Enthalpy is the total chemical energy stored inside a substance, also called its heat content.

What does Δ H represent in a chemical reaction?

Δ H represents the enthalpy change : the difference in chemical energy between the products and reactants.

What is the sign of Δ H for an exothermic reaction, and why?

Δ H is negative because the products have less energy than the reactants, so the system loses energy to the surroundings.

What is the sign of Δ H for an endothermic reaction, and why?

Δ H is positive because the products have more energy than the reactants, so the system gains energy from the surroundings.

True or False? In an exothermic reaction, the temperature of the surroundings decreases.

False. In an exothermic reaction, heat is released to the surroundings, so the temperature of the surroundings increases .

Why might an exothermic reaction still not occur even though it is thermodynamically possible?

If the activation energy is too large, the reaction rate may be too slow. The reaction is then described as kinetically controlled .

Define activation energy

Activation energy is the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction.

Define transition state

A transition state is a highly unstable arrangement of atoms at the peak of a reaction profile, where bonds are partially broken and partially formed.

In a reaction profile diagram, the activation energy is the energy difference between the .......... and the .......... of the curve.

In a reaction profile diagram, the activation energy is the energy difference between the reactants and the peak of the curve.

What is the key difference between an enthalpy level diagram and a reaction profile diagram?

A reaction profile diagram includes the activation energy and shows possible transition states , while an enthalpy level diagram shows only the overall Δ H between reactants and products.

What are the standard conditions used for enthalpy measurements?

Standard conditions are a pressure of 100 kPa, a temperature of 298 K (25°C) and each substance in its standard physical state .

Define standard enthalpy of combustion

The standard enthalpy of combustion is the enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions.

Define standard enthalpy of formation

The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

True or False? The standard enthalpy of formation of an element in its standard state is zero.

True. Elements in their standard state are the reference point for enthalpy measurements, so their Δ H f Ꝋ = 0 kJ mol -1 .

Define standard enthalpy of neutralisation

The standard enthalpy of neutralisation is the enthalpy change when one mole of water is formed from the reaction between an acid and an alkali under standard conditions.

Define standard enthalpy of atomisation

The standard enthalpy of atomisation is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state under standard conditions.

The symbol .......... is added to Δ H to show that a reaction was carried out under .......... conditions.

The symbol Ꝋ is added to Δ H to show that a reaction was carried out under standard conditions.

If the standard enthalpy of formation of one mole of water is -286 kJ mol -1 , what is Δ H Ꝋ for the formation of two moles of water?

Δ H Ꝋ = 2 × (-286) = -572 kJ mol -1 . The enthalpy change scales proportionally with the number of moles produced.

Why must physical states be specified in equations for enthalpy changes?

Changes of state involve significant enthalpy changes , so omitting states could give a very different value for Δ H .

Calorimetry is the experimental measurement of enthalpy changes in chemical reactions.

What equation is used to calculate the energy transferred in a calorimetry experiment?

The equation is q = m × c × Δ T , where q is the energy transferred, m is the mass, c is the specific heat capacity and Δ T is the temperature change.

What is the specific heat capacity of water and what are its units?

The specific heat capacity of water is 4.18 J g -1 K -1 . It is the energy required to raise the temperature of 1 g of water by 1 K.

True or False? When the temperature of the solution rises during a calorimetry experiment, Δ H is positive.

False. A rise in temperature indicates an exothermic reaction, so Δ H is negative .

Why is a temperature correction graph used in calorimetry?

Some reactions are not instantaneous, so heat is lost before the maximum temperature is reached. Extrapolating the cooling line back to the time of mixing gives the true maximum temperature change.

How is the enthalpy change per mole calculated from the energy transferred in a calorimetry experiment?

Δ H = q / n , where q is the energy transferred in kJ and n is the number of moles of the limiting reagent .

In a solution calorimetry experiment, the density of the solution is assumed to be .......... g cm -3 and the specific heat capacity is assumed to be .......... J g -1 K -1 .

In a solution calorimetry experiment, the density of the solution is assumed to be 1 g cm -3 and the specific heat capacity is assumed to be 4.18 J g -1 K -1 .

State two main sources of error in a combustion calorimetry experiment.

