Unit 11 Thermodynamics Chapter 16

Thermodynamics Definition Definition A study of heat transfer that accompanies chemical changes A study of heat transfer that accompanies chemical changes Concerned with overall chemical changes Concerned with overall chemical changes Chemical Change involves: Chemical Change involves: A change in energy A change in energy A degree of disorder A degree of disorder

In Thermodynamics… System refers to the reaction itself System refers to the reaction itself Surroundings refers to everything else Surroundings refers to everything else Standard Conditions Standard Conditions 25°C (298 K) 25°C (298 K) 1 atmosphere 1 atmosphere 1 molar solution 1 molar solution

Enthalpy Symbol:H Symbol:H Measure of heat content (energy) of a system at constant pressure Measure of heat content (energy) of a system at constant pressure Can’t be measure directly, can only measure the change in enthalpy. Can’t be measure directly, can only measure the change in enthalpy. We call this the Heat of Reaction, ΔH We call this the Heat of Reaction, ΔH Measure of the heat released or absorbed in a chemical reaction! Measure of the heat released or absorbed in a chemical reaction! ΔH rxn = ΔH products - ΔH reactants

Hess’s Law This summer your parents decide they are going to take you on a road trip to CA. You drive 400 mi day 1, 350 mi day 2, 275 mi day 3, and 100 mi day 4. What was the total mileage of the trip? This summer your parents decide they are going to take you on a road trip to CA. You drive 400 mi day 1, 350 mi day 2, 275 mi day 3, and 100 mi day 4. What was the total mileage of the trip? You add your daily totals, You add your daily totals, 400 mi mi mi mi = 1125 mi total 400 mi mi mi mi = 1125 mi total In the same way, if the reaction we want requires two or more equations we can sum the enthalpy changes to get the total energy change! In the same way, if the reaction we want requires two or more equations we can sum the enthalpy changes to get the total energy change!

Hess’s Law Say we want to find the ΔH of the reaction: A + D → E Say we want to find the ΔH of the reaction: A + D → E We know the following: We know the following: A + B → CΔH = 27 kJ C + D → B + EΔH = -15 kJ Using Hess’s Law we can calculate the ΔH we want by taking the sum of the reaction: (27 kJ) + (-15 kJ) = 12 kJ Using Hess’s Law we can calculate the ΔH we want by taking the sum of the reaction: (27 kJ) + (-15 kJ) = 12 kJ

Hess's law states that the change in enthalpy of the reaction equals the sum of the enthalpy change for the intermediate steps of the reaction. Hess's law could also be stated " as the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or several steps. Hess's law is also noted as the law of constant heat summation. Hess's law states that the change in enthalpy of the reaction equals the sum of the enthalpy change for the intermediate steps of the reaction. Hess's law could also be stated " as the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or several steps. Hess's law is also noted as the law of constant heat summation.

Standard Heat of Formation Change in enthalpy from the formation of 1 mol of a compound, in its standard state, from its elements. Change in enthalpy from the formation of 1 mol of a compound, in its standard state, from its elements. Symbol: Symbol: “  ” refers to standard conditions “  ” refers to standard conditions Units: kJ/mol Units: kJ/mol Example: Example: S (s) + O 2(g) → SO 2(g) -297 kJ/mol * Table 16-7 on page 510 lists Standard H f

Chemical Reactions Compare and contrast the following graphs: Compare and contrast the following graphs: What did you notice? How would you write these reactions in standard format?

