Chapter #7: Thermochemistry

What is thermochemistry?

Thermochemistry is the study of the relationships between chemistry and energy.

What is energy?

Energy is anything that has the capacity to do work.

What variable describes energy?

Internal energy is described by the variable E (or ΔE for energy change).

What is work?

Work is defined as a force acting over a distance.

What variable describes work?

Work is represented by the variable w.

What is heat?

Heat is the flow of energy caused by a difference in temperature.

What variable describes heat?

Thermal heat energy exchanged is represented by q, while total internal energy is represented by E.

How do energy, work, and heat relate to each other?

Energy is exchanged between a system and its surroundings through heat (q) and work (w), related by ΔE = q + w.

How do kinetic and potential energy relate to each other?

Kinetic energy (energy of motion) and potential energy (energy of position/composition) together comprise total energy.

What is thermal energy?

Thermal energy is energy associated with temperature.

How does thermal energy relate to kinetic energy?

Thermal energy is a form of kinetic energy because it arises from the movement of atoms and molecules.

What is chemical energy?

Chemical energy is energy associated with the positions of electrons and nuclei.

How does chemical energy relate to potential energy?

Chemical energy is a form of potential energy stored within the structure and bonds of molecules.

What does the law of conservation of energy state?

The law states that energy cannot be created or destroyed, only transferred or converted.

What is a system in thermochemistry?

A system is the specific material or process within which energy changes are studied.

What are the surroundings in thermochemistry?

The surroundings consist of everything else with which the system can exchange energy.

How does energy exchange between the system and surroundings relate to each other?

Energy lost by the system is gained by the surroundings, and vice versa (ΔEsystem = −ΔEsurroundings).

What is the equation for kinetic energy?

KE = ½mv².

What does each variable in the kinetic energy equation mean?

m represents mass; v represents velocity.

What is a joule?

A joule (J) is the SI unit of energy.

What is the equation for a joule?

1 J = 1 kg·m²/s².

What is a calorie?

A calorie (cal) is the heat required to raise the temperature of 1 g of water by 1°C.

What is a kilocalorie?

A kilocalorie (kcal or Cal) is the heat required to raise the temperature of 1000 g (1 kg) of water by 1°C.

How do calories relate to joules?

1 cal = 4.184 J; 1 kcal = 4184 J.

What is the application of the calorie?

Calories quantify energy transfer in chemical reactions, biological metabolism, and nutrition.

What is thermodynamics?

Thermodynamics is the study of energy and its interconversions.

How do the calorie and joule relate to thermodynamics?

Calories and joules are standard energy units used to quantify thermodynamic heat and work.

What is the first law of thermodynamics?

The first law states that the total amount of energy in the universe is constant.

How does the first law relate to the law of conservation of energy?

The first law is the application of the law of conservation of energy to thermodynamic systems.

What is internal energy?

Internal energy (E) is the sum of the kinetic and potential energies of all particles in a system.

What is a state function?

A state function is a property whose value depends only on the initial and final states, not the path taken.

What are some examples of state functions?

Internal energy (E), Enthalpy (H), Pressure (P), Volume (V), Altitude.

What is the equation for a state function?

ΔE = Efinal − Einitial; ΔErxn = Eproducts − Ereactants.

How does the first law of thermodynamics relate to conservation of energy?

It expresses energy conservation as: ΔEuniverse = 0 = ΔEsystem + ΔEsurroundings.

How do the system and surroundings relate to the first law of thermodynamics?

Energy gained by the system equals the energy lost by the surroundings.

How does this relate to chemical reactions?

During a reaction: ΔErxn = −ΔEsurroundings.

How do high and low internal energy relate to the system and surroundings?

If reactants have higher energy than products: ΔEsys < 0, and energy flows to the surroundings. If reactants have lower energy than products: ΔEsys > 0, and energy flows into the system.

How do heat energy and work relate to energy exchange?

Systems exchange energy with their surroundings through heat (q) and work (w). Together, they determine the change in internal energy: ΔE = q + w.

What is temperature's role in heat transfer?

Temperature measures thermal energy and determines the direction of heat flow.

How does heat transfer relate to thermal energy?

Heat naturally transfers from regions of higher thermal energy to regions of lower thermal energy.

What is thermal equilibrium?

Heat continues flowing until both objects reach the same temperature.

What is the heat transfer equation?

q = m × Cs × ΔT.

What does q represent and what are its units?

q = heat energy; Units: J.

What does m represent and what are its units?

m = mass; Units: g.

What does Cs represent and what are its units?

Cs = specific heat capacity; Units: J/(g·°C).

What does ΔT represent and what are its units?

ΔT = Tfinal − Tinitial; Units: °C.

How does heat capacity affect heat transfer?

A larger heat capacity results in a smaller temperature change for the same amount of heat absorbed.

What is specific heat capacity?

The heat required to raise the temperature of 1 g of a substance by 1°C.

What is molar heat capacity?

The heat required to raise the temperature of 1 mole of a substance by 1°C.

How are specific heat capacity and molar heat capacity similar and different?

Both measure a substance's ability to absorb heat. Specific heat capacity is measured per gram; molar heat capacity is measured per mole.

Why is water important for heat transfer?

Water has a high specific heat capacity (4.18 J/(g·°C)) and can absorb large amounts of heat with only a small temperature increase.

