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CH 101 Lecture 25: Thermochemistry 2

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24 Mar 2016

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Because e = q + w, we can determine e by measuring q and w. In practice, it is easiest to do a process in such a way that there is no change in volume, so w = 0. At constant volume, change in e of the system = q of the system. In practice, it is not possible to observe the temperature changes of the individual chemicals involved in a reaction, so instead we measure the temperature change in the surroundings. Used to measure change in e because it is a constant volume system. The heat capacity of the calorimeter is the amount of heat absorbed by the calorimeter for each degree rise in temperature and is called the calorimeter constant. The enthalpy, h, of a system is the sum of the internal energy of the system and the product of pressure and volume. When h is -, heat is being released by the system.

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Related Questions

Calorimetry is a method used to measure changes inenthalpy, or heat, that occur during chemical processes. Two commoncalorimeters are constant-pressure calorimeters and constant-volume(or "bomb") calorimeters. Bomb calorimeters are used to measurecombustion and other gas-producing reactions, in which the reactionis observed in a strong, sealed vessel. A simple constant-pressurecalorimeter can be made from a foam coffee cup and a thermometer,in which energy changes in a reaction are observed via the changein temperature of the solution in the cup. The idea behindcalorimeters is that if they are sufficiently insulated from theoutside environment, any energy gained or lost in the chemicalreaction will be directly observable as a temperature and/orpressure change in the calorimeter.

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Energy cannot be created nor destroyed, but it can be transferred between a reaction and its surroundings. The change in energy, Delta E, is positive if the reaction absorbs energy, and it is negative if the reaction releases energy. You may also see this expressed in terms of change in internal energy, Delta U. For the purposes of this question, Delta U and Delta E are equal. One way a reaction can transfer energy to or from the surroundings is by releasing or absorbing heat. A reaction can also transfer energy, in the form of work, through a change in volume. The total change in energy is the sum of the heat and work: Delta E = q + w At constant pressure, q = Delta H and w = -P Delta V and so Delta E = Delta H - P Delta V Assuming constant pressure, rank these reactions from most energy released by the system to most energy absorbed by the system, based on the following descriptions: Surroundings get colder and the system decreases in volume. Surroundings get hotter and the system expands in volume. Surroundings get hotter and the system decreases in volume. Surroundings get hotter and the system does not change in volume. Also assume that the magnitude of the volume and temperature changes are similar among the reactions. Rank from most energy released to most energy absorbed. To rank items as equivalent, overlap them. Energy cannot be created nor destroyed, but it can be transferred between a reaction and its surroundings. The change in energy, Delta E, is positive if the reaction absorbs energy, and it is negative if the reaction releases energy. You may also see this expressed in terms of change in internal energy, Delta U. For the purposes of this question, Delta U and Delta E are equal. One way a reaction can transfer energy to or from the surroundings is by releasing or absorbing heat. A reaction can also transfer energy, in the form of work, through a change in volume. The total change in energy is the sum of the heat and work: Delta E = q + w At constant pressure, q = Delta H and w = -P Delta V and so Delta E = Delta H - P Delta V A mole of X reacts at a constant pressure of 43.0 atm via the reaction X(g) + 4Y(g) - 2Z(g), Delta H degree = -75.0 kJ Before the reaction, the volume of the gaseous mixture was 5.00 L. After the reaction, the volume was 2.00 L Calculate the value of the total energy change. AE, in kilojoules. Express your answer with the appropriate units.