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Thermochemistry

AP Chemistry · Topic 6

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6.1

Endothermic and Exothermic Processes
Syllabus
Learning ObjectiveEssential Knowledge

6.1.A
Explain the relationship between experimental observations and energy changes associated with a chemical or physical transformation.

  • 6.1.A.1 Temperature changes in a system indicate energy changes.
  • 6.1.A.2 Energy changes in a system can be described as endothermic and exothermic processes such as the heating or cooling of a substance, phase changes, or chemical transformations.
  • 6.1.A.3 When a chemical reaction occurs, the energy of the system either decreases (exothermic reaction), increases (endothermic reaction), or remains the same. For exothermic reactions, the energy lost by the reacting species (system) is gained by the surroundings, as heat transfer from or work done by the system. Likewise, for endothermic reactions, the system gains energy from the surroundings by heat transfer to or work done on the system.
  • 6.1.A.4 The formation of a solution may be an exothermic or endothermic process, depending on the relative strengths of intermolecular/interparticle interactions before and after the dissolution process.

Source: College Board AP Course and Exam Description

Thermochemistry 热化学 tracks energy in reactions. A process is exothermic 放热 if it releases energy to the surroundings (feels hot, $\Delta H<0$) and endothermic 吸热 if it absorbs energy (feels cold, $\Delta H>0$). Breaking bonds costs energy; forming bonds releases it – the net decides the sign.

An exothermic reaction warms the surroundings; an endothermic one cools them An exothermic reaction warms the surroundings; an endothermic one cools them

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Compare endothermic and exothermic profiles

An exothermic reaction releases energy (products lower than reactants, $\Delta H<0$); an endothermic one absorbs it. The hump is the activation energy.

Vocabulary Train
English Chinese Pinyin
Thermochemistry 热化学 rè huà xué
exothermic 放热 fàng rè
endothermic 吸热 xī rè
Exercise sheet
6.2

Energy Diagrams
Syllabus
Learning ObjectiveEssential Knowledge

6.2.A
Represent a chemical or physical transformation with an energy diagram.

  • 6.2.A.1 A physical or chemical process can be described with an energy diagram that shows the endothermic or exothermic nature of that process.

Source: College Board AP Course and Exam Description

An energy diagram plots energy from reactants to products. Reactants above products means exothermic; below means endothermic. The vertical gap between them is the enthalpy change $\Delta H$.

Energy diagrams: exothermic products sit below the reactants, endothermic above Energy diagrams: exothermic products sit below the reactants, endothermic above

Exercise sheet
6.3

Heat Transfer and Thermal Equilibrium
Syllabus
Learning ObjectiveEssential Knowledge

6.3.A
Explain the relationship between the transfer of thermal energy and molecular collisions.

  • 6.3.A.1 The particles in a warmer body have a greater average kinetic energy than those in a cooler body.
  • 6.3.A.2 Collisions between particles in thermal contact can result in the transfer of energy. This process is called "heat transfer," "heat exchange," or "transfer of energy as heat."
  • 6.3.A.3 Eventually, thermal equilibrium is reached as the particles continue to collide. At thermal equilibrium, the average kinetic energy of both bodies is the same, and hence, their temperatures are the same.

Source: College Board AP Course and Exam Description

Heat 热量 flows from hot to cold until objects reach thermal equilibrium 热平衡 (equal temperature). Energy is conserved: the heat lost by the hot object equals the heat gained by the cold one.

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English Chinese Pinyin
Heat 热量 rè liàng
thermal equilibrium 热平衡 rè píng héng
Exercise sheet
6.4

Heat Capacity and Calorimetry
Syllabus
Learning ObjectiveEssential Knowledge

6.4.A
Calculate the heat $q$ absorbed or released by a system undergoing heating/cooling based on the amount of the substance, the heat capacity, and the change in temperature.

  • 6.4.A.1 The heating of a cool body by a warmer body is an important form of energy transfer between two systems. The amount of heat transferred between two bodies may be quantified by the heat transfer equation:

    • Equation: $q = mc\Delta T$.

