Steps chem exam 3

Stoichiometry (general)

Balance the equation, convert given to moles, use mole ratio to get desired substance, convert to final unit (grams, liters, etc.)

Limiting Reagent

Convert each reactant to moles, divide by coefficient, identify smallest value as limiting reagent, use limiting reagent for all calculations

Excess / Leftover Reactant

Find limiting reagent, use limiting reagent to find how much of other reactant is used, subtract from initial to find leftover

Solution Stoichiometry

Convert volume to liters, use M × V = moles, perform stoichiometry, convert to requested units

Acid–Base Neutralization

Balance the equation, convert solutions to moles, account for coefficients (H⁺ vs OH⁻), compare amounts or set equal at equivalence

Internal Energy (ΔU)

Identify q (heat) and w (work), assign correct signs, add: ΔU = q + w

Heat Calculation (q = mcΔT)

Write q = mcΔT, find ΔT = final − initial, plug in values, solve for missing variable

Final Temperature (two objects)

Write q_hot = -q_cold, expand both as m c (T_f - T_i), set equal, solve for T_f

Coffee Cup Calorimeter

Calculate q = mcΔT, assign correct sign for solution vs reaction, use q_rxn = -q_solution

Heat of Reaction per Mole/Gram

Find total heat q, divide by moles or grams, include correct sign

Hess’s Law

Write target equation, manipulate given equations (flip to change ΔH sign, multiply to scale ΔH), add equations, sum ΔH values

Enthalpy from Formation Values

Multiply each ΔHf by its coefficient, sum all products, sum all reactants, ΔH = products - reactants

Photon Energy

Use E = hc/λ, convert wavelength to meters, plug in constants, solve

Frequency

Use c = λν, rearrange for ν, plug in values

Number of Photons

Calculate energy per photon, divide total energy by energy per photon

Comparing Wavelength/Energy

Shorter wavelength corresponds to higher energy, longer wavelength corresponds to lower energy

Photoelectric Effect

Compare frequency to threshold frequency, if frequency < threshold then no emission, only increasing frequency can cause emission

Bohr Model / Electron Transition

Identify initial n and final n, use energy formula or concept, high to low = negative (emission), low to high = positive (absorption)

de Broglie Wavelength

Use λ = h / (mv), plug in mass and velocity, solve

Longest Wavelength Comparison

Identify smallest mass, identify slowest speed, that particle has largest λ