Thermochemistry Chapter 17

Thermochemistry Thermochemistry is the study of heat changes that occur during chemical reactions.

Energy is the ability to do work or cause change. Work is force applied over a distance Potential Energy is stored energy Kinetic Energy is energy of motion Chemical Potential Energy is stored within the structural units of chemical substances.

Heat Heat (q) is Energy that is transferred from one object to another Transfer is due to temperature difference between the two substances Heat ALWAYS travels from hot to cold Remember, temperature is a measure of the average kinetic energy of a substance

Heat CANNOT be detected. the changes CAUSED by heat however can be ex. Rise in temperature

Law of Conservation of Energy In any chemical or physical process, energy is neither created nor destroyed. All of the energy involved can be accounted for as Work, Potential Energy, or Heat.

Terms for studying heat: System – what you are focusing on Surroundings – everything else Universe = System + Surroundings Direction of heat flow is always described relative to the system. Endothermic process – Energy is absorbed by the system from the surroundings (Heats up) Exothermic process – Energy is released by the system into the surroundings (Cools down)

Endothermic vs. Exothermic Exothermic Lose Heat - q Endothermic Gain Heat + q

Units of Heat calorie- the quantity of heat needed to raise the temperature of 1 g of pure H2O 1 degree Celsius.   Calorie = 1000 calories = 1 kilocalorie => Dietary Calorie Joule – the quantity of heat needed to raise the temperature of 1 g of pure H2O, 0.2390 degrees Celsius.   4.184 J = 1 cal

Heat Capacity vs. Specific Heat Capacity Heat capacity – the amount of heat needed to raise the temperature 1 degree Celsius, for any substance Depends on mass and composition Specific heat capacity (specific heat) – the amount of heat it takes to raise the temperature of 1 g of a substance 1 degree Celsius. Depends on composition only

Calculation Specific heat = Heat______________ Mass x Change in Temperature Specific heat (C or Cp) Heat (q) Mass (m) Change in temperature ( ∆T) q = m C ∆T

Measuring and Expressing Heat Changes (change in temperature) Calorimetry- Accurate and precise measurement of heat change Instrument – Calorimeter For systems at constant pressure, the heat content is the same as enthalpy (H) Heat changes are the same as changes in enthalpy (ΔH) q = ΔH Since q = ΔH, then by substitution ΔH = m x C x ΔT

Heat in Changes of State Molar Heat of Fusion (ΔHfus) – Solid to Liquid Molar Heat of Vaporization (ΔHvap) – Liquid to Gas q = ΔH x mass OR q = ΔH x moles Depends on units of ΔH

Phase Change Diagram

Phase Changes

Group Concept Questions Your text defines energy as “the ability to do work or to cause change”. Another definition of energy is “the ability to resist a natural tendency”. Explain this definition and provide an example. A friend of yours reads that the process of water freezing is exothermic. This friend tells you that this can’t be true because exothermic implies “hot”, and ice is cold. Is the process of water freezing exothermic? If so, explain it so your friend can understand it. If not, explain why not.

Group Concept Questions You place hot metal into a beaker of cold water. Eventually what is true about the temperature of the metal compared to that of the water? Explain why this is true.  Label this process as endothermic or exothermic if we consider the system to be: the metal. Explain the water. Explain

Group Concept Questions The text describes the law of conservation of energy. Is there a law of conservation of heat? Explain why or why not. What does it mean when the heat for a process is reported with a negative sign? You place 100.0g of a hot metal in 100.0g of cold water. Which substance (metal or water) undergoes a larger temperature change? Why is this?

Group Concept Questions A desert is very hot during day but quite cold at night. In the Midwest of the United States, the temperature is more constant between day and night in the summer. Why is this? Explain why aluminum cans make good storage containers for soft drinks.

Calorimetry Problems: Changes in Temperature How much heat is absorbed by 60.0 g of copper when its temperature is raised from 20 oC to 80 oC? What is the specific heat of a 124 g sample of brass if 3.94 x 103 J raises the temperature of the brass from 12.5 oC to 97.0 oC? If 350 J of heat energy are added to 100 g of a metal and the temperature changes by 25 oC, what is the specific heat of the metal? What is the identity of the metal? 10g of an unknown metal requires 39J of energy to increase its temperature from 50 oC to 60 oC. What is the specific heat of the metal? Identify the metal.

