AP Chem Hess law practice questions

What is Hess's Law?

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken.

True or False: Hess's Law applies to any chemical reaction.

True. Hess's Law can be applied to any chemical reaction because enthalpy is a state function.

Calculate the enthalpy change: A + B → C (ΔH = 50 kJ), C → D (ΔH = -30 kJ)

ΔH for A + B → D = ΔH1 + ΔH2 = 50 kJ - 30 kJ = 20 kJ.

Fill in the blank: Hess's Law allows for the calculation of enthalpy changes using ________.

individual reaction steps.

What is a state function?

A state function is a property whose value does not depend on the path taken to reach that specific value, such as enthalpy.

If reaction A → B has ΔH = -100 kJ and B → C has ΔH = 50 kJ, what is ΔH for A → C?

ΔH for A → C = -100 kJ + 50 kJ = -50 kJ.

Comparison: Enthalpy vs. Internal Energy.

Enthalpy (H) includes pressure-volume work; Internal Energy (U) is total energy. ΔH = ΔU + PΔV.

What does ΔH represent in a chemical equation?

ΔH represents the change in enthalpy, indicating whether the reaction is exothermic (negative ΔH) or endothermic (positive ΔH).

Cause and effect: Reversing a reaction affects ΔH how?

Reversing a reaction changes the sign of ΔH; if forward is +100 kJ, reverse is -100 kJ.

What role do coefficients play in Hess's Law?

Coefficients in thermochemical equations multiply enthalpy changes; e.g., 2A → 2B means ΔH is doubled.

True or False: The enthalpy change is the same for all pathways.

True. The total enthalpy change is independent of the pathway between reactants and products.

Calculate ΔH for: 2H2 + O2 → 2H2O (ΔH = -484 kJ).

Enthalpy change for 1 mole of H2O = -484 kJ / 2 = -242 kJ.

Thermochemical equation definition?

An equation that includes the enthalpy change of a reaction, showing products and reactants along with their physical states.

True or False: Enthalpy is a state function.

True. Enthalpy depends only on the initial and final states, not the path taken.

Standard enthalpy of formation ($\displaystyle riangle H_f^ heta$) for elements?

Zero. The standard enthalpy of formation for any element in its standard state is defined as zero.

Given: C(s) + O2(g) → CO2(g) with $\displaystyle riangle H = -393.5 ext{ kJ}$; What is the reverse equation?

CO2(g) → C(s) + O2(g) with $\displaystyle riangle H = +393.5 ext{ kJ}$.

Fill in the blank: Hess's Law allows us to find ΔH for __________ reactions.

reactions that are difficult to measure directly.

Calculate ΔH for: A + B → C (ΔH = -200 kJ), C → D (ΔH = +150 kJ).

ΔH = -200 kJ + 150 kJ = -50 kJ for A + B → D.

Comparison: Hess's Law vs. Direct Measurement.

Hess's Law: uses known enthalpies; Direct Measurement: experimental, often challenging.

True or False: Heat of reaction depends on the amount of reactants.

True. The enthalpy change is proportional to the quantity of reactants used.

What is the enthalpy change in a reaction?

The heat absorbed or released at constant pressure, represented as ΔH.

If 2H2(g) + O2(g) → 2H2O(l); ΔH = -571.6 kJ, what's ΔH for H2 + 1/2 O2?

ΔH = -285.8 kJ for 1 mole of water formed.

What happens when reaction equations are reversed?

The sign of ΔH changes. For example, If A → B has ΔH = x, then B → A has ΔH = -x.

Fill in the blank: When adding reactions, ΔH is __________.

the sum of the individual ΔH values.

Given the reaction: 2C + D → E, and ΔH = -100 kJ. What's for C + 1/2 D?

ΔH = -50 kJ, half of the original since coefficients are halved.

What is the standard state for a gas?

1 atm pressure, 298 K (25 °C) temperature.

Cause → Effect: Increasing temperature of a reaction at constant pressure?

Increases enthalpy change (ΔH), as systems absorb heat.

What does a thermochemical equation include?

It includes the balanced equation, energy change (ΔH), and states of matter. Example: C(s) + O2(g) → CO2(g) ; ΔH = -393.5 kJ