Unit 3B Thermochemistry

Measuring Energy Changes A calorimeter is a device used to measure the energy given off or absorbed during chemical or physical changes. When the reaction occurs inside the sample cup, it causes a change in temperature of the surrounding water. The temperature change is used to measure the heat absorbed or released by the reaction. Click on the red box to see how a bomb calorimeter works. Click on part 18 in Saunders and click on red arrow to bomb calorimeter.

Specific Heat The heat needed to raise the temperature of one gram of a substance by one Celsius degree is called the specific heat (s or Cp) = J/g oC. Every substance has its own specific heat. For example, the heat required to raise the temperature of one gram of water one Celsius degree is 1 calorie or 4.184 joules. Therefore s or Cp = 1 cal/g . Co for water. Also, s or Cp = 4.184 J/g . Co for water. The ability of different substances to “take the heat” is similar to people being able to “take the pain”. Different people have different abilities to absorb pain, this is similar to how different substances have different abilities to absorb heat. The higher the heat capacity of the substance, the less it will “scream about the pain” or the less of an increase in heat it will have because it has a higher tolerance for the heat or “pain” Click on the red box and click on Saunders section 8 and then the red arrow at specific heat button in order to demonstrate the different heat capacities of substances.

The law of conservation of energy The law of conservation of energy says that in an enclosed system, energy will be conserved. From this law, we can see how a calorimeter works to measure energy. The energy that is released from the reaction is transferred into a change in temperature (thermal energy). From this law, the following relationship is used to calculate the heat that is gained or lost by a reaction inside a calorimeter. (heat gained or lost by water) = (mass in grams) x ( change in temperature) x (specific heat) q = (m) x ( Tfinal – Tinitial) x (s) OR Q = (m) x ( Tfinal – Tinitial) x (Cp )

Energy and Chemical Changes Chemical changes are always associated with a change in energy. Thermochemistry is the study of heat change in chemical reactions. Endothermic Reactions - If energy is absorbed in a reaction, the reaction is endothermic. When this happens, energy will be pulled from the surroundings into the reaction in order to form products. No heat, light or motion will be given off in the reaction, instead the reaction will turn cold because it will pull energy out of the surroundings in order to form the product. Reactant A + Reactant B + Energy Products I remember endothermic because energy is entering the reaction but it is not leaving. We all are familiar with cold packs, where we activate a cold pack by squeezing it. The cold pack turns cold because an endothermic reaction is taking place inside of the pack and it needs energy in order to form it’s products. Therefore, it will take energy from it’s surroundings and it will become cold. Because energy is taken in by the reactants in order to form the product. The products hold a higher energy value than the reactants do. Nature favors the easiest way. Therefore, because it takes energy in order to create endothermic reactions, they are not spontaneous and they are called reactant favored reactions.

Exothermic Reactions If energy is given off by a reaction, the reaction is exothermic. When this happens, energy will be given off by the system into the surroundings. Heat, motion or light will be given off in the reaction. Reactant A + Reactant B Products + Energy I remember exothermic because energy is exiting the reaction and leaving the system into the surroundings. Because the system has lost energy, the products of the reaction have less energy than the reactants did. We all remember the gummy bear experiment, Click on the red box to see it again, this is an example of an exothermic reaction. The bright light, movement of the reaction and the heat showed that energy was exiting the reaction. Click on the purple rectangle and let’s all decide which picture is exothermic and which is endothermic. Nature favors the easiest way. Therefore, because energy is not required in an exothermic reaction, they are spontaneous and they are called product favored reactions.

Exothermic Reaction Endothermic Reaction Click on the green button to view the endo and exothermic reaction on a macro and microscopic level for the melting and freezing of water. Exothermic Reaction Endothermic Reaction

Higher thermal energy = Higher movement of molecules H2O(s) H2O(l) Lower thermal energy = Lower movement of molecules Low

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) Higher thermal energy = Higher movement of molecules High CH4(g) + 2O2(g) CO2(g) + 2H2O(l) Lower thermal energy = Lower movement of molecules Low

Activation Energy, Ea Both endo and exothermic reactions require a minimum amount of energy in order to start a reaction. This is called the activation energy. If we do not have activation energy, the bonds between the reactants will never break and a reaction will never occur.

