Chapter 6.1

What is energy?

The ability to perform work

What are forms of energy?

• Thermal

• Light

• Sound

• Potential

• Mechanical

• Chemical

• Electrical

• Nuclear

What is thermal energy?

Heat energy

What is light energy?

Energy from electromagnetic waves

What is sound energy?

Energy from vibrations

What is potential energy?

Stored energy

What is mechanical energy?

Energy of motion

What is chemical energy?

Energy stored in chemical bonds

What is electrical energy?

Energy from moving charges

What is nuclear energy?

Energy store in the nucleus

What is energy transfer?

Movement of energy from one object to another

What is energy conversion?

Change of energy from one form to another

What happens during imperfect energy transfer?

Some energy is lost, usually as heat

What is enthalpy (H)?

The sum of internal energy and PV (pressure x volume)

What does enthalpy approximate?

Thermal energy of the system and surroundings

What is enthalpy of reaction (ΔH)?

Heat gained or lost during a reaction

What does a positive ΔH mean?

Energy is gained (endothermic)

What does a negative ΔH mean?

Energy is released (exothermic)

In A + B → C + D, why is ΔH = +5?

Products have higher enthalpy (15 − 10 = +5)

What does ΔH = +5 mean physically?

The reaction absorbs energy (endothermic)

In E + F → G + H, why is ΔH = −15?

Products have lower enthalpy (5 − 20 = −15)

What does ΔH = −15 mean physically?

The reaction releases energy (exothermic)

Key pattern for exams (ΔH sign)?

Positive = energy in, Negative = energy out

What does exothermic mean?

A reaction that releases heat (heat exits the system)

In an exothermic reaction, are products higher or lower in energy than reactants?

Lower in energy

What is the sign of ΔH in an exothermic reaction?

Negative

What happens to the temperature of the surroundings in an exothermic reaction?

It increases

What does “endothermic” mean?

A reaction that absorbs heat (heat enters the system)

In an endothermic reaction, are products higher or lower in energy than reactants?

Higher in energy

What is the sign of ΔH in an endothermic reaction?

Positive

What happens to the temperature of the surroundings in an endothermic reaction?

It decreases

What must happen for bonds to break during a reaction?

Energy must be absorbed

Do all reactions require energy to start?

Yes, activation energy is required

Explain what is happening in an exothermic energy diagram

The reactants start at a higher energy, require activation energy to reach the peak, and then release energy as they form lower-energy products; ΔH is negative because energy is released to the surroundings, increasing temperature.

Explain what is happening in an endothermic energy diagram

The reactants start at a lower energy, absorb energy to reach the activation energy peak, and form higher-energy products; ΔH is positive because energy is absorbed from the surroundings, decreasing temperature.

What is bond dissociation energy (BDE)?

The energy required to break a bond

Are electrons in a bond high or low in energy?

Low in energy (bonding orbital)

What must happen to electrons to break a bond?

They must move to a higher-energy state

Does breaking a bond release or require energy?

It requires energy

Why do bonds require energy to break?

Because bonded electrons are in a stable, low-energy state

What are the two ways bonds can break?

Homolytically and heterolytically

What happens in homolytic bond cleavage?

Electrons are split evenly between atoms

What is formed from homolytic cleavage?

Radicals

What happens in heterolytic bond cleavage?

One atom takes both electrons

What is formed from heterolytic cleavage?

Ions (cation and anion)

Which type of bond breaking does bond dissociation energy (BDE) refer to?

Homolytic cleavage

Do stronger bonds have higher or lower bond dissociation energy (BDE)?

Higher BDE

As bond strength increases, what happens to BDE?

It increases

Which bond is strongest: H–F, H–Cl, H–Br, or H–I?

H–F

Which bond is weakest: H–F, H–Cl, H–Br, or H–I?

H–I

How does bond strength change down the halogen group (F → I)?

It decreases

How does BDE change down the halogen group (F → I)?

It decreases

What equation is used to calculate ΔH using bond energies?

ΔH = BDE (bonds broken) − BDE (bonds formed)

Do bonds broken require or release energy?

Require energy

Do bonds formed require or release energy?

Release energy

What does a negative ΔH indicate?

Exothermic reaction

What does a positive ΔH indicate?

Endothermic reaction

In this example, why is the reaction exothermic?

More energy is released from forming bonds than required to break bonds

Are exothermic reactions always spontaneous?

No

Are endothermic reactions always nonspontaneous?

No

Does ΔH alone determine if a reaction is spontaneous?

No

No

Lower in energy

In an endothermic reaction, are products higher or lower in energy?

Higher in energy

Bonds broken (C–H and Cl–Cl) require less energy than bonds formed (C–Cl and H–Cl) release, so ΔH is negative and the reaction is exothermic.

What is entropy (ΔS)?

The disorder or randomness of a system

What does entropy relate to at the molecular level?

The number of possible arrangements (microstates)

What happens to entropy when volume increases?

It increases

Why does entropy increase when gas expands?

There are more possible positions for molecules

More volume for a gas leads to what change in entropy?

Greater entropy

What equation determines spontaneity using entropy?

ΔSₜₒₜ = ΔS_sys + ΔS_surr

What two components make up ΔSₜₒₜ?

System and surroundings

What does ΔS_sys represent?

Entropy change of the system (reaction)

What does ΔS_surr represent?

Entropy change of the surroundings

What does it mean if ΔSₜₒₜ is positive?

The process is spontaneous

What does the 2nd law of thermodynamics state about entropy?

Spontaneous processes increase the entropy of the universe

When is ΔS_sys positive based on number of moles?

When there are more moles of products than reactants

Why does more moles increase entropy?

More particles means more possible arrangements

What happens to entropy when a cyclic compound becomes acyclic?

It increases

Why does going from cyclic to acyclic increase entropy?

The molecule has more freedom of movement

What is a simple rule for predicting ΔS_sys?

More particles or more freedom = higher entropy

What equation relates total entropy to system and surroundings?

ΔSₜₒₜ = ΔS_sys + ΔS_surr

What is the equation for ΔS_surr?

ΔS_surr = −ΔH_sys / T

What equation combines ΔH and ΔS into one value?

ΔG = ΔH − TΔS

What does ΔG < 0 indicate?

Spontaneous

What does ΔG > 0 indicate?

Nonspontaneous

What determines spontaneity more directly: ΔH or ΔG?

ΔG

What is the equation for Gibbs Free Energy?

ΔG = ΔH − TΔS

What does ΔG determine?

Spontaneity of a reaction

What does ΔH represent?

Change in enthalpy (heat)

What does ΔS represent?

Change in entropy (disorder)

What does a negative ΔG indicate?

Spontaneous reaction

What does a positive ΔG indicate?

Nonspontaneous reaction

Why is ΔG useful?

It combines enthalpy and entropy to determine spontaneity

Exothermic

What does a negative ΔG indicate?

Spontaneous reaction

What is an exergonic reaction?

A reaction that releases free energy

Are spontaneous reactions exergonic or endergonic?

Exergonic

In an exergonic reaction, are products higher or lower in free energy?

Lower