# Thermochemistry: Enthalpy Changes in Simple Processes

Topics: Thermodynamics, Ammonia, Temperature Pages: 6 (2451 words) Published: May 20, 2013
Introduction
The primary objective of this lab is to be able to determine the specific heat of a reaction by using a calorimeter. A calorimeter is a device used to determine the specific heat of chemical reaction or a physical change. The specific heat a reactions is used to refer to the amount of heat that is lost or gained when one gram of a particular substance increases or decreases by one degree Celsius. When a chemical reaction occurs in an open container most of the energy gained or lost is in the form of heat. Almost no work is done (i.e. nothing is being moved). Heat flows between the system and surroundings until the two are at the same temperature, when a chemical reaction occurs in which the system absorbs heat, the process is endothermic (it feels cold). When a chemical reaction occurs in which the system produces heat it is exothermic (it feels hot) Under conditions of constant pressure the heat absorbed or released is termed enthalpy (or "heat content"). We do not measure enthalpy directly, rather we are concerned about the heat added or lost by the system, which is the change in enthalpy (or ΔH The quantity of heat gained or lost by a system, ΔH, is dependent upon, the mass, m, of the system: the more massive an object the more heat needed to raise its temperature, the change in temperature, (ΔT): the larger the temperature change in a system the more heat exchanged, and the nature of the substance(s) making up the system. The last quantity is defined by the heat capacity of the system. For a given substance, the specific heat capacity is defined as the quantity of heat needed to raise 1 gram of the substance by 1 degree Celcius. Specific heat capacity has units of joules per degree Celcius per gram, J.g-1.ºC-1.The three quantities combine to give the quantity of heat gained, or lost, by a system: ΔH = mass x Specific heat capacity x ΔT

ΔT represents the difference between the enthalpy of the system at the beginning of the reaction compared to what it is at the end of the reaction: ΔT = Tfinal - Tinitial
We are considering the enthalpic state of the system. If the temperature has higher enthalpy at the end of the reaction then the initial, then it absorbed heat from the surroundings (endothermic reaction). If the temperature has a lower enthalpy at the end of the reaction then the initial, then it gave off heat during the reaction (exothermic reaction) I. Determining the specific heat capacity of a metal

Fill a 1 L beaker with 900mL of water and place on a hot plate and bring to a boil. Then weigh out 31.00-34.00g of Mossy Tin and measure to the nearest 0.01 g. Put the mossy tin in a large test tube, and insert a thermometer into the center of the tin. When water is boiling, place the test tube in the boiling water clamping it so that the test tube is not touching the beaker and make the sure water level outside the test tube is higher than the top of the metal inside the test tube. Let the test tube of tin stay in the boiling water for 15 minutes. While waiting for the tin to be heated get the mass of a Styrofoam cup and pour 50 ml of water into it. Then measure the mass of water in the cup by assuming the density of water is 1.0 g/ml. When the tin has been in the boiling water for 15 minutes, record the temperature of the tin, and using a separate thermometer record the temperature of the water in the cup, the temperature will be the initial temperatures. Quickly pour the hot metal into the water in the styrofoam cup. Stir the mixture, then observe the rise in temperature of the water in the cup. When the temperature stops rising, record the temperature and those are the final temperature of both the water and the tin. Then with the givin infromation determine the specific heat capacity of tin. II. Determining the Enthalpy of solution of a salt.

Get the mass of a Styrofoam cup and pour 30 ml of water into it. Then measure the mass of water in the cup by assuming the density of water is 1.0 g/ml,...