Chem152: Chapter 6, Thermodynamics (& Chapter 9.10 Bond Energies and Lengths)

What are 2 important concepts related to heat capacity (C)? Think of this example: You put a steel saucepan on a kitchen flame. The saucepan's temperature rises rapidly as it absorbs heat from the flame. However, if you add some water to the saucepan, the temperature rises more slowly. Why?

1st important concept: Heat capacity is an extensive property, meaning it depends on the amount of matter being heated. The temperature rises more slowly when water is added to the steel saucepan, because now two things must be heated using the same amount of heat instead of just one thing. 2nd important concept: Heat capacity is also dependent on WHAT is being heated, because some things such as water have a much higher heat capacity than steel, meaning it takes a larger amount of heat to change the temperature of water than it does steel.

What does a positive delta H indicate?

A positive delta H indicates a positive q, since delta H = q. If q is positive, that means heat is entering the system as the reaction occurs!

What does it mean for a bond energy to be considered "stronger"? Is this change in enthalpy for the bond energy a large number?

A stronger bond energy will be larger in number! A bond that is considered "stronger" requires more energy to break it!

How does a system exchange energy with its surroundings?

A system exchanges energy with its surroundings through HEAT and WORK.

When energy is released from the system, how much of it goes into the surroundings?

According to the first law of thermodynamics, energy must be conserved. Therefore, the amount of energy lost by the system must exactly equal the amount gained by the surroundings. Delta E of the system = - Delta E of the surroundings

According to the first law of thermodynamics, the change in the internal energy of the system (delta E) is the sum...

According to the first law of thermodynamics, the change in the internal energy of the system (delta E) is the sum of the heat transferred (q) and the work done (w): Delta E = q + w

Describe how energy leaves a system through heat and work, using the example of a billiard ball traveling across a pool table and hitting another ball, then coming to a stop.

As the billiard ball moves across the table, there is friction between the ball and the table. Friction produces heat! So the ball is losing some of its kinetic energy through heat. When it hits the ball, it loses the rest of its kinetic energy through the work that was done to hit the ball. The total energy lost by the ball must equal the total energy gained by its surroundings, which in this case is the table (heat) and the other ball (work).

Does an endothermic reaction (+ delta H) feel hot or cold to the touch?

Because an endothermic reaction means the system is absorbing heat and the surroundings are losing heat, it would feel cool to the touch!

Does an exothermic reaction (- delta H) feel hot or cold to the touch?

Because an exothermic reaction means the system is losing heat and giving it off into the surroundings, it would feel hot to the touch!

Consider a balanced chemical equation that has 6 moles of gas on the reactant side and 7 moles of gas on the product side. Is delta H of the reaction negative or positive?

Because we went from fewer gas molecules to more gas molecules, there must have been heat given off, making this an exothermic reaction, meaning q is negative, and delta H is therefore negative.

Bomb calorimetry occurs at a constant ______ and measures delta ____ for the reaction.

Bomb calorimetry occurs at a constant VOLUME and measures delta E for the reaction.

Are bond energies positive or negative? Why?

Bond energies are ALWAYS POSITIVE, because it always takes energy to break a bond.

What two things does bond energy depend on?

Bond energies depend on 1. the kind of atoms involved in the bond, and 2. the type of bond (single, double, or triple)

Which REQUIRES ENERGY: breaking a bond or forming a bond?

Breaking a bond always requires energy! That makes breaking a bond endothermic!

(C.C. 6.3) Suppose you are cold-weather camping and decide to heat some objects to bring into your sleeping bag for added warmth. You place a large water jug and a rock of equal mass near the fire. Over time, both the rock and the water jug warm to about 38°C. If you could bring only one into your sleeping bag, which one should you choose to keep you the warmest? Why?

Bring in the water because it has the higher heat capacity, and will therefore release more heat as it cools! Recall C (heat capacity) = q (heat) / delta T (change in temperature). As temperature goes down, heat goes up! The higher the heat capacity, the more heat there is available!

What is the formula for C (heat capacity)?

C (heat capacity) = q (heat) / delta T (temperature)

What is temperature usually measured in?

Celsius (°C.)

What is the difference between change in enthalpy and change in internal energy?

