Enthalpy and Hess’s Law

Applying Hess’s Law, and Generalizing it to Different Physical Situations

Introduction

In a recent post, I introduced a thermodynamic state function called enthalpy and introduced something called Hess’s Law. That post was following up on two previous posts, the first of which covered the 1st and 2nd Laws of Thermodynamics, entropy, and the distinction between state functions vs path functions, and the second of which covered the concepts of work, heat transfer, reversibility, and internal energy in thermodynamic systems. Just to recap, enthalpy H is a state function that scientists and engineers use to analyze the thermodynamic properties of certain physical processes (particularly chemical reactions). More accurately, it’s the change in this function that we’re most interested in, particularly at constant pressure, which is the case for most biological processes as well as many experimental situations. You can check out the previous article for the gory details, but the take home message was that the change in enthalpy at constant pressure is equal to the heat transferred to or from the system, and that its status as a state function led to an important general result called Hess’s Law. The tl; dr version of Hess’s Law is that the change in enthalpy for a reaction is the sum of the enthalpies of formation of the products, each multiplied by its corresponding coefficient (n) from its balanced chemical equation, minus the enthalpies of formation of the reactants, each (again) multiplied by its corresponding coefficient. Hess’s Law can also be concisely summarized by the following equation which uses sigma (summation) notation:  (8). This is important to scientists and engineers because it has permitted the tabulation of many experimentally-derived ΔH values under a set of standardized conditions. Adding and subtracting various combinations of these can facilitate convenient thermodynamic predictions for a wide variety of reactions. The remainder of this article deals with how Hess’s Law is applied and generalized to a variety of physical situations. (more…)

By Credible Hulk, ago
Enthalpy and Hess’s Law

Enthalpy: Exothermic vs Endothermic Processes

Introduction

In a recent article, I talked about the 1st and 2nd Laws of Thermodynamics, entropy, and the distinction between state functions vs path functions. More recently, I wrote another one in which I talked about heat, work, reversibility, and internal energy in thermodynamic systems. At the end of the latter post, I mentioned in passing that the path-dependence of heat and work done by non-conservative forces makes it desirable to work with state functions whenever feasible, because it’s not always easy to know the precise path by which a system arrived at its current state from a prior one. That’s where a function called enthalpy comes into play. (more…)

By Credible Hulk, ago
Heat

Work, Heat, and Internal Energy

Heat, and internal energy

In a recent article, I mentioned in passing that the internal energy of a system is a state function. Just to quickly recap, state functions are properties of a physical system whose values do not depend on how they were arrived at from a prior state of the system. They depend only on the starting and ending states of the system. I then contrasted state functions with path-dependent functions, which can take on very different values depending on the path by which the system arrived at its current state from its previous state (the history of the system matters). Perhaps counter-intuitively, while it’s true that internal energy is a state function, the change in a system’s internal energy is the sum of two path-dependent functions. (more…)

By Credible Hulk, ago
Entropy

State Functions, Entropy, Path Dependence, and Energy Conservation in Thermodynamic Systems

State Functions vs Path Dependent Functions

In thermodynamics, scientists distinguish between what are called state functions vs path functions. State functions are properties of a system whose values do not depend on how they were arrived at from a prior state of the system. They depend only on the starting and ending states of the system. On the other hand, path functions can take on very different values depending on the path by which the system arrived at a state from its previous state. (more…)

By Credible Hulk, ago
Science Communication

Admin Reflections (July 2018)

Since I started doing science blogging and skeptical outreach, I’ve noticed that my most popular posts usually involve topics that are considered controversial by the general public, even if they aren’t considered as controversial by actual experts: i.e. genetically engineered foods, anthropogenic climate change, the safety and efficacy of municipal water fluoridation, the importance of vaccines, and anti-science conspiracy speculationism. (more…)

By Credible Hulk, ago
General Skepticism

The Streisand Threshold

The Streisand Threshold: Choosing one’s battles

There is a tremendous amount of pseudoscience and other misinformation that circulates-- seemingly unimpeded-- on the internet. Consequently, it can be a daunting task for those of us who fight against it to know which targets are an effective use of our time and effort. There are various considerations that might inform one’s decision about where (or toward whom) to direct one’s science advocacy or skeptical outreach: (more…)

By Credible Hulk, ago
Philosophy of Science

Incommensurability, The Correspondence Principle, and the “Scientists Were Wrong Before” Gambit

