Energetics-Physical chemistry Flashcards
What are the differences between endothermic and exothermic reactions?
Endothermic reactions absorb heat energy from the surroundings.
Temperature decreases in the surroundings.
∆H is positive (heat is taken in).
Example: Photosynthesis
Exothermic reactions release heat energy to the surroundings.
Temperature increases in the surroundings.
∆H is negative (heat is released).
Example: Combustion of fuels
What is enthalpy change (∆H)?
Enthalpy change (∆H) is the heat energy change during a reaction, measured at constant pressure.
∆H can be either positive (endothermic) or negative (exothermic).
Formula:Δ𝐻=𝐻products−𝐻reactants
What are standard enthalpy changes?
Standard enthalpy change refers to the enthalpy change under standard conditions:
Pressure = 100 kPa
Temperature = 298 K (25°C)
✔ Standard enthalpy changes are measured in kJ/mol.
What is the standard enthalpy of combustion (∆cHƟ)?
What is the standard enthalpy of combustion (∆cHƟ)?
A:
Standard enthalpy of combustion (∆cHƟ) is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
What is the standard enthalpy of formation (∆fHƟ)?
tandard enthalpy of formation (∆fHƟ) is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
What is the equation to calculate heat change (q) in a reaction?
The heat change
𝑞
q is given by the equation:
𝑞=𝑚𝑐Δ𝑇
q=mcΔT
Where:
q = heat change in Joules (J)
m = mass of the substance in grams (g)
c = specific heat capacity in J/g°C
ΔT = temperature change (final temperature - initial temperature) in °C
How do you use the equation
𝑞=𝑚𝑐Δ𝑇
q=mcΔT in calculations?
Step 1: Measure the mass of the substance (m) that is undergoing a temperature change.
Step 2: Measure the initial and final temperature to calculate ΔT.
Step 3: Use the substance’s specific heat capacity (c).
Step 4: Plug values into the equation and solve for q (the heat change).
How do you calculate the molar enthalpy change from the heat change?
After calculating the total heat change q, to find the molar enthalpy change (ΔH), use the formula:
Δ𝐻=𝑞/n
Where:
q = total heat change in Joules (J)
n = number of moles of the substance reacting
What types of related calculations can be done using the equation 𝑞=𝑚𝑐ΔT?
You can use this equation to calculate:
The amount of heat required to change the temperature of a substance.
The molar enthalpy change for reactions, given the moles of reactants or products involved.
The temperature change when a known amount of heat is supplied to a substance.
What is Hess’s Law?
Hess’s Law states that the total enthalpy change of a reaction is independent of the path taken, as long as the initial and final conditions are the same.
This means that enthalpy changes can be calculated by summing the enthalpy changes of individual steps of a reaction.
How can Hess’s Law be used to calculate enthalpy changes?
Hess’s Law allows us to calculate the total enthalpy change (∆H) by adding or subtracting the enthalpy changes of reactions, typically using either:
Enthalpies of combustion
Enthalpies of formation
How do you use Hess’s Law with enthalpies of combustion?
If you are given the enthalpies of combustion for reactants and products, you can apply Hess’s Law by:
Writing combustion reactions for all substances involved.
Combining these reactions to find the enthalpy change for the target reaction.
How do you use Hess’s Law with enthalpies of formation?
Write the balanced chemical equation for the reaction.
Enthalpy change of reaction (∆H) is calculated as:
▲
ΔfHƟ values are the standard enthalpies of formation for each substance.
Standard enthalpy of formation is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states.
What is the definition of mean bond enthalpy?
Mean bond enthalpy is the average enthalpy change required to break one mole of a specific type of bond in the gaseous state, averaged over a range of different molecules.
How does bond breaking and bond making affect enthalpy change?
Breaking bonds = Endothermic (+∆H) (energy absorbed)
Making bonds = Exothermic (-∆H) (energy released)
The overall enthalpy change of a reaction is determined by the difference between the energy required to break bonds and the energy released when new bonds form.
How do you calculate ∆H using mean bond enthalpies?
Use the equation:
Δ𝐻=∑BondEnthalpiesofBondsBroken− ∑BondEnthalpiesofBondsFormed
Why do values from mean bond enthalpy calculations differ from those determined using Hess’s Law?
Mean bond enthalpies are averages from different molecules, whereas Hess’s Law uses precise enthalpy values for specific compounds.
Mean bond enthalpies apply only to gaseous molecules, but reactions in Hess’s Law may involve liquids or solids, which have additional enthalpy changes.
Bond enthalpies vary slightly depending on the environment, so using an average introduces small inaccuracies.
What is activation energy?
Activation energy (Ea) is the minimum amount of energy that colliding particles must have for a reaction to occur.
Why do most collisions not lead to a reaction?
For a reaction to occur, two conditions must be met:
Particles must collide with enough energy (equal to or greater than the activation energy).
Particles must collide with the correct orientation to break and form bonds.
What factors influence whether a collision leads to a reaction?
Temperature → Higher temperature means particles have more kinetic energy, increasing successful collisions.
Concentration/Pressure → More particles in a given volume means more frequent collisions.
Catalysts → Provide an alternative reaction pathway with a lower activation energy.
Surface Area → More exposed particles in solids lead to more collisions.
What does the Maxwell-Boltzmann distribution describe?
The Maxwell-Boltzmann distribution describes the distribution of molecular energies in a gas. It shows that:
Most particles have a moderate amount of energy.
Some particles have low energy, and some have high energy.
A small proportion of particles have energy equal to or greater than activation energy (Ea), allowing reactions to occur.
What are the key features of a Maxwell-Boltzmann distribution curve?
No particles have zero energy (the curve starts at the origin).
Most particles have moderate energy (peak of the curve).
Few particles have very high energy (the curve has a long tail that never touches the x-axis).
The area under the curve represents the total number of molecules.
Particles with energy ≥ activation energy (Ea) are found in the high-energy tail.
How does increasing temperature affect the Maxwell-Boltzmann distribution?
The peak of the curve shifts to the right (higher energy).
The peak is lower (more molecules have higher energy).
More molecules have energy ≥ activation energy (Ea), leading to a higher reaction rate.
The total area under the curve stays the same (number of molecules remains constant).
Conclusion: At higher temperatures, more molecules have sufficient energy to react, increasing the reaction rate.
How does a catalyst affect the Maxwell-Boltzmann distribution?
A catalyst lowers the activation energy (Ea) by providing an alternative reaction pathway.
The curve itself does not change, but the position of Ea shifts to the left.
Since more molecules now have energy ≥ the new (lower) Ea, the reaction rate increases.
