5.2 Enthalpy cycles (SL)
Summary
TLDRThis video, presented by M Sjcam, explains Hess's Law and its application in enthalpy cycles. The law states that the enthalpy change of a chemical reaction is independent of the path taken. The video demonstrates this concept using examples involving the formation and combustion of compounds like benzene and ethene. By comparing different routes, the video shows how to calculate enthalpy changes using Hess's Law, highlighting key concepts such as the enthalpy change of formation and combustion in an easy-to-understand manner.
Takeaways
- 📚 Hess's law states that the enthalpy change for a chemical reaction is independent of the route taken.
- 🔄 In an enthalpy cycle, there are multiple routes from reactants to products, but the enthalpy change remains the same regardless of the route.
- ➡️ The equation ΔH3 = ΔH1 + ΔH2 represents the relationship between different routes in an enthalpy cycle.
- 🧪 An example involving carbon and hydrogen shows the enthalpy change for forming benzene from its elements.
- 🔥 The enthalpy change of combustion for benzene (ΔH2) is -3268 kJ/mol, and for the sum of 6 moles of carbon and 3 moles of hydrogen (ΔH3) is -3222 kJ/mol.
- 📝 Using Hess's law, the enthalpy change of formation for benzene (ΔH1) is calculated as +46 kJ/mol.
- 📉 An enthalpy level diagram for the combustion of ethene (C2H4) shows an enthalpy change of -1411 kJ/mol.
- ⚖️ The enthalpy change of formation for ethene can be determined by adding the enthalpy changes of carbon and hydrogen combustion and reversing the sign.
- 🟢 The enthalpy change of formation for ethene is calculated as +51 kJ/mol.
- 🔁 Hess's law is crucial in calculating unknown enthalpy changes by comparing different reaction routes.
Q & A
What is Hess's Law and how does it relate to enthalpy changes in chemical reactions?
-Hess's Law states that the enthalpy change for a chemical reaction is the same, regardless of the route by which the reaction occurs. It means that the total enthalpy change for a reaction is independent of the number of steps or the intermediates involved.
How is the enthalpy change for a reaction represented mathematically according to Hess's Law?
-According to Hess's Law, the enthalpy change for a reaction can be represented as ΔH3 = ΔH1 + ΔH2, where ΔH3 is the direct enthalpy change from reactants A to products C, and ΔH1 and ΔH2 are the enthalpy changes for the intermediate steps from A to B and from B to C, respectively.
What is the significance of the two possible routes in an enthalpy cycle?
-The two possible routes in an enthalpy cycle demonstrate that the overall enthalpy change for a reaction is the same, regardless of whether the reaction proceeds directly from reactants to products or through one or more intermediate steps.
What is the enthalpy change of formation for benzene, as mentioned in the script?
-The enthalpy change of formation for benzene, as calculated using Hess's Law, is found to be +46 kilojoules per mole.
How is the enthalpy change of combustion for benzene represented in the script?
-The enthalpy change of combustion for benzene is represented as ΔH2 and is given a value of -3268 kilojoules per mole.
What is the significance of the negative value for the enthalpy change of combustion for benzene?
-The negative value for the enthalpy change of combustion for benzene indicates that the reaction is exothermic, meaning it releases energy in the form of heat.
What does the enthalpy change of -3222 kilojoules represent in the script?
-The enthalpy change of -3222 kilojoules represents the sum of the enthalpy changes for the combustion of 6 moles of carbon and 3 moles of hydrogen.
Can you explain the concept of an enthalpy level diagram as presented in the script?
-An enthalpy level diagram is a graphical representation of the enthalpy changes for different steps in a chemical process. It helps visualize the energy changes associated with each step and the overall process.
What is the enthalpy change for the combustion of ethane (C2H4) as given in the script?
-The enthalpy change for the combustion of ethane is given as -1411 kilojoules per mole.
How is the enthalpy change for the formation of ethene (C2H4) from its elements calculated in the script?
-The enthalpy change for the formation of ethene from its elements is calculated by adding the enthalpy changes for the combustion of carbon and hydrogen and then reversing the sign, resulting in a positive value of 51 kilojoules per mole.
Why does the script mention reversing the sign of the enthalpy change when calculating for step Y?
-The sign of the enthalpy change is reversed when calculating for step Y because the calculation is going against the direction of the enthalpy level diagram arrow, which indicates an endothermic process (positive enthalpy change).
Outlines
🔍 Introduction to Enthalpy Cycles and Hess's Law
This paragraph introduces the concept of enthalpy cycles and Hess's Law. It explains that according to Hess's Law, the enthalpy change for a chemical reaction is independent of the path taken. The paragraph discusses a specific enthalpy cycle where there are two possible routes from reactants A to product C—either via an intermediate B or directly. The enthalpy change for both routes is the same, which can be expressed mathematically as ΔH3 (direct route) equals ΔH1 (A to B) plus ΔH2 (B to C).
💡 Example: Enthalpy Cycle with Benzene Formation
This paragraph provides an example of an enthalpy cycle involving the formation and combustion of benzene. It describes two routes to go from carbon and hydrogen (reactants) to carbon dioxide and water (products). Route one involves the formation of benzene (ΔH1), followed by its combustion (ΔH2). The enthalpy change for the entire process (ΔH3) can be calculated using Hess's Law, leading to the determination of ΔH1 as the enthalpy change of benzene formation.
📉 Energy Level Diagram: Ethene Combustion
This paragraph discusses an energy level diagram showing the combustion of ethene (C2H4). It explains the enthalpy change for the combustion of one mole of ethene to form carbon dioxide and water. The paragraph also covers the enthalpy changes for the combustion of carbon and hydrogen to form their respective products. Using Hess's Law, the enthalpy change for the formation of ethene from its elements is calculated, highlighting the importance of reversing the sign when going against an arrow in the diagram.