Change in internal energy is a measure of all of the energy exchanged with the surroundings (aka heat and work) while change in enthalpy is only the measure of the heat exchanged with the surroundings under conditions of constant pressure.

Coffee-cup calorimetry occurs at constant _______ and measures delta ____ for the reaction.

Coffee-cup calorimetry occurs at constant PRESSURE and measures delta H for the reaction.

So what is the formula for finding internal energy change (delta E)?

Delta E = (E products) - (E reactants) So delta E is the difference in internal energy between the products and the reactants

What does delta E stand for? What does its negative and positive form mean?

Delta E stands for change in internal energy! +delta E means energy is flowing INTO the system from the surroundings. -delta E means energy is flowing FROM the system into the surroundings.

Energy entering the system through heat or work carries a... negative or positive sign?

Energy entering the system through heat or work carries a positive sign.

Energy leaving the system through heat or work carries a... negative or positive sign?

Energy leaving the system through heat or work carries a negative sign.

Under what conditions does enthalpy occur?

Enthalpy occurs under conditions of constant pressure.

Which RELEASES ENERGY: breaking a bond or forming a bond?

FORMING a bond always RELEASES energy! That makes forming a bond exothermic!

Define "heat (q)"

Heat (q) is the exchange of thermal energy between a system and its surroundings caused by a temperature difference.

Define Hess's Law!

Hess's Law is the change in enthalpy for a stepwise process is the sum of the enthalpy changes of the steps.

If a chemical equation can be expressed as the sum of a series of steps, then delta H of the reaction for the overall equation is... What law does this describe?

If a chemical equation can be expressed as the sum of a series of steps, then delta H of the reaction for the overall equation is the sum of the heats of the reactions for each step. This is Hess's Law!

If a chemical equation is multiplied by some factor, then delta H of the reaction is...

If a chemical equation is multiplied by some factor, then delta H of the reaction is also multiplied by the same factor!

If a chemical equation is reversed, what happens to the delta H of the reaction?

If a chemical equation is reversed, then the sign of delta H of the reaction changes!

Does a negative delta H indicate an endothermic or exothermic reaction?

If delta H was negative, that would make q negative, meaning heat flows out of the system as the reaction proceeds. This means it'd be on the product side of the reaction, making it exothermic!

If the internal energy of the reactants is higher than the internal energy of the products, is the change in internal energy positive or negative? Is energy given off or gained?

If the internal energy of the reactants is higher than the internal energy of the products, then the change in internal energy is negative! If you're losing energy, then that means it must have been given off!

If the internal energy of the reactants is lower than the internal energy of the products, is the change in internal energy positive or negative? Is energy given off or gained?

If the internal energy of the reactants is lower than the internal energy of the products, then the change in internal energy is positive, because we're going from little to big! The system has gained energy from the surroundings.

Recall that delta E = q + w, and that w = (-P)(delta V). What happens if the volume is constant? This is referred to as "heat at constant volume."

If the volume is constant, than delta V = 0, which means w = 0. That would also then mean that delta E = q. This is called the heat at constant volume!

Where does the energy lost by the reactants (as they transform to products) go?

If we define the thermodynamic system as the reactants and products of the reaction, then energy flows out of the system and into the surroundings!

What is "calorimetry"?

In calorimetry, we can measure the thermal energy exchanged between the reaction (defined as the system) and the surroundings by observing the change in temperature of the surroundings.

What is a state function? What kind of energy is an example of one?

Internal energy is a state function, which means that its value depends ONLY on the state of the system, not on how the system arrived at that state.

What is heat (q) usually measured in?

Joules (J)

Does the value of delta E depend on the values of q and w?

NO, because delta E (change in internal energy) is a STATE FUNCTION, meaning its value does not depend on its surroundings, aka HOW it became that value. So whether q is higher than w, or w is higher than q... It doesn't matter, because either way they'll have to sum up to the same number in both cases to = delta E! So delta E (the sum of q and w) is constant.

Are q (heat) and w (work) state functions?

NO, q (heat) and w (work) are NOT state functions because their values DO depend on their surroundings. For example, if the billiard table is rough, then q is > w. If the billiard table is smooth, then q is < w.

