Define enthalpy change .

An enthalpy change (symbol: Δ H ) is the heat energy transferred between a chemical system and its surroundings during a reaction at constant pressure. It can be positive (endothermic) or negative (exothermic).

In an exothermic reaction, what happens to the temperature of the surroundings and what is the sign of ΔH?

The temperature of the surroundings increases because heat is released to the surroundings. ΔH is negative , as the products have less energy than the reactants.

True or False? In an endothermic reaction, ΔH is positive.

True. In an endothermic reaction, heat is absorbed from the surroundings. The products have more energy than the reactants, so ΔH is positive.

In an exothermic reaction, the temperature of the .......... increases, while the temperature of the .......... decreases. ΔH is .......... .

In an exothermic reaction, the temperature of the surroundings increases, while the temperature of the system decreases. ΔH is negative .

Define exothermic reaction .

An exothermic reaction is one in which energy is released to the surroundings as heat. The products have less enthalpy than the reactants, so ΔH is negative and the temperature of the surroundings rises.

Why must the physical states of all species be specified when writing an enthalpy equation?

Changes of state involve large enthalpy changes . For example, dissolving NaCl (s) has ΔH = +4 kJ mol -1 , but dissociating NaCl (g) has ΔH = +500 kJ mol -1 . Omitting states makes the value ambiguous.

True or False? An exothermic reaction is always fast enough to observe in practice.

False. An exothermic reaction is thermodynamically favourable, but the rate may be too slow to observe if the activation energy is high. In that case, the reaction is kinetically controlled.

The .......... of a reaction refers to the molecules that are reacting, while the .......... refers to everything else, such as the flask and solvent.

The system of a reaction refers to the molecules that are reacting, while the surroundings refers to everything else, such as the flask and solvent.

Define endothermic reaction .

An endothermic reaction is one in which energy is absorbed from the surroundings as heat. The products have more enthalpy than the reactants, so ΔH is positive and the temperature of the surroundings falls.

Define activation energy .

Activation energy ( E a ) is the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction. It is the energy difference between the reactants and the transition state.

On a reaction pathway diagram, what does the transition state represent and why can it not be isolated?

The transition state is a short-lived , high-energy arrangement in which bonds are partially broken and partially formed . It is the highest point on the energy curve and is too unstable to isolate.

True or False? In an exothermic reaction, the products are higher in energy than the reactants.

False. In an exothermic reaction, the products are lower in energy than the reactants. ΔH is negative and heat is released to the surroundings.

On a reaction pathway diagram, the .......... change (ΔH) is the energy difference between .......... and .......... . The activation energy is the energy difference from reactants to the .......... .

On a reaction pathway diagram, the enthalpy change (ΔH) is the energy difference between reactants and products . The activation energy is the energy difference from reactants to the transition state .

Define transition state .

A transition state is the highest-energy arrangement of atoms during a reaction, where bonds are partially broken and partially formed. It is unstable and cannot be isolated.

How do you determine the activation energy of the reverse reaction from a reaction pathway diagram?

The E a of the reverse reaction is the energy difference from the energy level of the products to the top of the hump ( transition state ), measured in the reverse direction.

True or False? Endothermic reactions generally have a higher activation energy than exothermic reactions.

False. Activation energy is independent of whether a reaction is endothermic or exothermic — it is determined by the energy of the transition state relative to the reactants. An endothermic reaction simply has products at higher energy than the reactants; this does not necessarily mean a higher activation energy.

The combustion of methane has ΔH = .......... kJ mol -1 . Because ΔH is .......... , the products are drawn .......... in energy than the reactants on the diagram.

The combustion of methane has ΔH = −890 kJ mol -1 . Because ΔH is negative , the products are drawn lower in energy than the reactants on the diagram.

What two axes must be labelled on a reaction pathway diagram?

The y-axis should be labelled energy (or enthalpy) and the x-axis should be labelled reaction coordinate (or extent of reaction / progress of reaction).

Define standard enthalpy change of combustion .

A standard enthalpy change of combustion (Δ H θ c ) is the enthalpy change when one mole of a substance in its standard state is burnt completely in excess oxygen under standard conditions. It is always exothermic.

What are the standard conditions used for enthalpy measurements?

