CHEM HW 3/28/26

formula for internal energy?

letters stand for?

ΔE = q + w

q is heat

w is work

when is w (work) negative?

when is w (work) positive?

work is done by system on surroundings (ex: system does 50 kJ of work)

work is done on system by surroundings (ex: system has 50 kcal work done to it)

# kcal = # J?

# J = # kJ?

1 kcal is 4184 J

1000 J = 1 kJ (divide J by 1000 to get kJ)

type of process is it when system temperature is increasing? (water boiling)

type of process is it when system temperature is decreasing? (water cooling)

endothermic (system absorbs heat)

exothermic (system releases heat)

formula for finding required heat for temp change?

q = mcΔT

each letter meaning in q = mcΔT

q = heat energy (J)

m = mass (g)

c = specific heat (J/g*C)

ΔT = temp change (final - initial)

_ mL of water = _ g of water

35.0 mL is…

1, 1

35.0 grams

specific heat of water (c)

4.184 J/g*C

if q is not given for metal, but q is obtained from water…what’s the formula to get q for the metal?

exception?

-q(water) = q(metal)

problem says to assume no heat is lost to surroundings

combusting of a hydrocarbon (usually reacting with oxygen) will…

always result to water and carbon dioxide

oxygen in combustion problems is…

o2 (not o)

each letter in q = n x ΔH?

q = heat absorbed or released

n = moles of substance reacting

ΔH = enthalpy change per mole

when is q = n x ΔH used?

Reactions where ΔH is given per mol (like kcal/mol, kJ/mol, J/mol)

to figure out how much heat (q) is produced when given grams of both reactants…

convert grams of reactants to moles then find limiting reactant and use moles of limiting reactant to find q (n x ΔH)

when given grams of element in equation and asked to find amount of heat (q), last step should be…

dividing or multiplying coefficent (unless element has coefficient of 1 in equation)

when problem gives grams of an element, it is usually

the amount for one, so it has not accounted for the coefficient

goal of Hess’s law is to…

match given equations to the equation we

rules of hess’s law?

Rule 1: when equation reversed, sign of enthalpy changes

Rule 2: when coefficients in equation multiplied or divided by factor, enthalpy value multiplied or divided by same factor

Rule 3: reactions are summed, enthalpy of equation we are trying to calculate is sum of enthalpies of the component reactions

standard enthalpy of reaction is calculated by

sum of the standard enthalpies of formation of the products - the sum of the standard enthalpies of formation of the reactants

Difference with these type of questions “What is the heat of combustion of ethane, C2H6 in kilojoules per mole of ethane?” to other enthalpy of formation questions?

need to write out balanced equation and divide all coefficients by 2 since question is asking for per mole

Open system?

Closed system?

Isolated system?

Open system: both matter and energy can move between the system and the surroundings

Closed system: energy but not matter can move between the system and the surroundings

Isolated system: neither matter nor energy can leave or enter the system

Path function?

Energy depends on the system process from initial state to final state

State function?

examples?

Describes the current state of a system and is independent of the path

Enthalpy (H), Internal energy (U), Temperature (T), Pressure (P), Volume (V)

Bomb calorimetry is done in? measures?

Coffee cup calorimetry is done in? measures?

Done in a sealed, rigid container

Measures reactions at constant volume (ΔE)

Done in an open container (like a Styrofoam cup)

Measures reactions at constant pressure (ΔH)

Objects with a ___ specific heat capacity will undergo a ___ temperature change.

low, greater

calculating for kinetic energy and given grams, remember that

kg is needed since 1 Joule (J) = 1 kg x m2/s2