    Calorimetry experiments are used to measure the transfer of heat.

  • 6.4.A.2 The first law of thermodynamics states that energy is conserved in chemical and physical processes.

  • 6.4.A.3 The transfer of a given amount of thermal energy will not produce the same temperature change in equal masses of matter with differing specific heat capacities.

  • 6.4.A.4 Heating a system increases the energy of the system, while cooling a system decreases the energy of the system.

  • 6.4.A.5 The specific heat capacity of a substance and the molar heat capacity are both used in energy calculations.

  • 6.4.A.6 Chemical systems change their energy through three main processes: heating/cooling, phase transitions, and chemical reactions.

  • 6.4.A.7 In calorimetry experiments involving dissolution, temperature changes of the mixture within the calorimeter can be used to determine the direction of energy flow. If the temperature of the mixture increases, thermal energy is released by the dissolution process (exothermic). If the temperature of the mixture decreases, thermal energy is absorbed by the dissolution process (endothermic).

Source: College Board AP Course and Exam Description

The heat to change a substance's temperature is

$$q=mc\,\Delta T,$$
where $c$ is the specific heat 比热容 (energy per gram per degree). Calorimetry 量热法 measures a reaction's heat by tracking the temperature change of surrounding water: the heat the water gains equals the heat the reaction releases.

Calorimetry: measure the temperature change of a known mass of solution Calorimetry: measure the temperature change of a known mass of solution

Worked example. A reaction in a coffee-cup calorimeter warms $100\ \text{g}$ of water by $8.0\,{}^{\circ}\text{C}$ ($c=4.18\ \text{J/(g}\,{}^{\circ}\text{C)}$). The heat absorbed by the water is

$$q=mc\,\Delta T=100\times4.18\times8.0=3.3\times10^{3}\ \text{J}.$$
By energy conservation the reaction released this $3.3\ \text{kJ}$, so it is exothermic ($q_{\text{rxn}}=-3.3\ \text{kJ}$).

Explore

Heat different materials

$Q=mc\Delta T$: a high specific heat (like water's) means a lot of energy for a small temperature rise. Compare materials for the same heat input.

Vocabulary Train
English Chinese Pinyin
specific heat 比热容 bǐ rè róng
Calorimetry 量热法 liàng rè fǎ
Exercise sheet
6.5

Energy of Phase Changes
Syllabus
Learning ObjectiveEssential Knowledge

6.5.A
Explain changes in the heat $q$ absorbed or released by a system undergoing a phase transition based on the amount of the substance in moles and the molar enthalpy of the phase transition.

  • 6.5.A.1 Energy must be transferred to a system to cause a substance to melt (or boil). The energy of the system therefore increases as the system undergoes a solid-to-liquid (or liquid-to-gas) phase transition. Likewise, a system releases energy when it freezes (or condenses). The energy of the system decreases as the system undergoes a liquid-to-solid (or gas-to-liquid) phase transition. The temperature of a pure substance remains constant during a phase change.
  • 6.5.A.2 The energy absorbed during a phase change is equal to the energy released during a complementary phase change in the opposite direction. For example, the molar enthalpy of condensation of a substance is equal to the negative of its molar enthalpy of vaporization. Similarly, the molar enthalpy of fusion can be used to calculate the energy absorbed when melting a substance and the energy released when freezing a substance.

Source: College Board AP Course and Exam Description

During a phase change 相变 (melting, boiling) the temperature stays constant while heat goes into breaking intermolecular forces, not raising kinetic energy. The energy needed is $q=n\,\Delta H_{\text{fus}}$ (melting) or $q=n\,\Delta H_{\text{vap}}$ (boiling) – the flat steps on a heating curve.

Explore

Heat through a phase change

During a phase change the temperature holds flat while energy breaks bonds — the latent heat. Watch the plateaus at melting and boiling.