Standard Heat of Formation What can we tell from values? What can we tell from values? Positive value means? Positive value means? Negative value means? Negative value means? Thermochemical equations Thermochemical equations 4Fe (s) + 3O 2(g) → 2Fe 2 O 3(s) kJ NH 4 NO 3(s) + 27 kJ → NH 4 + (aq) + NO 3 - (aq)

When you state the height of a mountain, it is relative to another point (usually sea level). When you state the height of a mountain, it is relative to another point (usually sea level). In the same way enthalpies of formation are stated based on the following arbitrary standard: In the same way enthalpies of formation are stated based on the following arbitrary standard: Every free element in its standard state has a value of exactly 0.0 kJ. Every free element in its standard state has a value of exactly 0.0 kJ. That way when the heat of formation is negative the system has lost heat, when positive the system has gained heat ! That way when the heat of formation is negative the system has lost heat, when positive the system has gained heat ! Where do Standard Heats of formation come from?

Check for understanding! Do elements in their standard states possess zero energy? Do elements in their standard states possess zero energy? Why are elements in their standard states assigned enthalpies of zero? Why are elements in their standard states assigned enthalpies of zero? What does the on the graph tell you? What does the on the graph tell you? What does the -396 on the graph tell you? What does the -396 on the graph tell you?

Enthalpy change from Standard Heat of Formation Use standard heat of formation to calculate ΔH rxn  for the combustion of methane Use standard heat of formation to calculate ΔH rxn  for the combustion of methane CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (l) We can summarize Hess’s Law into the following equation: We can summarize Hess’s Law into the following equation: ΔH rxn  = ΣΔH f  (products) - ΣΔH f  (reactants) The symbol Σ means “to take the sum of the terms.” The symbol Σ means “to take the sum of the terms.”

Enthalpy change from Standard Heat of Formation Use standard heat of formation to calculate ΔH rxn  for the combustion of methane Use standard heat of formation to calculate ΔH rxn  for the combustion of methane CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (l) First: look up ΔH f  values First: look up ΔH f  values Second: Use the formula and multiply each term by the coefficient of the substance in the balanced chemical equation Second: Use the formula and multiply each term by the coefficient of the substance in the balanced chemical equation Third: Do the math Third: Do the math

Enthalpy change from Standard Heat of Formation Use standard heat of formation to calculate ΔH rxn  for the combustion of methane Use standard heat of formation to calculate ΔH rxn  for the combustion of methane CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (l) ΔH f  (CO 2 ) = -394 kJΔH f  (H 2 O) = -286 kJ ΔH f  (CH 4 ) = -75 kJΔH f  (O 2 ) = 0.0 kJ ΔH f  (CO 2 ) = -394 kJΔH f  (H 2 O) = -286 kJ ΔH f  (CH 4 ) = -75 kJΔH f  (O 2 ) = 0.0 kJ ΔH rxn  = [(-394 kJ) + (2)(-286 kJ)] – [(-75 kJ) + (2)(0.0 kJ)] ΔH rxn  = [(-394 kJ) + (2)(-286 kJ)] – [(-75 kJ) + (2)(0.0 kJ)] products - reactants products - reactants ΔH rxn  = [-966 kJ] – [-75 kJ] = -891 kJ ΔH rxn  = [-966 kJ] – [-75 kJ] = -891 kJ Is this reaction endothermic or exothermic? Is this reaction endothermic or exothermic?

Exothermic Reactions

Potential Energy converts to Kinetic Energy as you release the energy to the surroundings. Potential Energy converts to Kinetic Energy as you release the energy to the surroundings. Where is the Potential energy stored? Where is the Potential energy stored? How do we know there is a shift to Kinetic energy? How do we know there is a shift to Kinetic energy? Most reactions are exothermic and spontaneous Most reactions are exothermic and spontaneous

Endothermic Reactions

Energy content of products is greater than reactants Energy content of products is greater than reactants If the products have more energy why are the surroundings cold? (Hint: PE↑) If the products have more energy why are the surroundings cold? (Hint: PE↑) The surroundings feel cold because the bonds absorb the heat energy from the surroundings, so The surroundings feel cold because the bonds absorb the heat energy from the surroundings, so Kinetic energy converts to Potential energy Kinetic energy converts to Potential energy

Reaction Spontaneity What happens when you leave a iron nail outside for a few months? What happens when you leave a iron nail outside for a few months? What happens when you light a gas stove? What happens when you light a gas stove? Do these reactions take place spontaneously? (without outside intervention) Do these reactions take place spontaneously? (without outside intervention) Will the reverse of these reactions take place spontaneously? Will the reverse of these reactions take place spontaneously? What about ice melting at room temperature? What about ice melting at room temperature? There is something more than ΔH determining spontaneity! There is something more than ΔH determining spontaneity!