Heat loss to the surroundings Incomplete combustion of the fuel

1.50 g of an organic liquid ( M r = 58.0) is burned completely, heating 100 g of water from 20°C to 75°C. Calculate the enthalpy of combustion.

Step 1: q = 100 × 4.18 × 55 = 22 990 J = 22.99 kJ Step 2: n = 1.50 / 58.0 = 0.0259 mol Step 3: Δ H = -22.99 / 0.0259 = -887 kJ mol -1 The value is negative as combustion is exothermic .

Hess's Law states that the total enthalpy change of a chemical reaction is independent of the route taken, provided the initial and final conditions are the same.

What fundamental law underpins Hess's Law?

Hess's Law is based on the Law of Conservation of Energy : energy cannot be created or destroyed, only changed in form.

Why is Hess's Law needed to calculate the enthalpy of formation of propane from its elements?

Hydrogen and carbon do not react together under standard conditions, so the enthalpy of formation of propane cannot be measured directly by calorimetry.

True or False? The enthalpy change of formation of any element in its standard state is zero.

True. Elements in their standard states are the reference point for enthalpy of formation, so their Δ H f Ꝋ is defined as 0 kJ mol -1 .

In a Hess cycle using enthalpies of formation, which direction do the arrows point and what does it mean to reverse an arrow?

Arrows point upwards from elements to each compound. If the cycle travels against an arrow, the sign of that enthalpy value must be reversed .

In a Hess cycle using enthalpies of combustion, the combustion products are written .......... the equation and arrows point .......... from each substance.

In a Hess cycle using enthalpies of combustion, the combustion products are written below the equation and arrows point downwards from each substance.

Using Δ H c Ꝋ values: C (s) = -394, H 2 (g) = -286, CH 3 COCH 3 (l) = -1821 kJ mol -1 , calculate Δ H f Ꝋ of propanone.

Δ H f Ꝋ = 3(-394) + 3(-286) - (-1821) = -1182 - 858 + 1821 = -219 kJ mol -1

True or False? When a Hess cycle travels in the same direction as an arrow, you reverse the sign of that enthalpy value.

False. You only reverse the sign when the cycle travels against the direction of the arrow.

State the relationship used to find Δ H r from a Hess cycle.

Δ H r = Δ H 2 − Δ H 1 , where Δ H 2 is the direct route from elements to products and Δ H 1 is the route from elements to reactants.

Define bond dissociation enthalpy

The bond dissociation enthalpy is the energy required to break one mole of a specific bond in a gaseous molecule into gaseous atoms.

True or False? Breaking a chemical bond is an exothermic process.

False. Bond breaking is endothermic : energy must be supplied to overcome the force of attraction between the atoms.

Why is an average bond enthalpy used rather than a specific bond enthalpy?

The same type of bond (e.g. C-H) has slightly different energies in different molecular environments. An average bond enthalpy is calculated across similar compounds to give a more representative value.

Define average bond enthalpy

The average bond enthalpy is the energy needed to break one mole of a particular bond in a gaseous molecule, averaged over a range of similar compounds.

How is the overall enthalpy change of a reaction found using bond enthalpies?

Δ H r Ꝋ = enthalpy change for bonds broken + enthalpy change for bonds formed . Bond breaking values are positive and bond forming values are negative.

In a bond enthalpy calculation, values for bonds .......... are positive and values for bonds .......... are negative.

In a bond enthalpy calculation, values for bonds broken are positive and values for bonds formed are negative.

What sign does bond breaking have in a bond enthalpy calculation and why?

Bond breaking has a positive sign because it is an endothermic process: energy must be absorbed from the surroundings to break the bond.

True or False? If more energy is released forming bonds than is required to break them, the reaction is endothermic.

False. If bond forming releases more energy than bond breaking requires, the reaction is exothermic and the products are more stable than the reactants.

In the Haber process, the total enthalpy for bonds broken is +2253 kJ mol -1 and for bonds formed is -2346 kJ mol -1 . Calculate Δ H r Ꝋ .

Δ H r Ꝋ = (+2253) + (-2346) = -93 kJ mol -1 The reaction is exothermic as more energy is released forming bonds than is required to break them.