Vocabulary Train
English Chinese Pinyin
phase change 相变 xiāng biàn
Exercise sheet
6.6

Introduction to Enthalpy of Reaction
Syllabus
Learning ObjectiveEssential Knowledge

6.6.A
Calculate the heat $q$ absorbed or released by a system undergoing a chemical reaction in relationship to the amount of the reacting substance in moles and the molar enthalpy of reaction.

  • 6.6.A.1 The enthalpy change of a reaction gives the amount of heat energy released (for negative values) or absorbed (for positive values) by a chemical reaction at constant pressure.
  • 6.6.A.2 When the products of a reaction are at a different temperature than their surroundings, they exchange energy with the surroundings to reach thermal equilibrium. Thermal energy is transferred to the surroundings as the reactants convert to products in an exothermic reaction. Thermal energy is transferred from the surroundings as the reactants convert to products in an endothermic reaction.
  • 6.6.A.3 The chemical potential energy of the products of a reaction is different from that of the reactants because of the breaking and forming of bonds. The energy difference results in a change in the kinetic energy of the particles, which manifests as a temperature change.
    • Exclusion Statement: The technical distinctions between enthalpy and internal energy will not be assessed on the AP Exam. Most reactions studied at the AP level are carried out at constant pressure, where the enthalpy change of the process is equal to the heat (and by extension, the energy) of reaction.

Source: College Board AP Course and Exam Description

The enthalpy of reaction 反应焓 $\Delta H_{\text{rxn}}$ is the heat released or absorbed at constant pressure. Because enthalpy is a state function 状态函数, $\Delta H$ depends only on the initial and final states, not the path taken – the key that makes the next three methods work.

Vocabulary Train
English Chinese Pinyin
enthalpy of reaction 反应焓 fǎn yìng hán
state function 状态函数 zhuàng tài hán shù
Exercise sheet
6.7

Bond Enthalpies
Syllabus
Learning ObjectiveEssential Knowledge

6.7.A
Calculate the enthalpy change of a reaction based on the average bond energies of bonds broken and formed in the reaction.

  • 6.7.A.1 During a chemical reaction, bonds are broken and/or formed, and these events change the potential energy of the system.
  • 6.7.A.2 The average energy required to break all of the bonds in the reactant molecules can be estimated by adding up the average bond energies of all the bonds in the reactant molecules. Likewise, the average energy released in forming the bonds in the product molecules can be estimated. If the energy released is greater than the energy required, the reaction is exothermic. If the energy required is greater than the energy released, the reaction is endothermic.

Source: College Board AP Course and Exam Description

One way to estimate $\Delta H$: sum the energy to break all reactant bonds, then subtract the energy released forming product bonds:

$$\Delta H \approx \sum (\text{bonds broken}) - \sum (\text{bonds formed}).$$
This is an approximation, since bond enthalpies are averages.

Breaking bonds takes in energy; making bonds releases it Breaking bonds takes in energy; making bonds releases it

Worked example. Estimate $\Delta H$ for $\text{H}_2+\text{Cl}_2\rightarrow2\text{HCl}$ using bond enthalpies H–H $=436$, Cl–Cl $=242$, H–Cl $=431\ \text{kJ/mol}$. Break both reactant bonds ($436+242=678$) and form two H–Cl bonds ($2\times431=862$):

$$\Delta H\approx 678-862=-184\ \text{kJ},$$
exothermic, because the strong H–Cl bonds formed release more than the reactant bonds cost.

Exercise sheet
6.8

Enthalpy of Formation
Syllabus
Learning ObjectiveEssential Knowledge

6.8.A
Calculate the enthalpy change for a chemical or physical process based on the standard enthalpies of formation.

