unit 3 | pt. 1

🔥 What does thermochemistry study?

Heat absorbed or released during chemical reactions and physical changes.

🏃 How do kinetic and potential energy differ?

Kinetic energy is energy of motion; potential energy is stored because of position, composition, or arrangement.

🌡️ How do heat and temperature differ?

Heat (q) is thermal energy transferred because of a temperature difference. Temperature measures the average kinetic energy of particles.

🧊 What kinds of processes can involve energy changes?

Phase changes, chemical reactions, and nuclear reactions can all transfer or transform energy.

♻️ State the first law of thermodynamics.

Energy cannot be created or destroyed, only transferred or transformed; the total energy of the universe is conserved.

🌀 What does the second law say about a spontaneous process?

A spontaneous process increases the total entropy of the universe: ΔS(universe) > 0. The system's entropy alone does not always have to increase.

❄️ What does the third law of thermodynamics state?

The entropy of a perfect crystal approaches zero as its temperature approaches absolute zero. It does not say every substance has zero entropy at 0 K.

🧪 What are the system and surroundings?

The system is the matter or reaction being studied; the surroundings are everything outside it that can exchange energy or matter with it.

🚪 Compare open, closed, and isolated systems.

Open: exchanges matter and energy. Closed: exchanges energy but not matter. Isolated: exchanges neither; perfect isolation is an idealization.

🧣 How do endothermic and exothermic processes affect the surroundings?

Endothermic processes absorb heat from the surroundings, which tend to cool. Exothermic processes release heat to the surroundings, which tend to warm.

➕ What are the signs of ΔH for endothermic and exothermic processes?

Endothermic: ΔH > 0. Exothermic: ΔH < 0, using the system as the reference.

☀️ When does a nuclear reaction release energy?

When its products have greater total binding energy and slightly less mass than the reactants; the mass difference is released as energy through E = mc². Not every possible nuclear reaction is exothermic.

🌞 What is the net idea behind fusion in stars?

Light nuclei combine into more tightly bound nuclei. In the Sun, the overall proton-proton process converts four protons into one helium-4 nucleus plus positrons, neutrinos, and energy—not helium plus one free neutron.

⚛️ What is nuclear fission?

A heavy nucleus splits, often after absorbing a neutron, producing smaller nuclei, additional neutrons, and energy; the neutrons can sustain a chain reaction.

🥄 What is specific heat capacity?

The heat required to raise 1 g of a substance by 1 °C (or 1 K), usually measured in J/(g·°C).

💧 What is notable about water's specific heat capacity?

Water's value, about 4.18 J/(g·°C), is high compared with most metals and many common substances, but it is not the highest of all materials.

🧮 What equation relates heat, mass, specific heat capacity, and temperature change?

q = mcΔT, where ΔT = T(final) - T(initial).

📈 In q = mcΔT, what does a positive or negative q mean?

Positive q means the object or system absorbs heat; negative q means it releases heat.

⚖️ What energy-balance equation is used in an ideal calorimeter?

q(system) + q(surroundings) = 0, so q(system) = -q(surroundings).

🧪 What common assumptions are made in simple solution calorimetry?

Negligible heat exchange with the external environment, negligible calorimeter heat capacity unless given, solution density near 1.00 g/mL, and solution specific heat near water's 4.18 J/(g·°C).

💣 What is a bomb calorimeter mainly used for?

Measuring heat released by combustion at constant volume in a sealed, strong container; it minimizes matter loss and contains high pressures.

📦 What is enthalpy, and when does ΔH equal transferred heat?

Enthalpy is the state function H = U + PV. At constant pressure, ΔH equals the heat exchanged by the system, qₚ.

🧮 How are total heat and molar enthalpy related?

q = nΔHᵣ, so ΔHᵣ = q/n, with consistent units such as kJ and mol.

✖️ If a thermochemical equation is multiplied by 3, what happens to ΔH?

ΔH is multiplied by 3 because enthalpy change scales with the amount reacting.

↩️ If a thermochemical equation is reversed, what happens to ΔH?

Its sign reverses because heat flow is reversed.

📉 How do exothermic and endothermic potential-energy diagrams differ?

Exothermic: products lie below reactants and ΔH < 0. Endothermic: products lie above reactants and ΔH > 0.

↕️ How is ΔH read from a potential-energy diagram?

ΔH = H(products) - H(reactants), the vertical difference between the product and reactant energy levels.

🔨 What is bond dissociation energy?

The energy required to break one mole of a specified covalent bond in the gas phase; tabulated average bond energies are approximate across different molecules.

🪢 How do single and multiple bonds generally compare?

For the same atoms, a double or triple bond is shorter and stronger overall than a single bond. A π component is weaker than a σ component, but σ + π is stronger than σ alone.

✂️ Why does breaking bonds require energy while forming bonds releases energy?

Energy must be supplied to separate attracted atoms; when a stable bond forms, the system falls to lower potential energy and releases the difference.

🧮 What equation estimates reaction enthalpy from bond energies?

ΔH(reaction) ≈ ΣD(bonds broken) - ΣD(bonds formed).

🔍 Why is a bond-energy ΔH usually an estimate?

Tables often give average gas-phase bond energies, while actual bond strength depends on the molecule's environment and physical states.

🧭 State Hess's law.

Because enthalpy is a state function, the total ΔH depends only on initial and final states, not on the reaction path.

🧩 How is a target reaction constructed using Hess's law?

Reverse and/or scale known equations so unwanted species cancel; apply the same changes to their ΔH values, then add the equations and enthalpies.