Activation Energy, Ea When molecules collide, they need enough energy from their collision to break the bonds in order for a chemical reaction to occur. If there is little amount of energy held in their kinetic energy from motion, then the bonds will not break and the molecules will simply bounce off one another. We postulate that in order to react, the colliding molecules must have a total kinetic energy that is greater than or equal to the activation energy, Ea, which is the minimum amount of energy required to initiate a chemical reaction. When molecules collide, they form an activated complex (also known as the transition state), which is a temporary species formed by the reactant molecules as a result of the collision before they form the product.

Exothermic Reaction Endothermic Reaction

Enthalpy Almost every reaction that we see will take place at constant pressure. Beakers being open in the lab, etc. The pressure is usually equally to the atmospheric pressure of 1 atm. Enthalpy (H) - is the heat flow into or out of a system in a constant-pressure process. (Meaning the heat flow that we normally experience inside a lab with open beakers.) Remember that heat is defined as the transfer of thermal energy between two bodies at different temperatures. Thermal energy is the energy associated with the random motion of atoms and molecules. Enthalpy can therefore also be thought of as the transfer of energy, into or out of a system, that is associated with the random motion of atoms and molecules at a constant-pressure process.

The Enthalpy of Reaction H Because enthalpy is the transfer of energy as heat flow into or out of a system, it is impossible to determine the enthalpy, H, of a substance. For this reason, we measure the enthalpy of reaction, H. This is the difference between the enthalpies (heat flow) of the products and the enthalpies (heat flow) of the reactants: H = H (products) – H (reactants) H represents the heat given off or absorbed during a reaction. This can be a + or – value. For endothermic reactions, heat is absorbed by the system from the surroundings and it is therefore a + H value. For exothermic reactions, heat is released by the system into the surroundings and it is therefore a - H value.

Thermochemical Equations The previous two examples would have the following thermochemical equations: H2O(s) H2O(l) H = + 6.01kJ CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890.4 kJ What would happen to the enthalpy if the combustion of methane went straight into making a gaseous water form for a product? CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H = -802.4 kJ This shows us the value of knowing the term thermal energy and how it relates to enthalpy and it also shows us that writing the physical states of each reactant and product is necessary in a thermochemical equation.

Thermochemical equations The stoichiometric coefficients always refer to the number of moles of a substance. H2O(s) H2O(l) H = + 6.01kJ Therefore, the equation for the melting of ice may be “read” as follows: one mole of liquid water is formed from one mole of ice at 0 degrees Celsius, the enthalpy change is 6.01 kJ. CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890.4 kJ For the combustion of methane, we interpret the equation this way: when 1 mole of gaseous methane reacts with 2 moles of gaseous oxygen to form 1 mole of gaseous carbon dioxide and 2 moles of liquid water, the enthalpy change is -890.4 kJ.

Mass-Energy Problems Given the thermochemical equation 2SO2(g) + O2(g) 2SO3(g) + heat(198.2 kJ) Calculate the heat evolved when 74.6 g of SO2 (molar mass = 64.07 g/mol) is converted to SO3. 74.6 g SO2 1 mol SO2 -198.2 kJ = -115 kJ 64.07 g SO2 2 mol SO2 Another example: Calculate the heat evolved when 266 g of white phosphorus (P4) burn in air according to the equation: P4(s) + 5O2(g) P4O10(s) + heat(3013 kJ)

Thermochemical Equation Example Problem Pentaborane-9, B5H9, is a colorless, highly reactive liquid that will burst into flame or even explode when exposed to oxygen. The rxn is: 2 B5H9(l) + 12O2(g) 5B2O3(s) + 9H2O(l) + heat(9036.6 kJ) Calculate the heat released from the reaction if 100 g of B5H9 react with oxygen. 100 g B5H9 1 mol B5H9 -9036.6 kJ = -7158.3 kJ 63.12 g B5H9 2 mol B5H9