We know delta E equals the sum of heat and work. Are the values of heat and work equal to each other? Use the billiard table analogy.

Not necessarily. If a billiard ball is traveling across a very smooth pool table, little of its kinetic energy will be lost due to friction (heat, q). Which leaves the majority of its energy to be transferred to the other billiard ball (work, w). The opposite would be true if the billiard ball were to travel across a very rough pool table.

Does a positive delta H indicate an endothermic or exothermic reaction?

Positive delta H means q is positive, so heat is flowing into the system as the reaction proceeds, aka heat is absorbed from the surroundings. So heat is on the reactants side, making it an endothermic reaction!

When does "pressure-volume work" occur?

Pressure-volume work occurs when the force is caused by a volume change against and external pressure.

Why does Sacramento, an inland city not near water, experience higher temperatures than San Diego, a city near the water?

San Fransisco is surrounded by water. Water, with its high heat capacity, absorbs much of the sun's heat without undergoing a large increase in temperature, keeping San Fransisco cool. Sacramento, by contrast, doesn't have water nearby to absorb the sun's heat! So Sacramento's heat capacity is lower, so it's hotter.

How do you calculate a change in a state function? Give an example, using the mountain climb analogy.

Since state functions depend only on the state of the system, the value of a change in a state function is always the difference between its final and initial values. For example, if we start climbing a mountain at an elevation of 3,000ft and reach the summit at 10,000ft, then our elevation change is 7,000ft, regardless of what path we took.

What is the difference between heat and temperature?

Temperature is a measure of the thermal energy within a sample of matter. Heat is the TRANSFER of thermal energy.

What is "bond energy"?

The bond energy of a chemical bond is the energy required to break 1 mole of the bond in a gas phase. For example, the bond energy of the Cl-Cl bond in Cl2 is 243kJ/mol. Cl2(g) --> 2Cl(g) delta H = 243kJ

How do you find the change in enthalpy of a reaction, using bond energies? (equation)

The change in enthalpy of a reaction can be found using bond energies by summing the change in enthalpy for the bonds broken (positive) and then adding the sum of the enthalpies for the bonds formed (negative). delta H rxn = Σ (ΔHrxn's bonds broken) + Σ (ΔHrxn's bonds formed)

When is the change in enthalpy of formation (delta H°f) negative?

The change in enthalpy of formation (delta H°f) is negative when compound --> elements (when a compound breaks down to form its elements).

When is the change in enthalpy of formation (delta H°f) positive?

The change in enthalpy of formation (delta H°f) is positive when elements --> compound (when elements react to form a compound).

For an example, is the combustion of natural gas an endothermic or exothermic reaction?

The combustion of a natural gas is exothermic because as it combusts, it's RELEASING heat!

What is the constant of proportionality between q (heat) and delta T (change in temperature) known as?

The constant of proportionality between q (heat) and delta T (change in temperature) is the system's "heat capacity (C)"

How do we define "enthalpy (H)" and what is its equation?

The enthalpy (H) of a system is the sum of its internal energy and the products of its pressure and volume. H = E + (PV)

The enthalpy change for a chemical reaction is an extensive property, the opposite of a state function. What does this mean?

The enthalpy change for a chemical reaction is an extensive property, meaning it depends on its surroundings, aka the amount of material undergoing the reaction! In other words, the amount of heat generated or absorbed when a chemical reaction occurs depends on the AMOUNTS of reactants that actually react!

The enthalpy change for a chemical reaction is also known as...

The enthalpy of reaction OR heat of reaction

What is the first law of thermodynamics?

The first law of thermodynamics is the law of energy conservation, which states: "The total energy of the universe is constant, since energy is neither created nor destroyed; only exchanged in forms."

How is the heat absorbed by a system related to its corresponding temperature change?

The heat absorbed by a system is directly proportional to its corresponding temperature change! So if a system gains x amount of heat, its temperature changes by x amount.

Describe the concept of thermal energy transfer (from the system to its surroundings).

The heat lost from one substance exactly equals the heat gained by the other, aka the heat lost from the system will equal the heat gained by the surroundings or vise versa. q (system) = -q (surroundings)

Thermal energy from a coffee cup is transferred as heat to its surroundings are the coffee cup cools down. When does the transfer of heat stop? What is this called?

