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Purpose of the Experiment Thermochemistry (Heat of Reaction) Determine the heat of neutralization for the reaction of a strong acid and base; and for a weak acid with a strong base. Determine the heat of fusion of ice.

Thermodynamic Definition of Enthalpy (H): H = E + PV E = energy of the system P = pressure of the system V = volume of the system Definition of Enthalpy What is the Heat of Reaction?

Recall, by definition a change in energy equals heat transferred (q) plus work (w):  E = q + w Consider a process carried out at constant pressure. At constant pressure, work involves only a change in volume. We can then substitute -P  V for w.  E = q p - P  V Then if we want to solve for the heat transferred, q p, at constant pressure, we simply rearrange the equation. q p =  E + P  V At Constant Pressure

Recall our original definition of enthalpy: H = E + PV Then for a change in enthalpy:  H =  E +  (PV) If we set P constant, then:  H =  E + P  V Since q p =  E + P  V Then  H = q p The change in enthalpy,  H, is then equal to the heat transferred at constant pressure, q p. Enthalpy = Heat Transferred

In a chemical reaction  H = H products – H reactants If  H >0, then q p >0 The reaction is Endothermic. Heat goes from the surroundings into the system. If  H <0, then q p <0 The reaction is Exothermic. Heat goes from the system into the surroundings. An example of an exothermic reaction: An example from the S&T mining dept:

Heat Capacity, C “C” is an extensive property; so a large object has a larger heat capacity than a small object made of the same material. Using the Equation: Looking at the figures on the left, it can be seen that the temperature change is constant, but the heat absorbed by the larger object is greater. This results in a larger heat capacity for the larger object because more heat is absorbed.

 Specific heat capacity: The energy (joules) required to raise the temperature of 1 gram of substance by 1  C Unit: J g -1 K -1 or J g -1  C -1  Molar heat capacity: The energy (joules) required to raise the temperature of 1 mol of substance by 1  C Unit: J mol -1 K -1 or J mol -1  C -1

Substance Specific Heat, Cs (cal/gram°C)(J/kg °C) Pure water1.004,186* Wet mud0.602,512 Ice (0 °C)0.502,093 Sandy clay0.331,381 Dry air (sea level)0.241,005 Quartz sand Granite calorie = joules *The high heat capacity of water makes it ideal for storing heat in solar heating systems.

Neutralization HCl aq + NaOH aq  NaCl aq + H 2 O The reaction between an acid and a base which results in a salt plus water. Another example, cyanic acid and a hydroxide ion. For example, hydrochloric acid and sodium hydroxide: acid + base  salt + water

Heat of Neutralization Energy released by reaction = Energy absorbed by solution C s = q / [(mass) (T final -T initial )] Net ionic equation for neutralization: H + (aq) + OH - (aq)  H 2 O(l) Specific heat capacity, C s, is defined as the quantity of heat transferred, q, divided by the mass of the substance times the change in temperature. A value of C s is specific to the given substance. q = C s (mass) (T final -T initial ) This can then be rearranged to solve for the heat transferred.

Enthalpy of Fusion (Melting) Enthalpy of Fusion is defined as the heat that is absorbed when the melting occurs at constant pressure. If the substance freezes, the reaction is reversed, and an equal amount of heat is given off to the surroundings; i.e., ΔH freez = - ΔH fus Melting (fusion) is an endothermic process solidliquid

Calorimetry Science of measuring heat based on observing the temperature change when a body absorbs or loses energy as heat. A calorimeter can be created by doing something as simple as inserting one Styrofoam cup inside another.

A Calorimeter may be used to determine the Heat Capacity, Cs, of a material by measuring the temperature change when a known mass of the material at a higher temperature is placed in a known mass of water, usually at room temperature, and the system is allowed to reach a final intermediate temperature. Heat lost by hot object = Heat gained by cold water Cs material (mass) material (T final -T initial ) material = Cs water (mass) water (T final -T initial ) water Note: The heat capacity is related to the atomic mass and the intermolecular forces in the material. Calorimetry

A Calorimeter may be used in a similar manner to determine the enthalpy change associated with other processes, such as:  Chemical reactions* (bond energies)  Phase changes* (intermolecular forces)  Mixing (intermolecular forces)  Solvation (intermolecular forces) Calorimetry *These are the processes you will be learning today.

A Bomb Calorimeter is used to determine the caloric value of food and of fuels, by burning them in excess oxygen and measuring the amount of heat evolved. A basic combustion reaction: C x H y + O 2 (excess) --> x CO 2 + y / 2 H 2 O + heat

The Computer Display Setup for the Experiments Parameters: Temperature: o C Time: seconds (Check: Probe should display o C resting on lab bench and should read higher when warmed by hand.)

Temperature change is important. Exact time is not important. Temperature will drift toward ambient before and after reaction Transition will be faster if NaOH is added rapidly and well stirred. (That is you will have a more nearly vertical temp. rise) HCl (or acetic acid) and NaOH mixed, reaction begins Reaction is completed, heat released, begin slow cooling to ambient The Heat of Neutralization Experiments

IMPORTANT: Use only 1 ice cube, the entire cube must melt. The Heat of Fusion Experiment Ice cube added Melting complete, begin slow warming

Checkout 1-Calorimeter – Return to stockroom. 1-styrofoam cup – Return to stockroom. Reagents in Lab _____M HCl (record) _____M CH 3 CO 2 H (record) _____M NaOH (record) Important: Use distilled water from carboys*, NOT from the tap. (*Distilled water from the tap is normally not at room temp.)

Hazards HCl, strong acid, corrosive CH 3 CO 2 H, weak acid, corrosive (neutralize acid spills with solid NaHCO 3 ) NaOH, strong base, pH>14, corrosive Waste Liquid waste labeled “Calorimetry” or “Heat of Neutralization”’ For October *Thermochemistry pp 9, 11, 13, & 17 and a calculations page are due. *Read over “Radiochemistry” pp & remember to bring your student id.