol or kJ/mol and are referred to as ...

Reaction Rate

The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time ...

Measuring Reaction Rates

Polarimetry finds application in chemical kinetics to measure the concentration and reaction kinetics of optically active substances during a chemical ...

Concentration and Rate Law

The rate of a reaction is affected by the concentrations of reactants. Rate laws (differential rate laws) or rate equations are mathematical expressions ...

Determining Order of Reaction

Rate laws describe the relationship between the rate of a chemical reaction and the concentration of its reactants. In a rate law, the rate constant k and ...

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The Integrated Rate Law: The Dependence of Concentration on Time

While the differential rate law relates the rate and concentrations of reactants, a second form of rate law called the integrated rate law relates ...

Half-life of a Reaction

The half-life of a reaction (t1/2) is the time required for one-half of a given amount of reactant to be consumed. In each succeeding half-life, half of ...

Temperature Dependence on Reaction Rate

The Collision Theory
Atoms, molecules, or ions must collide before they can react with each other. Atoms must be close together to form chemical bonds. ...

Arrhenius Plots

The Arrhenius equation relates the activation energy and the rate constant, k, for chemical reactions. In the Arrhenius equation, k = Ae&#8722;Ea/RT, R is ...

Reaction Mechanisms

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates ...

Rate-Determining Steps

Relating Reaction Mechanisms
In a multistep reaction mechanism, one of the elementary steps progresses significantly slower than the others. This slowest ...

Catalysis

The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed ...

Enzymes

Inside living organisms, enzymes act as catalysts for many biochemical reactions involved in cellular metabolism. The role of enzymes is to reduce the ...

Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.  Clarifying Statement: Emphasis is on the idea that a chemical reaction is a system that affects the energy change.  Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.  Assessment Boundry: Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.

Students who demonstrate understanding can apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.  Clarifying Statement: Emphasis is on student reasoning that focuses on the number and energy of collisions between molecules.  Assessment Boundry: Assessment is limited to simple reactions in which there are only two reactants;  evidence from temperature, concentration, and rate data; and qualitative relationships between rate and temperature.

3.2.9-12.E

Dynamic Equilibrium

A reversible chemical reaction represents a chemical process that proceeds in both forward (left to right) and reverse (right to left) directions. When ...

The Equilibrium Constant

Consider the oxidation of sulfur dioxide:

For a reaction that begins with a mixture of reactants only, the product concentration is initially equal to ...

homogeneous-equilibria-for-gaseous-reactions

Homogeneous Equilibria for Gaseous Reactions

Homogeneous Equilibria for Gaseous Reactions
For gas-phase reactions, the equilibrium constant may be expressed in terms of either the molar ...

Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.  Clarifying Statement: Emphasis is on the application of Le Chatelier's Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level.  Examples of designs could include different ways to increase product formation including adding reactants or removing products.  Assessment Boundry: Assessment is limited to specifying the change in only one variable at a time.  Assessment does not include calculating equilibrium constants and concentrations.

3.2.9-12.F

Reaction Heat, Hess’s Law, and Calorimetry

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