International A Level (IAL)ChemistryEdexcelFlashcards2. Energetics, Group Chemistry, Halogenoalkanes & AlcoholsEnergetics

Energetics (Edexcel International A Level (IAL) Chemistry): Flashcards

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Define enthalpy
Enthalpy is the total chemical energy stored inside a substance, also called its heat content.
What does ΔH represent in a chemical reaction?
ΔH represents the enthalpy change: the difference in chemical energy between the products and reactants.
What is the sign of ΔH for an exothermic reaction, and why?
ΔH is negative because the products have less energy than the reactants, so the system loses energy to the surroundings.
What is the sign of ΔH for an endothermic reaction, and why?
ΔH is positive because the products have more energy than the reactants, so the system gains energy from the surroundings.
True or False?
In an exothermic reaction, the temperature of the surroundings decreases.
False.
In an exothermic reaction, heat is released to the surroundings, so the temperature of the surroundings increases.
Why might an exothermic reaction still not occur even though it is thermodynamically possible?
If the activation energy is too large, the reaction rate may be too slow. The reaction is then described as kinetically controlled.
Define activation energy
Activation energy is the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction.
Define transition state
A transition state is a highly unstable arrangement of atoms at the peak of a reaction profile, where bonds are partially broken and partially formed.
In a reaction profile diagram, the activation energy is the energy difference between the .......... and the .......... of the curve.
In a reaction profile diagram, the activation energy is the energy difference between the reactants and the peak of the curve.
What is the key difference between an enthalpy level diagram and a reaction profile diagram?
A reaction profile diagram includes the activation energy and shows possible transition states, while an enthalpy level diagram shows only the overall ΔH between reactants and products.
What are the standard conditions used for enthalpy measurements?
Standard conditions are a pressure of 100 kPa, a temperature of 298 K (25°C) and each substance in its standard physical state.
Define standard enthalpy of combustion
The standard enthalpy of combustion is the enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions.
Define standard enthalpy of formation
The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
True or False?
The standard enthalpy of formation of an element in its standard state is zero.
True.
Elements in their standard state are the reference point for enthalpy measurements, so their ΔH_f^Ꝋ = 0 kJ mol^-1.
Define standard enthalpy of neutralisation
The standard enthalpy of neutralisation is the enthalpy change when one mole of water is formed from the reaction between an acid and an alkali under standard conditions.
Define standard enthalpy of atomisation
The standard enthalpy of atomisation is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state under standard conditions.
The symbol .......... is added to ΔH to show that a reaction was carried out under .......... conditions.
The symbol Ꝋ is added to ΔH to show that a reaction was carried out under standard conditions.
If the standard enthalpy of formation of one mole of water is -286 kJ mol^-1, what is ΔH^Ꝋ for the formation of two moles of water?
ΔH^Ꝋ = 2 × (-286) = -572 kJ mol^-1.
The enthalpy change scales proportionally with the number of moles produced.
Why must physical states be specified in equations for enthalpy changes?
Changes of state involve significant enthalpy changes, so omitting states could give a very different value for ΔH.
Define calorimetry
Calorimetry is the experimental measurement of enthalpy changes in chemical reactions.
What equation is used to calculate the energy transferred in a calorimetry experiment?
The equation is q = m × c × ΔT, where q is the energy transferred, m is the mass, c is the specific heat capacity and ΔT is the temperature change.
What is the specific heat capacity of water and what are its units?
The specific heat capacity of water is 4.18 J g^-1 K^-1.
It is the energy required to raise the temperature of 1 g of water by 1 K.
True or False?
When the temperature of the solution rises during a calorimetry experiment, ΔH is positive.
False.
A rise in temperature indicates an exothermic reaction, so ΔH is negative.
Why is a temperature correction graph used in calorimetry?
Some reactions are not instantaneous, so heat is lost before the maximum temperature is reached. Extrapolating the cooling line back to the time of mixing gives the true maximum temperature change.
How is the enthalpy change per mole calculated from the energy transferred in a calorimetry experiment?