Why is water used as a coolant?

Water absorbs and carries away large amounts of thermal energy without overheating quickly.

What is pressure-volume (PV) work?

PV work occurs when a gas expands or compresses against an external pressure.

What is the equation for PV work?

w = −PΔV.

What does P represent and what are its units?

P = external pressure; Units: atm.

What does ΔV represent and what are its units?

ΔV = change in volume; Units: L.

What does w represent and what are its units?

w = work; Units: J or L·atm.

What is the mechanism for PV work?

Gas expansion or compression moves a piston or boundary against an external pressure.

What is an important factor required for PV work to occur?

A non-zero change in volume (ΔV ≠ 0) under an external pressure.

How do you convert PV work from L·atm to joules?

1 L·atm = 101.3 J.

How do heat and work interact within a system and its surroundings?

Heat (q) and work (w) are the two ways energy is transferred between a system and its surroundings.

What is a bomb calorimeter?

A bomb calorimeter is a rigid, sealed, constant-volume container used to measure the heat released during combustion reactions.

What is the mechanism of a bomb calorimeter?

Place a weighed sample inside the rigid bomb container filled with high-pressure oxygen gas. Submerge the sealed bomb into a known quantity of water within an insulated jacket. Ignite the sample electrically to initiate complete combustion. Record the temperature change (ΔT) of the surrounding water as heat transfers from the bomb to the water.

What is ΔE?

ΔE is the change in internal energy of a system, representing the total kinetic and potential energy gained or lost.

What is the equation for ΔE?

ΔE = q + w.

How does ΔE relate to the heat equation in a bomb calorimeter?

In a rigid bomb calorimeter, the volume is constant (ΔV = 0). Therefore, no pressure-volume work is done (w = 0). As a result: ΔE = qv.

What is the equation for qcal?

qcal = Ccal × ΔT.

How is this equation different from the specific heat equation?

The specific heat equation accounts for the mass of the substance.

What is the specific heat equation?

q = m × Cs × ΔT

What is the calorimeter equation?

qcal = Ccal × ΔT

What is enthalpy?

Enthalpy (H) is the total heat content of a system at constant pressure.

Why is enthalpy a state function?

It depends only on the current state of the system, not the path taken.

What is the equation for enthalpy?

H = E + PV

What is the equation for changes in enthalpy?

ΔH = ΔE + PΔV

What is ΔHrxn?

ΔHrxn is the heat absorbed or released during a chemical reaction at constant pressure.

How does ΔHrxn relate to heat (q)?

At constant pressure: ΔHrxn = qp

What is the equation for ΔHrxn?

ΔHrxn = qrxn / moles of reactant

What are the units for ΔHrxn?

kJ/mol

What is the relationship between ΔH and ΔE?

ΔH = ΔE + PΔV

What is the approximation for reactions with little or no volume change?

ΔH ≈ ΔE

What are exothermic reactions?

Reactions that releases heat to the surroundings (ΔH < 0).

What are endothermic reactions?

Reactions that absorbs heat from the surroundings (ΔH > 0).

How does ΔH relate to heat transfer?

ΔH < 0: Heat flows from the system to the surroundings; ΔH > 0: Heat flows into the system.

What is an example of an exothermic reaction?

Combustion of propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + heat

What is an example of an endothermic reaction?

Dissolving ammonium nitrate in water.

What is the thermal decomposition of calcium carbonate?

CaCO₃ + heat → CaO + CO₂

How do exothermic reactions occur?

Bonds in reactants break (absorbing energy), new bonds form in products (releasing more energy).

How do endothermic reactions occur?

Bonds in reactants break (absorbing energy), new bonds form in products (releasing less energy).

How does enthalpy change relate to a chemical reaction?

It reflects the difference in chemical potential energy between reactant bonds and product bonds.

How do you calculate the enthalpy change of a chemical reaction?

1. Calculate qsoln = m × Cs × ΔT; 2. qrxn = −qsoln; 3. ΔHrxn = qrxn / moles.

What is coffee-cup calorimetry?

A constant-pressure method using an insulated cup to measure heat changes during reactions.

How does coffee-cup calorimetry work?

Mix reactants in the cup, measure temperature change to calculate heat transferred.

How does coffee-cup calorimetry relate to bomb calorimetry?

Coffee-cup: constant pressure, measures ΔH; Bomb: constant volume, measures ΔE.

What are the rules involving ΔHrxn?

Multiplying an equation by n multiplies ΔHrxn by n; reversing changes the sign; adding reactions adds ΔHrxn values.

What is Hess's Law?

ΔHoverall = ΣΔHsteps; overall enthalpy change equals the sum of enthalpy changes for each step.

How do you solve Hess's Law problems?

Identify target reaction, adjust given reactions, add equations, and sum adjusted ΔH values.

What is the standard state?

Reference condition for thermodynamic measurements: pure gases at 1 atm, liquids/solids at 1 atm and 25°C.

How does the standard state relate to absolute enthalpy values?

Standard states provide a reference point; pure elements in standard states have ΔHf° = 0.

What is the standard enthalpy change (ΔH°)?

Enthalpy change when reactants and products are in their standard states.

What is the standard enthalpy of formation (ΔHf°)?

Enthalpy change when 1 mole of a compound is formed from its constituent elements in standard states.