Standard conditions are a pressure of 101 kPa and a temperature of 298 K (25 °C) , with each substance in its normal physical state.

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

True. By definition, elements in their standard state are the reference point. No reaction is needed to form them, so Δ H θ f = 0 kJ mol -1 .

The standard enthalpy change of neutralisation (Δ H θ neut ) is the enthalpy change when .......... mole of .......... is formed by reacting an acid and an alkali. It is always .......... .

The standard enthalpy change of neutralisation (Δ H θ neut ) is the enthalpy change when one mole of water is formed by reacting an acid and an alkali. It is always exothermic .

Define standard enthalpy change of formation .

A standard enthalpy change of formation (Δ H θ f ) is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

The Δ H θ f [Fe 2 O 3 (s)] = -824.2 kJ mol -1 . What is Δ H for the reaction 4Fe (s) + 3O 2 (g) → 2Fe 2 O 3 (s)?

Δ H = 2 x (-824.2) = -1648.4 kJ mol -1 . The Δ H θ f value applies to one mole of Fe 2 O 3 , so multiply by 2.

True or False? The standard enthalpy change of formation can be either exothermic or endothermic.

True. Unlike combustion and neutralisation, formation enthalpy changes can be positive or negative depending on whether the compound is less or more stable than its elements.

The reaction C (graphite) + O 2 (g) → CO 2 (g) can be classified as both Δ H θ f and .......... because one mole of CO 2 is .......... from its elements and one mole of carbon is .......... completely in oxygen.

The reaction C (graphite) + O 2 (g) → CO 2 (g) can be classified as both Δ H θ f and Δ H θ c because one mole of CO 2 is formed from its elements and one mole of carbon is burnt completely in oxygen.

The Δ H θ r for H 2 (g) + ½O 2 (g) → H 2 O (l) is -286 kJ mol -1 . What is Δ H r for 2H 2 (g) + O 2 (g) → 2H 2 O (l)?

Δ H r = 2 x (-286) = -572 kJ mol -1 . Doubling the equation doubles the enthalpy change.

Define average bond energy .

An average bond energy is the average amount of energy required to break one mole of a particular type of covalent bond across a range of different molecules. It differs from exact bond dissociation energy because bond strength is affected by the surrounding atoms.

What is the formula used to calculate heat transferred in a calorimetry experiment?

q = m x c x Δ T , where m = mass of water (g), c = specific heat capacity (J g -1 °C -1 ) and Δ T = temperature change (°C or K).

True or False? Bond breaking is exothermic and bond forming is endothermic.

False. Bond breaking is endothermic (energy is absorbed to overcome attractive forces). Bond forming is exothermic (energy is released to the surroundings).

Δ H θ r = enthalpy change for bonds .......... + enthalpy change for bonds .......... . Values for bond breaking are .......... and values for bond forming are .......... .

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

Define specific heat capacity .

Specific heat capacity ( c ) is the energy required to raise the temperature of 1 g of a substance by 1 °C. For water, c = 4.18 J g -1 °C -1 .

In the Haber process (N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g)), bonds broken = +2253 kJ mol -1 and bonds formed = -2346 kJ mol -1 . What is ΔH θ r ?

Δ H θ r = +2253 + (-2346) = -93 kJ mol -1 . The negative value confirms the reaction is exothermic.

True or False? If more energy is released forming new bonds than is absorbed breaking bonds, the reaction is exothermic.

True. When bond forming releases more energy than bond breaking absorbs, the net energy change is negative and the reaction releases heat to the surroundings.

When using Δ H = - m x c x Δ T for a calorimetry experiment, a rise in temperature gives a .......... value of Δ H , indicating an .......... reaction.

When using Δ H = - m x c x Δ T for a calorimetry experiment, a rise in temperature gives a negative value of Δ H , indicating an exothermic reaction.

Why are average bond energies used instead of exact bond dissociation energies for most enthalpy calculations?

Bond strength is affected by the surrounding atoms in a molecule. Average bond energies account for this variation by averaging the energy across many different molecular environments for the same bond type.

A LevelChemistryCambridge (CIE)Flashcards5. Chemical EnergeticsEnthalpy Change, ΔH

Enthalpy Change, ΔH (Cambridge (CIE) A Level Chemistry): Flashcards

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Define enthalpy change.