ΔH = q / n, where q is the energy transferred in kJ and n is the number of moles of the limiting reagent.
In a solution calorimetry experiment, the density of the solution is assumed to be .......... g cm^-3 and the specific heat capacity is assumed to be .......... J g^-1 K^-1.
In a solution calorimetry experiment, the density of the solution is assumed to be 1 g cm^-3 and the specific heat capacity is assumed to be 4.18 J g^-1 K^-1.
State two main sources of error in a combustion calorimetry experiment.
1. Heat loss to the surroundings
2. Incomplete combustion of the fuel
1.50 g of an organic liquid (M_r = 58.0) is burned completely, heating 100 g of water from 20°C to 75°C. Calculate the enthalpy of combustion.
Step 1: q = 100 × 4.18 × 55 = 22 990 J = 22.99 kJ
Step 2: n = 1.50 / 58.0 = 0.0259 mol
Step 3: ΔH = -22.99 / 0.0259 = -887 kJ mol^-1
The value is negative as combustion is exothermic.
Define Hess's Law
Hess's Law states that the total enthalpy change of a chemical reaction is independent of the route taken, provided the initial and final conditions are the same.
What fundamental law underpins Hess's Law?
Hess's Law is based on the Law of Conservation of Energy: energy cannot be created or destroyed, only changed in form.
Why is Hess's Law needed to calculate the enthalpy of formation of propane from its elements?
Hydrogen and carbon do not react together under standard conditions, so the enthalpy of formation of propane cannot be measured directly by calorimetry.
True or False?
The enthalpy change of formation of any element in its standard state is zero.
True.
Elements in their standard states are the reference point for enthalpy of formation, so their ΔH_f^Ꝋ is defined as 0 kJ mol^-1.
In a Hess cycle using enthalpies of formation, which direction do the arrows point and what does it mean to reverse an arrow?
Arrows point upwards from elements to each compound.
If the cycle travels against an arrow, the sign of that enthalpy value must be reversed.
In a Hess cycle using enthalpies of combustion, the combustion products are written .......... the equation and arrows point .......... from each substance.
In a Hess cycle using enthalpies of combustion, the combustion products are written below the equation and arrows point downwards from each substance.
Using ΔH_c^Ꝋ values: C (s) = -394, H₂ (g) = -286, CH₃COCH₃ (l) = -1821 kJ mol^-1, calculate ΔH_f^Ꝋ of propanone.
ΔH_f^Ꝋ = 3(-394) + 3(-286) - (-1821)
= -1182 - 858 + 1821
= -219 kJ mol^-1
True or False?
When a Hess cycle travels in the same direction as an arrow, you reverse the sign of that enthalpy value.
False.
You only reverse the sign when the cycle travels against the direction of the arrow.
State the relationship used to find ΔH_r from a Hess cycle.
ΔH_r = ΔH₂ − ΔH₁, where ΔH₂ is the direct route from elements to products and ΔH₁ is the route from elements to reactants.
Define bond dissociation enthalpy
The bond dissociation enthalpy is the energy required to break one mole of a specific bond in a gaseous molecule into gaseous atoms.
True or False?
Breaking a chemical bond is an exothermic process.
False.
Bond breaking is endothermic: energy must be supplied to overcome the force of attraction between the atoms.
Why is an average bond enthalpy used rather than a specific bond enthalpy?
The same type of bond (e.g. C-H) has slightly different energies in different molecular environments. An average bond enthalpy is calculated across similar compounds to give a more representative value.
Define average bond enthalpy
The average bond enthalpy is the energy needed to break one mole of a particular bond in a gaseous molecule, averaged over a range of similar compounds.
How is the overall enthalpy change of a reaction found using bond enthalpies?
ΔH_r^Ꝋ = enthalpy change for bonds broken + enthalpy change for bonds formed.
Bond breaking values are positive and bond forming values are negative.
In a bond enthalpy calculation, values for bonds .......... are positive and values for bonds .......... are negative.
In a bond enthalpy calculation, values for bonds broken are positive and values for bonds formed are negative.
What sign does bond breaking have in a bond enthalpy calculation and why?
Bond breaking has a positive sign because it is an endothermic process: energy must be absorbed from the surroundings to break the bond.
True or False?
If more energy is released forming bonds than is required to break them, the reaction is endothermic.
False.
If bond forming releases more energy than bond breaking requires, the reaction is exothermic and the products are more stable than the reactants.
In the Haber process, the total enthalpy for bonds broken is +2253 kJ mol^-1 and for bonds formed is -2346 kJ mol^-1. Calculate ΔH_r^Ꝋ.
ΔH_r^Ꝋ = (+2253) + (-2346)
= -93 kJ mol^-1
The reaction is exothermic as more energy is released forming bonds than is required to break them.

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