AP Chemistry Test B

How do you calculate \(\Delta H_{rxn} \degree\)?

Subtract the enthalpies of formation of the reactants multiplied by their stoichiometric coefficients from the enthalpies of formation of the products multiplied by their stoichiometric coefficients

If a chemical equation is reversed, what happens to \(\Delta H_{rxn}\)?

It changes sign

Definition of molar heat capacity

Amount of heat required to raise the temperature of 1 mole of a substance by \(1 \degree C\)

Standard state

• For a Gas: The standard state for a gas is the pure gas at a pressure of exactly 1 atm.
• For a Liquid or Solid: The standard state for a liquid or solid is the pure substance in its most stable form at a pressure of 1 atm and at the temperature of interest (often taken to be 25 °C).
• For a Substance in Solution: The standard state for a substance in solution is a concentration of exactly 1 M.

Negative \(\Delta H\)

Exothermic reaction

When energy is transferred to a substance, is \(\Delta E\) positive or negative?

Positive

Standard Enthalpy Change (\(\Delta H \degree\))

The change in enthalpy for a process when all reactants and products are in their standard states. The degree sign indicates standard states.

When a substance is warmed, is \(\Delta E\) positive or negative?

Positive

Definition of heat capacity

Quantity of heat required to change its temperature by \(1 \degree C\)

The amount of energy lost by the system must __ the amount gained by the surroundings

equal

Bomb calorimetry occurs at constant __ and measures __ for a reaction

volume, \(\Delta E\)

Positive \(\Delta H\)

Endothermic reaction

Sign Conventions for \(q\), \(w\), and \(\Delta E\)

If no heat escapes from the calorimeter, the amount of heat gained by the calorimeter __ that released by the reaction

equals

At constant volume, what is \(\Delta V\)?

$$0$$

Definition of specific heat capacity

Amount of heat required to raise the temperature of \(1\) gram of the substance by \(1 \degree C\)

$$E$$

Internal energy - the sum of the kinetic and potential energies of all of the particles that compose the system

Definition of \(\Delta E\)

$$\Delta E = q + w$$

Where \(q\) is heat and \(w\) is work.

The standard enthalpy of formation of pure elements in their standard state is __

$$0$$

Standard enthalpies of formation at \(298 K\)

What is \(q_{soln}\)?

The heat absorbed by or lost from the solution (which is acting as the surroundings)

Standard Enthalpy of Formation (\(\Delta H_f \degree\))

• For a Pure Compound: The change in enthalpy when 1 mol of the compound forms from its constituent elements in their standard states.
• For a Pure Element in Its Standard State: \(\Delta H_f \degree = 0\).

Relate the heat absorbed, heat capacity, and temperature change of a calorimeter

$$q_{cal} = C_{cal} \cdot \Delta T$$

Hess's law

Get \(\Delta H_{rxn}\) for \(A + 2B \rightarrow 2D\) using the equations \(A + 2B \rightarrow C\) and \(C \rightarrow 2D\)

What is \(\Delta H_{rxn}\)?

Enthalpy of reaction or heat of reaction and depends on the amount of material undergoing the reaction

Relate work to external pressure and change in volume

$$w = −P_{ext} \cdot \Delta V$$

Molar heat capacity

$$\frac{J}{\text{mol} \cdot C \degree}$$

Thermal equilibrium

No additional net transfer of heat

Convert from specific heat to molar heat capacity

Multiply by the molar mass (\(\frac{g}{\text{mol}}\))

At constant volume, what is \(w\)?

$$0$$

If a chemical equation is multiplied by some factor, what happens to \(\Delta H_{rxn}\)?

It is also multiplied by the same factor

Define change in enthalpy (\(\Delta H\))

$$\Delta H = \Delta E + P \Delta V = q_p$$

How do we measure the thermal energy exchanged between the reaction (defined as the system) and the surroundings?

We observe the change in temperature of the surroundings

Coffee-cup calorimetry occurs at constant __ and measures __ for a reaction

pressure, \(\Delta H\)

State function

Does not depend on how the system arrived at that state

Define enthalpy (\(H\))

$$H = E + PV$$

When work is done on the surroundings by the system, \(w\) is

negative

When two substances are mixed and allowed to come to thermal equilibrium, what is the relation between their \(q\) values?

Equal in magnitude, but in opposite directions for each substance.

What is \(\Delta E_{rxn}\)?

$$\Delta E_{rxn} = q_v + w$$

If a chemical equation can be expressed as the sum of a series of steps, what is \(\Delta H_{rxn}\) for the overall equation?

The sum of the heats of reaction for each step

What is \(q_v\)?

Heat at constant volume

The equation relating heat, temperature change, mass, and specific heat.

$$Q = m C \Delta T$$

Heat capacity of a calorimeter

$$\frac{\text{Total amount of heat}}{\text{Temperature change}}$$

Units of specific heat

$$\frac{\text{energy}}{\text{mass} \cdot \text{temperature}}$$

Typically,

$$\frac{J}{g \cdot C \degree}$$

How do we calculate \(q_{soln}\)?

$$q_{soln} = m_{soln} \cdot C_{s, soln} \cdot \Delta T$$

The standard enthalpy of formation is also called __

Standard heat of formation

What is \(q_p\)?

Heat at constant pressure

Relate \(q_{rxn}\) and \(q_{soln}\)

$$q_{rxn} = −q_{soln}$$