Entropy Symbol: S Symbol: S Measure of the disorder or randomness of the particles that make up the system Measure of the disorder or randomness of the particles that make up the system Molecules are more likely to exist in a high state of disorder than in a low state of disorder. Molecules are more likely to exist in a high state of disorder than in a low state of disorder. Change in entropy is similar to change in enthalpy Change in entropy is similar to change in enthalpy ΔS system =S products - S reactants

Entropy Do you expect the ΔS of the phase change shown below to be positive or negative? Do you expect the ΔS of the phase change shown below to be positive or negative? S products > S reactants ΔS system positive S products < S reactants ΔS system negative

Entropy What happens to molecules when you increase their temperature? What happens to molecules when you increase their temperature? Do you think this will increase or decrease their entropy? Do you think this will increase or decrease their entropy? ΔS positive = more entropy, ie more disorder ΔS positive = more entropy, ie more disorder ΔS negative = less entropy, ie less disorder ΔS negative = less entropy, ie less disorder Reactions tend to go spontaneous towards increased entropy. Reactions tend to go spontaneous towards increased entropy.

Entropy Practice Predict the ΔS for the following changes: Predict the ΔS for the following changes: 1. H 2 O(l) → H 2 O(g) 2. CO 2 (g) → CO 2 (aq) 3. 2SO 3 (g) → 2SO 2 (g) + O 2 (g) 4. NaCl(s) → Na + (aq) + Cl - (aq) 5. CH 3 OH(l) → CH 3 OH(aq)

Gibbs Free Energy By calculating free energy (energy that is available to do work) we can determine if a reaction is spontaneous. By calculating free energy (energy that is available to do work) we can determine if a reaction is spontaneous. Just like Enthalpy and Entropy we can only measure the free energy as a change. Just like Enthalpy and Entropy we can only measure the free energy as a change. ΔG system = ΔH system – TΔS system The sign of ΔG system tells you if the reaction is spontaneous: The sign of ΔG system tells you if the reaction is spontaneous: Negative = spontaneous, will occur Negative = spontaneous, will occur Postive = nonspontaneous, will NOT occur Postive = nonspontaneous, will NOT occur

Gibbs Free Energy How do the enthalpies and entropies affect reaction spontaneity? How do the enthalpies and entropies affect reaction spontaneity? What is happening if ΔG system = 0? What is happening if ΔG system = 0? -ΔH system +ΔH system +ΔS system Always spontaneous, -ΔG system -ΔG system Spontaneous only at high temperatures, + or - ΔG system -ΔS system Spontaneous only at low temperatures, + or - ΔG system Never spontaneous +ΔG system

Review of Symbols ΔH – Tells us if a reaction is endothermic or exothermic (measure of change in energy) ΔH – Tells us if a reaction is endothermic or exothermic (measure of change in energy) Postive = endothermic Postive = endothermic Negative = exothermic Negative = exothermic ΔS – Tells us if the reaction is more or less ordered (randomness of particles) ΔS – Tells us if the reaction is more or less ordered (randomness of particles) Positive = more disordered Positive = more disordered Negative = more ordered Negative = more ordered ΔG – Tells us if the reaction is spontaneous by determining the amount of energy available to do work. ΔG – Tells us if the reaction is spontaneous by determining the amount of energy available to do work. Positive = nonspontaneous Positive = nonspontaneous Negative = spontaneous Negative = spontaneous