True or False? Bond breaking is an exothermic process.

False. Bond breaking is an endothermic process.

Define the term bond dissociation enthalpy .

Bond dissociation enthalpy is the energy required to break a particular chemical bond.

State the equation for calculating enthalpy change of reaction using bond energies.

The equation for calculating enthalpy change of reaction using bond energies is: Δ H = Σ(bonds broken) - Σ(bonds formed)

Define average bond energy .

Average bond energy is the energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds.

True or False? A reaction is exothermic if more energy is released when new bonds are formed than the energy required to break bonds.

True. A reaction is exothermic if more energy is released when new bonds are formed than the energy required to break bonds.

In terms of bond making and breaking, why is a reaction endothermic?

A reaction is endothermic because more energy is required to break bonds than the energy released when new bonds are formed.

Hess's Law states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.

What does Hess's Law allow us to calculate?

Hess's Law allows us to calculate the standard enthalpy change of a reaction from known standard enthalpy changes.

True or False? In Hess's Law energy cycles, the direct route always has a larger enthalpy change than the indirect route.

False. In Hess's Law energy cycles, the enthalpy change of the direct route is equal to the enthalpy change of the indirect route.

True or False? Hess's Law can be used to calculate enthalpy changes that can be found experimentally using calorimetry.

False. Hess's Law is used to calculate enthalpy changes which cannot be found experimentally using calorimetry.

Give the two common methods used to solve Hess's law problems.

The two common methods for solving Hess's Law problems are: Using Hess's law cycles Using equations

True or False? When using cycles to solve Hess's Law problems, if you follow the direction of the arrow you subtract the quantity.

False. When using cycles to solve Hess's Law problems, if you follow the direction of the arrow you add the quantity.

True or False? In Hess's Law calculations, you always need to adjust for different molar amounts.

True. In Hess's Law calculations, you always need to adjust for different molar amounts.

State the general equation for solving Hess's Law problems using equations.

The general equation for solving Hess's Law problems using equations is: ΔH(reaction) = Σ(ΔH products) - Σ(ΔH reactants)

IBChemistryDPSLFlashcardsWhat Drives Chemical Reactions?Energy Cycles in Reactions

Energy Cycles in Reactions (DP IB Chemistry)

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FrontBond Enthalpy Calculations
True or False?
Bond breaking is an exothermic process.

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True or False?
Bond breaking is an exothermic process.
False.
Bond breaking is an endothermic process.
Define the term bond dissociation enthalpy.
Bond dissociation enthalpy is the energy required to break a particular chemical bond.
State the equation for calculating enthalpy change of reaction using bond energies.
The equation for calculating enthalpy change of reaction using bond energies is:
ΔH = Σ(bonds broken) - Σ(bonds formed)
Define average bond energy.
Average bond energy is the energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds.
True or False?
A reaction is exothermic if more energy is released when new bonds are formed than the energy required to break bonds.
True.
A reaction is exothermic if more energy is released when new bonds are formed than the energy required to break bonds.
In terms of bond making and breaking, why is a reaction endothermic?
A reaction is endothermic because more energy is required to break bonds than the energy released when new bonds are formed.
Calculate the enthalpy change of reaction for this equation:
N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
Bond
Average Bond Energy (kJ mol^-1)
N $\equiv$ N
945
H $-$ H
436
N $-$ H
391
The enthalpy change of reaction is:
- Bonds broken = (1 x 945) + (3 x 436) = +2253
- Bonds formed= 6 x 391 = -2346
- ΔH_r^ꝋ = +2253 - 2346 = -93 kJ mol^-1
Define Hess's Law.
Hess's Law states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.
What does Hess's Law allow us to calculate?
Hess's Law allows us to calculate the standard enthalpy change of a reaction from known standard enthalpy changes.
True or False?
In Hess's Law energy cycles, the direct route always has a larger enthalpy change than the indirect route.
False.
In Hess's Law energy cycles, the enthalpy change of the direct route is equal to the enthalpy change of the indirect route.
Give the equation that would be used to calculate the enthalpy change for the conversion of graphite to diamond.
The equation that would be used to calculate the enthalpy change for the conversion of graphite to diamond is ΔH_r = ΔH₁ - ΔH₂
True or False?
Hess's Law can be used to calculate enthalpy changes that can be found experimentally using calorimetry.
False.
Hess's Law is used to calculate enthalpy changes which cannot be found experimentally using calorimetry.
Give the two common methods used to solve Hess's law problems.
The two common methods for solving Hess's Law problems are:
- Using Hess's law cycles
- Using equations
True or False?
When using cycles to solve Hess's Law problems, if you follow the direction of the arrow you subtract the quantity.
False.
When using cycles to solve Hess's Law problems, if you follow the direction of the arrow you add the quantity.
True or False?
In Hess's Law calculations, you always need to adjust for different molar amounts.
True.
In Hess's Law calculations, you always need to adjust for different molar amounts.
State the general equation for solving Hess's Law problems using equations.
The general equation for solving Hess's Law problems using equations is:
ΔH(reaction) = Σ(ΔH products) - Σ(ΔH reactants)
Use the Hess cycle to calculate the enthalpy change for the reaction.
The enthalpy change for the reaction is:
ΔH_r= - ( - 544 x 2) + (- 1648) = - 560 kJ

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What Drives Chemical Reactions?

How Much, How Fast & How Far?

What are the Mechanisms of Chemical Change?

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Bond	Average Bond Energy (kJ mol^-1)
N $\equiv$ N	945
H $-$ H	436
N $-$ H	391