Pre-Lab Questions 1. Review the Background section. Arrange Equations A-C in such a way that they add up to Equation 1. 2. Use Hess's Law to express the heat of reaction for Equation 1 as the appropriate algebraic sum of the heats of reaction for Equations A-C. 3. The heat of reaction for Equation C is equal to the standard heat of formation of water. The heat of formation of a compound is defined as the enthalpy change for the preparation of one mole of a compound from its respective elements in their standard states at 25 °C. Chemical reference sources contain tables of heats of formation for many compounds. Look up the heat of formation of water in your textbook or in a chemical reference source such as the CRC Handbook of Chemistry and Physics.
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
![• Heat of reaction
• Heat of formation
• Hess's Law
• Calorimetry
round
Magnesium reacts with oxygen in air to form magnesium oxide, according to Equation 1.
Mg(s) + 20,) → MgO(s) + heat
Equation 1
As mentioned above, a great deal of heat and light are produced-the temperature of
flame can reach as high as 2400 °C. The amount of heat energy produced in this reaction
cannot be measured directly in the high school lab. It is possible, however,
amount of heat produced by an indirect method, using Hess's Law.
determine the
The heat or enthalpy change for a chemical reaction is called the heat of reaction (AH).
The enthalpy change-defined as the difference in enthalpy between the products and
reactants-is equal to the amount of heat transferred at constant pressure and does not
depend on how the transformation occurs. This definition of enthalpy makes it possible to
determine the heats of reaction for reactions that cannot be measured directly. According to
Hess's Law, if the same overall reaction is achieved in a series of steps, rather than in one
step, the enthalpy change for the overall reaction is equal to the sum of the enthalpy
changes for each step in the reaction series. There are two basic rules for calculating the
enthalpy change for a reaction using Hess's Law.
Equations can be "multiplied" by multiplying each stoichiometric coefficient in the bal-
anced chemical equation by the same factor. The heat of reaction (AH) is proportional to
the amount of reactant. Thus, if an equation is multiplied by a factor of two to increase
the number of moles of product produced, then the heat of reaction must also be multi-
plied by a factor of two](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2F05763ed4-533b-42c3-8ef5-e76671003502%2Fa5b1ea6d-bf01-4f87-84a3-c1144269e967%2Fsktfrus_processed.jpeg&w=3840&q=75)
![Consider the following three reactions:
Mg(s) + 2HCI(aq) → MgCl,(aq) + H,()
Equation A
MgO(s) + 2HCI(aq) → MgCl,(aq) + H,0(1)
Equation B
H,(g) + 20,(g) → H,0(1)
Equation C
It is possible to express the combustion of magnesium (Equation 1) as an algebraic sum of
Equations A, B, and C. Applying Hess's Law, therefore, it should also be possible to deter-
mine the heat of reaction for Equation 1 by combining the heats of reaction for Equations
A-C in the same algebraic manner. Note: Chemical equations may be combined by addition,
subtraction, multiplication, and division.
Experiment Overview
The purpose of this experiment is to use Hess's Law to determine the heat of reaction for the
combustion of magnesium (Equation 1). The heats of reaction for Equations A and B will be
measured by calorimetry. The heats of reaction for these two reactions will then be combined
algebraically with the heat of formation of water (Equation C) to calculate the heat of reaction
for the combustion of magnesium.
Pre-Lab Questions
1. Review the Background section. Arrange Equations A-C in such a way that they add up to
Equation 1.
2. Use Hess's Law to express the heat of reaction for Equation 1 as the appropriate algebraic
sum of the heats of reaction for Equations A-C.
3. The heat of reaction for Equation C is equal to the standard heat of formation of water. The
heat of formation of a compound is defined as the enthalpy change for the preparation of
one mole of a compound from its respective elements in their standard states at 25 °c.
Chemical reference sources contain tables of heats of formation for many compounds.
Look up the heat of formation of water in your textbook or in a chemical reference source
such as the CRC Handbook of Chemistry and Physics.
Materials *](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2F05763ed4-533b-42c3-8ef5-e76671003502%2Fa5b1ea6d-bf01-4f87-84a3-c1144269e967%2F1it1eab_processed.jpeg&w=3840&q=75)
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