Introduction

One of the intrinsic features of the scientific process is that it leads to modifications to previously accepted knowledge over time. Those modifications come in many forms. They may involve simply tacking on new discoveries to an existing body of accepted knowledge without really contradicting prevailing theoretical frameworks. They may necessitate making subtle refinements or adjustments to existing theories to account for newer data. They may involve the reformulation of the way in which certain things are categorized within a particular field so that the groupings make more sense logically, and/or are more practical to use. In rare cases, scientific theories are replaced entirely and new data can even lead to an overhaul of the entire conceptual framework in terms of which work within a particular discipline is performed. In his famous book, The Structure of Scientific Revolutions, physicist, historian, and philosopher of science, Thomas Kuhn referred to such an event as a “paradigm shift.” [1],[2]. This tendency is a result of efforts to accommodate new information and cultivate as accurate a representation of the world as possible. (more…)

By Credible Hulk, ago
Chemophobia

Glyphosate and the Gut Microbiome: Another Bad Argument Annihilated

Introduction:

Glyphosate is a broad spectrum herbicide that was first introduced by the Monsanto company in the 1970s under the brand name Roundup. The already popular product grew even more popular among farmers upon the introduction of various commodity crops which were genetically engineered to resist the herbicide while it killed the surrounding weeds with which the crops would otherwise compete for water and nutrients. Glyphosate went off patent back in the year 2000, and since then many manufacturers have cashed in on its popularity [1]. Although it is of unusually low toxicity, glyphosate receives a level of scrutiny and vehemence of criticism that is disproportionate to its actual established risks [2],[3],[4]. This is attributable in part to its ubiquity in modern conventional farming, but it's likely even more attributable to its association with Monsanto, against which a large and well-organized counter-movement has emerged [5]. (more…)

By Credible Hulk, ago
Genetic Engineering and Agricultural Science

Genetically Engineering Foods Involves Greater Precision and Lower Risk of Unintentional Changes Than Traditional Breeding Methods

Introduction

There exists an international scientific consensus that existing genetically engineered foods are at least as safe as their closest corresponding non-GMO counterparts [1]. This consensus is drawn on decades of research and thousands of studies. Despite this fact, there still exists a broad gap between the science and public perception of the topic. According to PEW reports, this gap is wider than on any other topic for which a strong scientific consensus exists [2]. This is significant because we know several other scientific topics have remained highly controversial in the court of public opinion for decades after having achieved mainstream acceptance among experts. Young Earth Creationists have been trying out various strategies for roughly a century to undermine the teaching of evolution in public schools in the US [3]. Rejection of Anthropogenic Climate change is ubiquitous in the US, and even extends to the POTUS himself, despite the weight of the evidence and resulting scientific consensus to the contrary [4],[5],[6],[7]. Although its prevalence has waxed and waned over time, vaccine opposition has been a near constant presence ever since the discovery of the cowpox vaccine [8],[9]. So, for there to exist an even bigger gap between science and public perception on GE foods than for any of these other topics, is no trivial matter. One of the most common reasons given for trepidation with respect to GE foods is the idea that it's a more radical way of altering our foods than more traditional methods of artificial selection, and that we therefore can't know whether or not GE foods are bad until they've been around for several more human generations. This view relies on an argument from ignorance logical fallacy, and presents a double standard with no biologically plausible justification. One fact that proponents of this view rarely acknowledge is that it also leads to the generation of testable hypotheses, the results of which actually suggest the exact opposite. Allow me to explain. (more…)

By Credible Hulk, ago
Alternative Medicine

Scientific Consensus isn’t a “Part” of the Scientific Method: it’s a Consequence of it

Although conceptually simple, the term “scientific consensus” is often misused and misunderstood. It can get confused with appeals to popular opinion or erroneously conflated with “consensus” in the colloquial sense of the word. These misunderstandings can lead to things like opinion polls, often predominated by unqualified individuals, being misconstrued as evidence that no scientific consensus exists on some topic for which it clearly does, or that it leans towards a different conclusion than it actually does. In some cases, the very concept itself invokes resentment or even retaliatory commentary from people whose views are threatened by its implications. The purpose of this article is to clarify the concept that the term scientific consensus is meant to refer to and address some of the arguments commonly leveled against it.

Defining Scientific Consensus

Just as the term “theory” has a different meaning in science than its colloquial usage, the term scientific consensus means something different than “consensus” in the usual colloquial sense. The latter typically refers to a popular opinion, and needn't necessarily be based on knowledge or evidence. On the other hand, a scientific consensus is, by definition, an evidence-based consensus. A convergence of the weight of existing evidence is a prerequisite which distinguishes a knowledge-based scientific consensus from mere agreement. This is critical, because the scientific enterprise is essentially a meritocracy. As a result, it doesn't matter if a few contrarians on the fringe disagree with the conclusions unless they can marshal up evidential justification of comparable weight or explain the existing data better. The weight of the evidence is paramount. (more…)

By Credible Hulk, ago