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Define enthalpy change.
An enthalpy change (symbol: ΔH) is the heat energy transferred between a chemical system and its surroundings during a reaction at constant pressure. It can be positive (endothermic) or negative (exothermic).
In an exothermic reaction, what happens to the temperature of the surroundings and what is the sign of ΔH?
The temperature of the surroundings increases because heat is released to the surroundings. ΔH is negative, as the products have less energy than the reactants.
True or False?
In an endothermic reaction, ΔH is positive.
True.
In an endothermic reaction, heat is absorbed from the surroundings. The products have more energy than the reactants, so ΔH is positive.
In an exothermic reaction, the temperature of the .......... increases, while the temperature of the .......... decreases. ΔH is .......... .
In an exothermic reaction, the temperature of the surroundings increases, while the temperature of the system decreases. ΔH is negative.
Define exothermic reaction.
An exothermic reaction is one in which energy is released to the surroundings as heat. The products have less enthalpy than the reactants, so ΔH is negative and the temperature of the surroundings rises.
Why must the physical states of all species be specified when writing an enthalpy equation?
Changes of state involve large enthalpy changes. For example, dissolving NaCl (s) has ΔH = +4 kJ mol^-1, but dissociating NaCl (g) has ΔH = +500 kJ mol^-1. Omitting states makes the value ambiguous.
True or False?
An exothermic reaction is always fast enough to observe in practice.
False.
An exothermic reaction is thermodynamically favourable, but the rate may be too slow to observe if the activation energy is high. In that case, the reaction is kinetically controlled.
The .......... of a reaction refers to the molecules that are reacting, while the .......... refers to everything else, such as the flask and solvent.
The system of a reaction refers to the molecules that are reacting, while the surroundings refers to everything else, such as the flask and solvent.
Define endothermic reaction.
An endothermic reaction is one in which energy is absorbed from the surroundings as heat. The products have more enthalpy than the reactants, so ΔH is positive and the temperature of the surroundings falls.
Define activation energy.
Activation energy (E_a) is the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction. It is the energy difference between the reactants and the transition state.
On a reaction pathway diagram, what does the transition state represent and why can it not be isolated?
The transition state is a short-lived, high-energy arrangement in which bonds are partially broken and partially formed. It is the highest point on the energy curve and is too unstable to isolate.
True or False?
In an exothermic reaction, the products are higher in energy than the reactants.
False.
In an exothermic reaction, the products are lower in energy than the reactants. ΔH is negative and heat is released to the surroundings.
On a reaction pathway diagram, the .......... change (ΔH) is the energy difference between .......... and .......... . The activation energy is the energy difference from reactants to the .......... .
On a reaction pathway diagram, the enthalpy change (ΔH) is the energy difference between reactants and products. The activation energy is the energy difference from reactants to the transition state.
Define transition state.
A transition state is the highest-energy arrangement of atoms during a reaction, where bonds are partially broken and partially formed. It is unstable and cannot be isolated.
How do you determine the activation energy of the reverse reaction from a reaction pathway diagram?
The E_a of the reverse reaction is the energy difference from the energy level of the products to the top of the hump (transition state), measured in the reverse direction.
True or False?
Endothermic reactions generally have a higher activation energy than exothermic reactions.
False.
Activation energy is independent of whether a reaction is endothermic or exothermic — it is determined by the energy of the transition state relative to the reactants. An endothermic reaction simply has products at higher energy than the reactants; this does not necessarily mean a higher activation energy.
The combustion of methane has ΔH = .......... kJ mol^-1. Because ΔH is .......... , the products are drawn .......... in energy than the reactants on the diagram.
The combustion of methane has ΔH = −890 kJ mol^-1. Because ΔH is negative, the products are drawn lower in energy than the reactants on the diagram.
What two axes must be labelled on a reaction pathway diagram?