Calorimetry Problems: Heat Lost = Heat Gained When 80.0 grams of a certain metal at 90.0 °C was mixed with 100.0 grams of water at 30.0 °C, the final equilibrium temperature of the mixture was 36.0 °C. What is the specific heat of the metal? Calculate the specific heat of a metal if a 55.0 g sample of an unknown metal at 99.0 °C causes a 1.7 °C temperature rise when added to 225.0 g of water at 22.0 °C.

Calorimetry Problems: Heat Lost = Heat Gained A piece of an unknown metal with mass 23.8 g is heated to 100.0ºC and dropped into 50.0 cm3 of water at 24.0ºC. The final temperature of the system is 32.5ºC. What is the specific heat of the metal? A blacksmith heated an iron bar to 1445ºC. The blacksmith then tempered the metal by dropping it into 42,800 mL of water that had a temperature of 22ºC. The final temperature of the system was 45ºC. What was the mass of the bar? Note: Specific heat of iron is 0.4494 J/g·Cº.

Energy and Reactions How does ENERGY affect REACTIONS

Enthalpy The amount of energy gained or released in a reaction is the ENTHALPY (∆H). ALL reactions require energy to occur. The amount of energy needed to occur is called the ACTIVATION ENERGY. The more energy required for the reaction to occur, the less likely the reaction will happen. Because of the energy released in exothermic reactions, exothermic reactions are more likely to occur than endothermic reactions. In other words, they are more spontaneous.

Exothermic and Endothermic Reactions

Catalysts Catalysts speed up reactions by lowering the activation energy required for the reaction to occur. Enzymes are biological catalysts. Most catalysts work by helping ions and molecules to “line up” the right way so they can react. http://www.dlt.ncssm.edu/tiger/Flash/kinetics/EnzymeCatalyst.html

Reaction Path

Entropy Whether or not a reaction will occur depends on both ENTHALPY and ENTROPY. Entropy is a measure of disorder. Increasing disorder is a spontaneous process. Entropy of States: solid < liquid < gas Kinetic ..Energy – Gas Kinetic Energy – Liquid Kinetic Energy – Solid Mixing substances, combining or separating elements, and changing the temperature can affect the level of disorder as well. Mixing gases

Spontaneous? If energy is released and disorder is increased, the reaction will be spontaneous. In other words, it will happen “on its own.” If energy is absorbed and disorder decreases, the reaction will be non-spontaneous. These reactions will need “help” to occur. They can’t do it by themselves. If one factor is favorable and one is unfavorable, then spontaneity will depend on the values of enthalpy and entropy.

Reaction Rates How fast will the reaction happen?

Rates of Reaction A rate is the measure of change over time. Reaction rates measure the change in reactants over time. The rate of a reaction is governed by collision theory. Collision theory states that in order for a reaction to occur, the reactants must collide and collide with enough energy to overcome the activation energy barrier to form products.

Collision Theory So…anything that affects the number of collisions that occur in a reaction or the amount of energy the reactants have will affect the rate of reaction. The rate of a reaction is dependent on several factors. Properties of the elements and compounds in the reaction Temperature of the reaction Amount of reactants present Size of the particles Presence of catalysts or inhibitors

Properties The combination of elements and compounds in a reaction will affect the rate of reaction. Some substances are more reactive than others. Ex. Metals rusting Also, some substances are more reactive when in the presence of other substances. Ex. Baking soda in water vs. baking soda in vinegar

Temperature Measures the average kinetic energy of a substance Raising the temperature, raises the kinetic energy of the substance Increasing kinetic energy makes it MORE likely a reaction will occur, thereby INCREASING the reaction rate Raising the temperature also increases the number of collisions that occur Increasing the collisions makes it MORE likely a reaction will occur, thereby INCREASING the reaction rate

Concentration Measures the amount of reactants that are present Moles / Liter (molarity) Increasing the concentration, increases the number of collisions that occur Increasing the collisions makes it MORE likely a reaction will occur, thereby INCREASING the reaction rate

Particle Size Measures the size of the particles of the substance Ex. powder vs. crystal vs. chunks Decreasing the particle size, increases the surface area of the substance. The increased surface area increases the number of collisions that occur Increasing the collisions makes it MORE likely a reaction will occur, thereby INCREASING the reaction rate

Catalysts and Inhibitors Alter the activation energy or the ability of substances to collide Catalysts affect the activation energy Inhibitors affect the substances’ ability to interact Catalysts lower the activation energy making it easier for the reactions to happen Lowering the activation energy makes it MORE likely a reaction will occur, thereby INCREASING the reaction rate Inhibitors make it difficult for reactants to interact with each other Decreasing the collisions makes it LESS likely a reaction will occur, thereby DECREASING the reaction rate