Measuring Energy Changes

1 Measuring energy changes - the enthalpy changes from 

When heat flows from one object to another object, they are said to be in thermal contact. During thermal contact, heat will flow from the hotter object to the cooler object until they are both at the same temperature. This is known as thermal equilibrium and the rates of heat flow between the two bodies are the same. Heat can never flow on its own from a cooler object to a hotter object. This is against the laws of thermodynamics. 

Physicists use the concept of internal energy, which is the sum of all types of energies inside a substance, including potential energy due to forces between molecules. A substance does not contain heat but it does contain internal energy. 

The random kinetic energy of particles in matter can be increased in a number of ways. For example, solar energy from the Sun being absorbed by sea water, striking a piece of metal with a hammer many times, compressing the air in a tyre pump for a bicycle or simply using a flame. 

Temperature is direcdy related to the random motion of particles in substances at temperatures above absolute zero. In the case of an ideal gas, the absolute temperature (in Kelvin) is direcdy proportional to the average kinedc energy of translational motion (from one place to another). In liquids and solids there is still a similar relationship. So the warmth you feel when you touch a hot surface is the kinetic energy transferred by molecules in the hot surface to your colder fingers. 

Diet drinks are now labeled as low joule instead of low calorie in European countries. 

You need to be able to convert between calories and joules. We will consider that conversion process in Example 10.1. 

Note that the 1 in the denominator is an exact number by definition and so does not limit the number of significant figures. 

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Convenience and speed are the watchwords of our modern society. One new product that fits these requirements is a container of coffee that heats itself with no batteries needed. Consumers can now buy a 10-ounce container of Wolfgang Puck gourmet latte that heats itself to 145 °F in 6 minutes and stays hot for 30 minutes. What kind of chemical magic makes this happen Pushing a button on the bottom of the container. This action allows water to mix with calcium oxide, or quicklime (see accompanying figure). The resulting reaction 

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An example of a chemical reaction with a measurable energy change is the reaction of an acid and a base. In this activity, you will determine whether the reaction of hydrochloric acid (HCI) with sodium hydroxide (NaOH) absorbs or liberates heat. 

In this review we consider large-scale polymer motions which naturally occur on mesoscopic time scales. In order to access such times by neutron scattering a very high resolution technique is needed in order to obtain times of several tens of nanoseconds. Such a technique is neutron spin echo (NSE), which can directly measure energy changes in the neutron during scattering [32,33]. 

Einstein would have disagreed he would calculate the total internal energy from where m is the mass of the system. He would agree, though, that, in practice, we can only measure energy changes in chemistry. 

The total energy of a system is a difficult quantity to measure directly. It is much easier to measure energy changes dE/dT—for example, the number of joules necessary to raise the temperature of one mole of gas by one degree Kelvin. If the gas is kept in a constant volume container, this is called the constant-volume molar heat capacity cv, and equals 3R/2 (independent of temperature) for a monatomic gas. Each possible direction of motion (x,y, or z) contributes RT / 2 to the total energy per mole, or R/2 to the heat capacity. 

A bomb calorimeter measures energy change at constant volume. A bomb calorimeter tells us the internal energy change in a reaction. (Recall that at constant volume q - ALL) In a bomb calorimeter, a steel container full of reactants is placed mside another rigid, thermally insulated container. 

The following sample problem shows how calorimetry can be used to measure energy change in a chemical reaction. 

Measuring Energy Changes Calorimetry Energy Value of Food Energy Economics MiniLab 20.2 Heat In, Heat Out CbemLab Energy Content of Some Common Foods 719... 

Because all forms of energy are Interconvertible, they can be expressed in the same units of measurement. Although the standard unit of energy is the joule, biochemists have traditionally used an alternative unit, the calorie (1 joule = 0.239 calories). Throughout this book, we use the kilocalorie to measure energy changes (1 kcal = 1000 cal). 

Most physical and chemical changes occur at virtually constant atmospheric pressure—a reaction in an open flask, the freezing of a lake, a drug response in an organism. In this section, we define a thermodynamic variable that makes it much easier to measure energy changes at constant pressure. 

How do we measure energy changes in biochemistry It is possible to measure amounts of energy released or absorbed in a process to see whether it is likely to occur or not. Reactions that release energy are energetically favored, whereas those that require energy are disfavored. 

At this level of interpretation nothing is yet postulated about the forces, except that they manifest themselves in the measurable energy changes which accompany almost every kind of physical and chemical transformation. It is necessary, therefore, to describe the state of a chaotic system of particles primarily in terms of its energy. Such a description is provided in statistical theory. 

The Nature of Energy Temperature and Heat Exothermic and Endothermic Processes Thermodynamics Measuring Energy Changes... 

Measuring energy changes associated with chemical reactions enables us to understand why they happen and what exactly occurs between reactant particles during a reaction. The main reason chemical reactions occur is that the amount of disorder involved always increases as a result. Disorder can increase in two ways either energy is more spread out after the reaction than before, or the number of ways the particles in the reaction are arranged is greater than before. Some chemical reactions involve both of these, others just one. 

A major problem arises from the definition of enthalpy, Equation (3.15). The problem is that we cannot measure it. This arises from the nature of energy itself, because we can only measure energy changes, not absolute energies. Therefore we can only measure enthalpy changes, and changes in any other property which includes the energy U (or U). 

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