The heat transfer from the coffee distributes itself to the molecules of its surroundings. The heat transfer from the coffee to the surroundings stops when the two reach the same temperature, a condition called "thermal equilibrium."

When a system has a higher heat capacity (C), what does that mean for its change in temperature for a given amount of absorbed heat?

The higher the heat capacity of a system, the smaller the change in temperature for a given amount of absorbed heat. This makes sense because if the heat capacity is high, it can withstand absorbing more heat before a significant change in temperature happens.

For an example, when you apply an ice pack to your bruised wrist, is this an endothermic or exothermic reaction? Consider the ice pack the system!

The ice pack (the system) is gaining heat from its surroundings (your wrist) and the surroundings are losing heat! Because the system is gaining heat in this reaction, it's endothermic!

What is internal energy (E)?

The internal energy (E) of a system is the sum of the kinetic and potential energies of all of the particles that compose the system.

How do you calculate the standard enthalpy change (delta H°rxn) of a reaction?

The standard enthalpy change (delta H°rxn) of a reaction is equal to the sum of the products' standard enthalpy change (given by a chart) minus the sum of the reactants' standard enthalpy change. Remember to multiply the chart numbers by the coefficients in front of the element/compound!

How does thermal energy ALWAYS flow? Give an example, using a hot cup of coffee as an analogy.

Thermal energy ALWAYS flows from matter at higher temperatures to matter at lower temperatures. For example, a hot cup of coffee transfers thermal energy -- as heat -- to the lower temperature surroundings as it cools down.

Define "thermal equilibrium"

Thermal equilibrium is when there is no additional net transfer of heat; the temperature between the system and its surroundings are equal.

What is thermodynamics the study of?

Thermodynamics is the general study of energy and its interconversions.

List the strength of bonds, in order from strongest to weakest

Triple bond (strongest) Double bond Single bond (weakest)

Which compounds usually tend to have stronger bonds?

Usually compounds with stronger bonds tend to be more chemically stable

What is "heat capacity (C)"? What is its formula?

We define the heat capacity (C) of a system as the quantity of heat required to change its temperature by 1 °C. Heat capacity (C) is a measure of the system's ability to absorb thermal energy without undergoing a large change in temperature. Its formula is: q (heat) = C (heat capacity) * delta T (change in temperature).

When a chemical reaction occurs in a sealed container under conditions of constant volume, how does the energy evolve and why?

When a chemical reaction occurs in a sealed container under conditions of constant volume, that means that the change in volume is zero, which makes the value of work equal to zero. The change in energy is defined as heat + work, and if work is zero, that means that the change in energy evolves only as heat!

When a system absorbs heat (q), its temperature changes by... +delta T or -delta T?

When a system absorbs heat (q), its temperature changes by +delta T.

When bonds form, is the process endothermic or exothermic? Does it have a positive or negative bond energy?

When bonds FORM, the process is exothermic because energy is released when bonds are formed! So the bond energy is negative.

When bonds break, is the process endothermic or exothermic? Does it have a positive or negative bond energy?

When bonds break, the process is endothermic because it TAKES energy to break a bond! So the bond energy is positive.

With bond energies, a reaction is endothermic when...

With bond energies, a reaction is endothermic when strong bonds break and weak bonds form (because it takes more energy to break a strong bond)

With bond energies, a reaction is exothermic when...

With bond energies, a reaction is exothermic when weak bonds break and strong bonds form (because it takes less energy to break a weak bond)

How is "work" defined?

Work is force acting through a distance.

In an open atmosphere, is pressure constant?

YES

What is the formula for "change in enthalpy (delta H)"?

delta H = delta E + (P)(delta V) This can also be written as delta H = (q + w) + (-w), which reduces to delta H = q. So change in enthalpy is equal to the heat at constant pressure.

What does q stand for? What does its negative and positive form mean?

q stands for heat/thermal energy! +q means that the system GAINS thermal energy. -q means that the system LOSES thermal energy.

What is the equation for work (w)?

w = -P (pressure) * delta V (change in volume)

What does w stand for? What does its negative and positive form mean?

w stands for work done on the system! +w means work has been done ON the system. -w means work is done BY the system.