🧹 What should you check after adding Hess's-law equations?

All intermediates must cancel, the remaining equation must exactly match the target including coefficients and states, and the ΔH arithmetic must follow every reversal and scaling.

🏗️ What is the standard enthalpy of formation, ΔH°f?

The enthalpy change when exactly 1 mol of a compound forms from its constituent elements in their standard states under standard conditions, commonly 25 °C and 100 kPa.

0️⃣ Why is ΔH°f of an element in its standard state equal to zero?

It is defined as zero by convention, providing a common reference—not because all formation or phase changes physically require no energy.

🧮 What is the correct formation-enthalpy equation for a reaction?

ΔH°rxn = ΣnΔH°f(products) - ΣnΔH°f(reactants), including stoichiometric coefficients n.

🧠 When using ΔH°f values, why do coefficients and physical states matter?

Coefficients multiply each molar formation enthalpy, and different states or forms—such as H₂O(l) versus H₂O(g)—have different ΔH°f values.

🧰 Match each enthalpy method with the information it uses.

Calorimetry: m, c, and ΔT. Bond energies: bonds broken and formed. Hess's law: other reactions and their ΔH values. Formation enthalpies: tabulated ΔH°f values.

⛽ What are fossil fuels and natural gas chemically?

Fossil fuels formed from ancient biomass over geological time. Petroleum contains mixtures of hydrocarbons of varied sizes; natural gas is mainly methane with smaller amounts of other light hydrocarbons.

💨 Is hydrogen an energy source in the same sense as fossil fuel?

Hydrogen is mainly an energy carrier: energy is required to produce it. It has high energy per mass but low energy per volume unless compressed or liquefied.

🌱 What are biofuels?

Fuels made from recently living biomass, including plant sugars, oils, and organic wastes; their sustainability and net emissions depend on how they are produced.

⚙️ How is energy efficiency calculated?

Efficiency = useful energy output ÷ total energy input × 100%.

What are the types of molar enthalpy?

Molar enthalpy of solidification (freezing), condesation, deposition (gas to solid), sublimation, fusion (solid → liquid) & vaporization

Which phase change is endothermic?

Fusion (melt), Vaporization (boil) & Sublimation (solid → gas)

Why is temperature constant at phase change?

During a phase change, temperature remains constant because all the added thermal energy is used to break or form intermolecular bonds, rather than increasing the kinetic energy of the molecules.

Why are reactions generally fastest at the beginning?

Concentrations are initially highest, causing more frequent collisions and more effective collisions per second. As reactants are consumed, their concentrations decrease, slowing the reaction.

What are the five factors affecting reaction rates, and why?

• Temperature: Higher kinetic energy means more particles overcome activation energy.

• Concentration: More particles per volume increases collision frequency.

• Surface area: More exposed particles increase effective collisions.

• Catalyst: Provides an alternative pathway with lower activation energy.

• Nature of reactants: Different substances have different chemical reactivities.

Why are strong acids generally more corrosive than weak acids at equal concentrations?

Strong acids ionize completely, producing higher concentrations of H₃O⁺ ions. Weak acids only partially ionize, producing fewer H₃O⁺ ions.

How does increasing temperature speed up reaction differently from concentration of reactants?

Temperature increases average kinetic energy and the fraction of particles exceeding activation energy. Concentration only increases the number of particles per volume, resulting in more frequent collisions.

Which factors increase collision frequency, and why?

• Temperature ↑: Particles move/collide faster, increasing collision frequency.

• Concentration ↑: More particles per unit volume, increasing collisions.

• Surface area ↑: More exposed particles, increasing collisions at the surface.

• Pressure ↑ (gases): Particles are closer together, increasing collision frequency

Which factors increase the fraction of effective collisions, and why?

• Temperature ↑: More particles have sufficient energy to overcome activation energy.

• Catalyst: Lowers activation energy, allowing more collisions to succeed.

• Nature of reactants: Different substances have different activation energies and orientation requirements, affecting collision effectiveness

What is the difference between collision frequency and the fraction of effective collisions?

• Collision frequency: How often particles collide per second.

• Fraction of effective collisions: The proportion of collisions that successfully produce a reaction

What is the difference between individual reaction order and overall reaction order?

Individual order is the exponent of a specific reactant's concentration in the rate law. Overall order is the sum of all exponents.

How can you find reaction orders?

Reaction orders are determined experimentally. You cannot use the coefficients of the balanced chemical equation to determine the rate law unless the reaction is an elementary step.

Why does temperature increase rate constant but not concentration?

Temperature increases the reaction rate, but concentration does not increase; thus, it must be due to a change in the rate constant.

Second-order reaction VS. Second-order of reaction with respects to the reactants

One is the overall order (sum of all orders of reactants), one is limited to the order of only one reactant

How can you tell the rate-determing step from the Rate Law equation?

The exponents of each concentration of reactant must match teh coefficients of it’s stoichiometric balanced equation of the rate determing elementary step.

How can you predict the rate-determining step using collision theory?

Step requiring the most (reactant) particles to collide simultaneously is often the slowest because effective collisions become less probable as w/ particles.

Why are collisions involving 3 or more reactant particles less likely to be successful?

Requires particles must collide simultaneously with the correct orientation and sufficient energy. The more particles involved, the less probable it is that all these conditions will be met at once, making successful collisions rarer.

How can you determine the rate-determining step from potiental energy diagrams?

The reaction wiith the largest activation energy barrier (largest kJ difference)

WHat does empirical rate law mean?

It is determined from experimental data