Thermochemistry: Energetics, Enthalpy and Hess’s Law

These flashcards cover fundamental concepts from Chapter 4 on thermochemistry, including definitions of energy forms, internal energy, the first law, enthalpy, calorimetry, Hess’s Law, standard states, and enthalpy calculations.

What branch of science deals specifically with the heat involved in chemical and physical changes?

Thermochemistry.

In thermochemistry, what do we call the part of the universe under study?

The system.

What term describes everything outside the system that can exchange energy with it?

The surroundings.

Which two fundamental forms can all energy be classified into?

Potential energy and kinetic energy.

Define potential energy.

Energy an object possesses because of its position or composition.

Define kinetic energy.

Energy an object possesses due to its motion.

What is the symbol and formula for a system’s internal energy change?

ΔE = Efinal – Einitial = Eproducts – Ereactants.

State the mathematical relationship that connects heat, work, and the change in internal energy.

ΔE = q + w.

In the expression ΔE = q + w, what does a positive q signify?

The system gains heat from the surroundings.

In the sign convention for work, what does w > 0 mean?

Work is done on the system by the surroundings.

State the first law of thermodynamics in words.

Energy is conserved; it can change form but cannot be created or destroyed.

Give the SI unit of energy.

The joule (J).

How many joules are in one calorie?

4.18 J.

What quantity of energy is represented by a nutritional Calorie (with a capital C)?

One kilocalorie (1 kcal = 1000 cal).

Define enthalpy (H).

A thermodynamic quantity defined as H = E + PV.

Provide the expression for the change in enthalpy (ΔH).

ΔH = ΔE + PΔV.

At constant pressure, what is the relationship between heat and enthalpy change?

q_p = ΔH.

When is ΔH approximately equal to ΔE?

When no gases are involved, when the number of moles of gas does not change, or when PΔV is negligible compared with q.

What sign of ΔH corresponds to an exothermic reaction?

Negative (ΔH < 0).

What sign of ΔH corresponds to an endothermic reaction?

Positive (ΔH > 0).

Give the specific heat capacity formula for calculating heat transfer.

q = c × m × ΔT.

What is the specific heat capacity of liquid water in J/g·K?

4.184 J/g·K.

Name the simple calorimeter often used for reactions in solution at constant pressure.

A coffee-cup calorimeter.

Which calorimeter operates at constant volume and is typically used for combustion reactions?

A bomb calorimeter.

State Hess’s Law.

The enthalpy change for an overall process equals the sum of enthalpy changes for its individual steps.

List the four key steps for applying Hess’s Law to find an unknown ΔH.

1) Identify the target equation, 2) manipulate known equations (reverse/scale) to match target, 3) add equations so unwanted species cancel, 4) sum their ΔH values.

Define ‘standard state’ for a gas in thermochemistry.

The pure gas at 1 atm behaving ideally.

What notation indicates a standard enthalpy of reaction?

ΔH°_rxn.

Write the general formula for ΔH°_rxn in terms of heats of formation.

ΔH°rxn = Σ m ΔH°f(products) – Σ n ΔH°_f(reactants).

What is the ΔH°_f value for any element in its standard state?

Zero kJ/mol.

Give the balanced formation equation for AgCl(s).

Ag(s) + ½ Cl₂(g) → AgCl(s).

Identify four commonly referenced enthalpy changes besides ΔH_rxn.

Heat of combustion (ΔHcomb), heat of formation (ΔHf), heat of fusion (ΔHfus), and heat of vaporization (ΔHvap).

What is pressure–volume work, and what sign does it carry when the system expands?

Work done when a gas changes volume against an external pressure; it is negative because the system loses energy.

Convert 1 kilowatt-hour (kWh) of energy to joules.

1 kWh = 3.6 × 10⁶ J.

If ΔE is negative and q = 0, what must be true about w?

w is negative; the system did work on the surroundings.

During a reaction at constant pressure, what experimental quantity do we typically measure to obtain ΔH?

The temperature change of the surroundings (to find q_p).

For the reaction 2H₂O(l) → 2H₂(g) + O₂(g), is it endothermic or exothermic and what is the sign of ΔH?

Endothermic; ΔH is positive.

State the relation between Celsius and Kelvin temperatures.

T(K) = T(°C) + 273.15.

What is the purpose of the insulated jacket around a bomb calorimeter?

To minimize heat exchange with the external environment so measured temperature change corresponds to the reaction heat.

How does the magnitude of ΔH change when the stoichiometric coefficients of a thermochemical equation are doubled?

ΔH also doubles; it is proportional to the amount of substance.

In calorimetry problems, why can the density of dilute aqueous solutions be approximated as 1.00 g/mL?

Because at low solute concentrations the solution’s density is very close to that of pure water.