Chapter 9 - Enthalpy Changes Flashcards
Define average bond enthalpy
energy required to break one mole of a specified type of bond in a gaseous molecule.
What are the limitations of bond enthalpy
• Actual bond enthalpy can vary depending on the chemical environment of the bond
• Average bond enthalpy is calculated from the actual bond enthalpies in different chemical environments.
What is bond breaking
Endothermic
What is the H value for endothermic
ΔH is positive
What is bond making
Exothermic
What is the H value for Exothermic
ΔH is negative
Formula for ΔH
ΔH = Σ(bond enthalpies in reactants) – Σ(bond enthalpies in products)
What happens to the temp of the surroundings in an Exothermic reaction
Increases
What happens to the temp of the surroundings in an endothermic reaction
Decreases
What does this mean ΔH⦵
Standard conditions
What are the standard conditions
• Pressure: 100 kPa
• Temperature: 298 K
• Concentration: 1 mol dm-3
• State: the physical state under standard conditions
Define enthalpy change of reaction
• enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.
Define enthalpy change of formation
• enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.
Define enthalpy change of combustion
• enthalpy change that takes place when one mole of a substance is reacted completely with oxygen under standard conditions, with all reactants and products in their standard states.
Define enthalpy change of neutralisation
• enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) under standard conditions, with all reactants and products in their standard states.
Formula for calorimetry / heat energy released
q = mcΔT
What is q in q = mcΔT
heat energy – J
What is m in q = mcΔT
mass of SURROUNDINGS (material changing temp) – g
What is c in q = mcΔT
specific heat capacity = water = 4.18 J/gK
What is T in q = mcΔT
• Temp change = ΔT = T(final) – T(initial)
Accuracy of experimental ΔCH?
Experimental values are LESS exothermic than the data values
Why are experimental values are LESS exothermic than the data values
• Heat is lost to surroundings other than water
• Incomplete combustion may occur
• May have evaporation of the fuel from the wick
• Non-standard conditions
How can a simply calorimeter be made
from a polystyrene drinking cup, a vacuum flask or metal can
Symbol for standard enthalpy change of reaction
Symbol for standard enthalpy change of formation
Symbol for standard enthalpy change of combustion
Symbol for standard enthalpy change of neutralisation
Define Hess’ law
states that the overall enthalpy change for a reaction is independent of the route the reaction takes
How to draw Enthalpy Cycles – Using ΔfH
- Write the balanced equation for the reaction at the top
- Write the balanced elements at the bottom
- Draw arrows up to the reactants/ products.
- Add in any ΔH values you know.
- Use Hess’ Law to calculate any known values.
Using vectors, what’s the equation for the Enthalpy Cycles – Using ΔfH
ΔrH = ΣΔfH(products) – ΣΔfH(reactants)
A = - b + c
A = c - b
What’s m in q=mc^T
MASS OF MATERIAL CHANGING TEMP
What moles should you calculate in a ΔrH⦵ question when reactants have different number of moles?
Either is fine, but you must use the correct molar ratio
Example: if one mole of X reacts with 2 moles of Y:
1X + 2Y → …..
- If you have information for X, work out as normal
- If you have information for Y, find n(Y), then you must:
- Work as normal finding the enthalpy change per mole of Y
- At the end, multiply enthalpy change per mole of Y by number of moles