The y-axis should be labelled energy (or enthalpy) and the x-axis should be labelled reaction coordinate (or extent of reaction / progress of reaction).
Define standard enthalpy change of combustion.
A standard enthalpy change of combustion (ΔH^θ_c) is the enthalpy change when one mole of a substance in its standard state is burnt completely in excess oxygen under standard conditions. It is always exothermic.
What are the standard conditions used for enthalpy measurements?
Standard conditions are a pressure of 101 kPa and a temperature of 298 K (25 °C), with each substance in its normal physical state.
True or False?
The standard enthalpy change of formation of an element in its standard state is zero.
True.
By definition, elements in their standard state are the reference point. No reaction is needed to form them, so ΔH^θ_f = 0 kJ mol^-1.
The standard enthalpy change of neutralisation (ΔH^θ_neut) is the enthalpy change when .......... mole of .......... is formed by reacting an acid and an alkali. It is always .......... .
The standard enthalpy change of neutralisation (ΔH^θ_neut) is the enthalpy change when one mole of water is formed by reacting an acid and an alkali. It is always exothermic.
Define standard enthalpy change of formation.
A standard enthalpy change of formation (ΔH^θ_f) is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
The ΔH^θ_f [Fe₂O₃ (s)] = -824.2 kJ mol^-1. What is ΔH for the reaction 4Fe (s) + 3O₂ (g) → 2Fe₂O₃ (s)?
ΔH = 2 x (-824.2) = -1648.4 kJ mol^-1. The ΔH^θ_f value applies to one mole of Fe₂O₃, so multiply by 2.
True or False?
The standard enthalpy change of formation can be either exothermic or endothermic.
True.
Unlike combustion and neutralisation, formation enthalpy changes can be positive or negative depending on whether the compound is less or more stable than its elements.
The reaction C (graphite) + O₂ (g) → CO₂ (g) can be classified as both ΔH^θ_f and .......... because one mole of CO₂ is .......... from its elements and one mole of carbon is .......... completely in oxygen.
The reaction C (graphite) + O₂ (g) → CO₂ (g) can be classified as both ΔH^θ_f and ΔH^θ_c because one mole of CO₂ is formed from its elements and one mole of carbon is burnt completely in oxygen.
The ΔH^θ_r for H₂ (g) + ½O₂ (g) → H₂O (l) is -286 kJ mol^-1. What is ΔH_r for 2H₂ (g) + O₂ (g) → 2H₂O (l)?
ΔH_r = 2 x (-286) = -572 kJ mol^-1. Doubling the equation doubles the enthalpy change.
Define average bond energy.
An average bond energy is the average amount of energy required to break one mole of a particular type of covalent bond across a range of different molecules. It differs from exact bond dissociation energy because bond strength is affected by the surrounding atoms.
What is the formula used to calculate heat transferred in a calorimetry experiment?
q = m x c x ΔT, where m = mass of water (g), c = specific heat capacity (J g^-1 °C^-1) and ΔT = temperature change (°C or K).
True or False?
Bond breaking is exothermic and bond forming is endothermic.
False.
Bond breaking is endothermic (energy is absorbed to overcome attractive forces). Bond forming is exothermic (energy is released to the surroundings).
ΔH^θ_r = enthalpy change for bonds .......... + enthalpy change for bonds .......... . Values for bond breaking are .......... and values for bond forming are .......... .
ΔH^θ_r = enthalpy change for bonds broken + enthalpy change for bonds formed. Values for bond breaking are positive and values for bond forming are negative.
Define specific heat capacity.
Specific heat capacity (c) is the energy required to raise the temperature of 1 g of a substance by 1 °C. For water, c = 4.18 J g^-1 °C^-1.
In the Haber process (N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)), bonds broken = +2253 kJ mol^-1 and bonds formed = -2346 kJ mol^-1. What is ΔH^θ_r?
ΔH^θ_r = +2253 + (-2346) = -93 kJ mol^-1. The negative value confirms the reaction is exothermic.
True or False?
If more energy is released forming new bonds than is absorbed breaking bonds, the reaction is exothermic.
True.
When bond forming releases more energy than bond breaking absorbs, the net energy change is negative and the reaction releases heat to the surroundings.
When using ΔH = -m x c x ΔT for a calorimetry experiment, a rise in temperature gives a .......... value of ΔH, indicating an .......... reaction.
When using ΔH = -m x c x ΔT for a calorimetry experiment, a rise in temperature gives a negative value of ΔH, indicating an exothermic reaction.
Why are average bond energies used instead of exact bond dissociation energies for most enthalpy calculations?
Bond strength is affected by the surrounding atoms in a molecule. Average bond energies account for this variation by averaging the energy across many different molecular environments for the same bond type.

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