  • 6.8.A.1 Tables of standard enthalpies of formation can be used to calculate the standard enthalpies of reactions.
    • Equation: $\Delta H^{\circ}_{reaction} = \Sigma \Delta H^{\circ}_{f\ products} - \Sigma \Delta H^{\circ}_{f\ reactants}$

Source: College Board AP Course and Exam Description

The standard enthalpy of formation 生成焓 $\Delta H_f^\circ$ is the enthalpy to make one mole of a compound from its elements (zero for an element in its standard state). Then

$$\Delta H_{\text{rxn}}^\circ = \sum \Delta H_f^\circ(\text{products}) - \sum \Delta H_f^\circ(\text{reactants}).$$

Formation makes a compound from its elements; combustion burns it in oxygen Formation makes a compound from its elements; combustion burns it in oxygen

Worked example. Find $\Delta H_{\text{rxn}}^\circ$ for burning methane, $\text{CH}_4+2\text{O}_2\rightarrow\text{CO}_2+2\text{H}_2\text{O}$, given $\Delta H_f^\circ$: CH$_4=-75$, CO$_2=-394$, H$_2$O$=-286\ \text{kJ/mol}$ (O$_2=0$). Products minus reactants:

$$\Delta H_{\text{rxn}}^\circ=[-394+2(-286)]-[-75+0]=-966+75=-891\ \text{kJ},$$
a large release, as expected for a combustion.

Vocabulary Train
English Chinese Pinyin
standard enthalpy of formation 生成焓 shēng chéng hán
Exercise sheet
6.9

Hess's Law
Syllabus
Learning ObjectiveEssential Knowledge

6.9.A
Represent a chemical or physical process as a sequence of steps.

  • 6.9.A.1 Many processes can be broken down into a series of steps. Each step in the series has its own energy change.

6.9.B
Explain the relationship between the enthalpy of a chemical or physical process and the sum of the enthalpies of the individual steps.

  • 6.9.B.1 Because total energy is conserved (first law of thermodynamics), and each individual reaction in a sequence transfers thermal energy to or from the surroundings, the net thermal energy transferred in the sequence will be equal to the sum of the thermal energy transfers in each of the steps. These thermal energy transfers are the result of potential energy changes among the species in the reaction sequence; thus, at constant pressure, the enthalpy change of the overall process is equal to the sum of the enthalpy changes of the individual steps.
  • 6.9.B.2 The following are essential principles of Hess's law:
    • i. When a reaction is reversed, the enthalpy change stays constant in magnitude but becomes reversed in mathematical sign.
    • ii. When a reaction is multiplied by a factor $c$, the enthalpy change is multiplied by the same factor $c$.
    • iii. When two (or more) reactions are added to obtain an overall reaction, the individual enthalpy changes of each reaction are added to obtain the net enthalpy change of the overall reaction.
    • Exclusion Statement: The concept of state functions will not be assessed on the AP Exam.

Source: College Board AP Course and Exam Description

Hess's law: the route does not matter

Hess's law 盖斯定律: if a reaction is the sum of several steps, its $\Delta H$ is the sum of the steps' $\Delta H$ values. Reverse a step and flip the sign; scale a step and scale its $\Delta H$. This lets you find a hard-to-measure $\Delta H$ by combining known reactions – a frequent exam calculation.

Hess's law: the direct and indirect routes give the same total enthalpy change Hess's law: the direct and indirect routes give the same total enthalpy change

Vocabulary Train
English Chinese Pinyin
Hess's law 盖斯定律 gài sī dìng lǜ
Exercise sheet
6.9

Exam tips
  • An exothermic reaction has $\Delta H<0$ (feels hot); endothermic has $\Delta H>0$ — always give $\Delta H$ a sign and units.
  • In calorimetry use $q=mc\,\Delta T$ with the mass of the water/solution; the reaction releases what the water gains.
  • Bond enthalpies: $\Delta H\approx\sum(\text{bonds broken})-\sum(\text{bonds made})$ — breaking is endothermic, making is exothermic (the classic sign trap).
  • Hess's law: the total $\Delta H$ is the sum of the steps' — reverse a step and flip the sign, scale a step and scale $\Delta H$.
  • During a phase change the temperature stays constant while energy goes into the forces between particles.

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