🔬 Final Enthalpy Calculation for Ethene Formation
This final paragraph concludes the calculation of the enthalpy change for the formation of ethene. It summarizes the calculated value as +51 kJ/mol, derived from combining the enthalpy changes of carbon and hydrogen combustion with the enthalpy change of ethene combustion. This value represents the enthalpy change of formation for ethene.
Mindmap
Keywords
💡Hess's Law
💡Enthalpy
💡Enthalpy Cycle
💡Enthalpy Change of Formation
💡Enthalpy Change of Combustion
💡Chemical Reaction Pathways
💡Reactants and Products
💡Standard Conditions
💡Energy Level Diagram
💡Kilojoules per Mole (kJ/mol)
Highlights
Introduction to Hess's Law and its significance in determining enthalpy changes for chemical reactions.
Explanation that the enthalpy change for a chemical reaction is independent of the route taken, according to Hess's Law.
Description of an enthalpy cycle with two possible routes: from reactants A to product C directly, or via an intermediate B.
Introduction of the equation: ΔH3 (direct route A to C) = ΔH1 (route A to B) + ΔH2 (route B to C).
Application of Hess's Law to a specific example involving the formation and combustion of benzene (C6H6).
Calculation of the enthalpy change for the formation of benzene (ΔH1) using Hess's Law: ΔH1 = ΔH3 - ΔH2.
Explanation of how to rearrange the equation to solve for ΔH1, leading to the value of 46 kJ/mol.
Introduction to an energy level diagram for the combustion of ethene (C2H4).
Explanation of the enthalpy change of combustion for ethene, with a detailed description of the energy levels involved.
Description of the upward arrow representing the positive enthalpy change for the formation of ethene from carbon and hydrogen.
Calculation of the enthalpy change for the combustion of carbon and hydrogen to form carbon dioxide and water.
Demonstration of the use of Hess's Law to calculate the enthalpy change for the formation of ethene.
Clarification that the enthalpy change sign must be reversed when going against the direction of the arrow in the energy level diagram.
Final calculation of the enthalpy change of formation for ethene as +51 kJ/mol.
Summary of the application of Hess's Law to both enthalpy cycles and energy level diagrams to determine enthalpy changes in chemical reactions.
Transcripts
this is M sjcam in this video I'll be
looking at enthalpy cycles we'll start
by looking at Hess's law
Hess's law states that the enthalpy
change for a chemical reaction is
independent of the route by which the
chemical reaction occurs in this
enthalpy cycle there are two possible
routes to get from reactants a to
product c one route is to go from a via
B to C the other route is to go directly
from A to C so according to Hess's law
the enthalpy change for the reaction
will be the same if we go from A to B to
C or if we go from a directly to C so
this can be represented in equation form
as Delta H 3 which is the direct route
from A to C is equal to Delta H 1 which
is the route from A to B plus Delta H 2
which is the route from B to C so
regardless of the route that we take to
get from reactants a to product C the
enthalpy change will be the same next
we'll have a look at an example in this
enthalpy cycle there are two possible
routes to go from the reactants which
are carbon and hydrogen to the products
which are carbon dioxide and water the
first route starts with the reaction of
carbon and hydrogen to form one mole of
C 6 H 6 which is benzene this represents
the enthalpy change of formation of
benzene and we'll call this Delta H 1
next we have the combustion of one mole
of benzene to form carbon dioxide and
water which we will call Delta H 2 the
value of Delta H 2 which is the enthalpy
change of combustion for benzene is
negative 3268 kilojoules per mole Delta
H 3 which is the sum of the entropy
change of combustion for 6 moles of
carbon and 3 moles of hydrogen is
negative 3222 kilojoules
according to Hess's law the
to be changed for the reaction is
independent of the route taken so an
equation form Delta H 3 is equal to
Delta H 1 plus Delta H 2 if we rearrange
the equation we get Delta H 1 equals
Delta H 3 minus Delta H 2 this gives us
Delta H 1 equals negative 3222
minus negative 3268 which gives us an
entropy change for Delta H 1 of positive
46 kilojoules per mole which is the
entropy change of formation of benzene
in our next example we look at an
enthalpy or energy level diagram so
we'll start with this downwards arrow on
the right which is for the enthalpy
change of combustion of C 2 H 4 which is
e thin so in the reaction 1 mole of
ethane reacts with 3 moles of oxygen to
form two moles of carbon dioxide and 2
moles of water and the enthalpy change
for this reaction is negative 1411
kilojoules per mole next we look at the
enthalpy change given the letter Y this
enthalpy change represents the formation
of one mole of ethene from its elements
carbon and hydrogen which is the
enthalpy change of formation note that
this arrow is pointing in the upwards
direction which means it has a positive
enthalpy change next we look at the two
arrows pointing downwards the first
arrow represents the combustion of 2
moles of carbon to form two moles of
carbon dioxide to calculate this
enthalpy change we multiply the enthalpy
change of combustion of carbon by 2 to
give us negative 788 kilojoules the next
arrow represents the combustion of 2
moles of hydrogen to form two moles of
water to calculate this we multiply the
enthalpy change of combustion of
hydrogen by 2 to give us negative 572
kilojoules a-kor
Hess's law the entropy change for
reaction is independent of the route
that is taken so to calculate the ant be
change for step y we can add together
the ant be changes for these three steps
note that because we are going against
this arrow we need to reverse the sign
of the enthalpy change so the enthalpy
change for step y equals negative 788
plus negative 572 Plus 1411 which gives
us positive 51 kilojoules per mole this
is the enthalpy change of formation
value for e thene
Ver Más Videos Relacionados
5.0 / 5 (0 votes)