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Hess's Law is one of the topic that we 12-STEM encountered for the last semester of our Chemistry Class. Discussing it was a bit confusing since it requires a lot of practices in order to solve each problem and one must really understood the concept for him to arrive at the right answer. Similarly, our class used up a lot of time just for the whole to fully understand and be able to solve the problem solving without the help of our teacher. We would admit that at first we really found it very difficult but with the help of our classmates, subject teacher and even from other sources like Khan academy; soon we were able to overcome those struggles and be familiar to the process of solution.

This blog will focus on helping the upcoming Grade 12 or for those who wanted to understand the Concept and solution of Hess's Law since this will give you some overview of the solutions and guidelines on how to easily solved a problem solution of Hess's Law.

But first, there are a couple notes to keep straight before beginning. 1. If a reaction is reversed, the sign of the change in enthalpy (ΔHf) changes. For example: the reaction C(s) + O2(g) → CO2(g) has an ΔHf of -393.5 kJ/mol. The reverse reaction CO2(g) → C(s) + O2(g) has a ΔHf of +393.5 kJ/mol.

2. If a reaction is multiplied by a constant, the change in enthalpy is changed by the same constant. Example, for the previous reaction, if three times the reactants are allowed to react, ΔHf is changed by three times.

3. If ΔHf is positive, the reaction is endothermic. If ΔHf is negative, the reaction is exothermic.

EXAMPLE:

Find the enthalpy change for the reaction

CS2(l) + 3 O2(g) → CO2(g) + 2 SO2(g)

when:

C(s) + O2(g) → CO2(g); ΔHf = -393.5 kJ/mol

S(s) + O2(g) → SO2(g); ΔHf = -296.8 kJ/mol

C(s) + 2 S(s) → CS2(l); ΔHf = 87.9 kJ/mol

SOLUTION:

Hess’s Law problems can take a little trial and error to get started. One of the best places to begin is with a reaction with only one mole of reactant or product in the reaction. Our reaction needs one CO2 in the product and the first reaction also has one CO2 product. C(s) + O2(g) → CO2(g) ΔHf = -393.5 kJ/mol This reaction gives us the CO2 needed on the product side and one of the O2 needed on the reactant side. The other two O2 can be found in the second reaction. S(s) + O2(g) → SO2(g) ΔHf = -296.8 kJ/mol Since only one O2 is in the reaction, multiply the reaction by two to get the second O2. This doubles the ΔHf value. 2 S(s) + 2 O2(g) → 2 SO2(g) ΔHf = -593.6 kJ/mol Combining these equations gives 2 S(s) + C(s) + 3 O2(g) → CO2(g) + SO2(g) The enthalpy change is the sum of the two reactions: ΔHf = -393.5 kJ/mol + -593.6 kJ/mol = -987.1 kJ/mol This equation has the product side needed in the problem but contains an extra two S and one C atom on the reactant side. Fortunately, the third equation has the same atoms. If the reaction is reversed, these atoms are on the product side. When the reaction is reversed, the sign of the change in enthalpy is reversed. CS2(l) → C(s) + 2 S(s); ΔHf = -87.9 kJ/mol Add these two reactions together and the extra S and C atoms cancel out. The remaining reaction is the reaction needed in the question. Since the reactions were added together, their ΔHf values are added together. ΔHf = -987.1 kJ/mol + -87.9 kJ/mol ΔHf = -1075 kJ/mol Answer: The change in enthalpy for the reaction CS2(l) + 3 O2(g) → CO2(g) + 2 SO2(g) is ΔHf = -1075 kJ/mol. Hess’s law problems require reassembling the component reactions until the needed reaction is achieved. While Hess’s law applies to changes in enthalpy, this law can be used for other thermodynamic state equations such as Gibbs energy and entropy.

Hoping that through this blog you are able to understand the problem solving in relation to Hess’s Law. This will be a very big contribution of us If you can share your experience upon reading this tips and guidelines by commenting on this blog and by giving this a heart.

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Written by: Erica Cuizon

Learning about thermodynamics and calorimetry was not easy most especially with poor consistency in study habits. Let me explain further. The first lecture period was the time of my attentive listening as well as active note-taking but being an inconsistent student, I tend to doze off and be preoccupied with unrelated things from the subject and unfortunately, it did happen. During the next lecture days with Calorimetry as the topic, I was completely inattentive, unparticipating, and too carefree. Then came the first Chemistry quiz, one which I realized the value of being consistent and motivated.

After the lesson with Calorimetry, a quiz the next day was announced and I was but a weary student with insufficient knowledge, lacking notes, and unstimulated brain. Everything seemed hopeless; nonetheless, I still studied for the test as soon as I got home. Despite my effort in coping with what I’ve missed, the last question in the problem solving portion was challenging. I wasn’t able to write anything for a long duration of time and only wrote unsure calculations. It was during these moments when I only stared at the yellow paper and somehow contemplate with my lacking as a student. The blank yellow pad seemed to be a glaring mirror which tells me that I have no one to blame but myself in my failure. The vague problem didn’t get any clearer but with every ticking second, the things that got clearer were my incompetent decisions. In relation with endothermic and exothermic processes, which are a part of the lessons, I realized that the more energy I lose or the lesser my consistency with studying is, the more negative the results will be; however, the more energy absorbed or to put it in another way, the greater my consistency and motivation with studying, the more positive the outcomes will be. From this experience I have learned the essentialities in being a student and being a pursuer of my passion and success in my future work field.

The last question was indeed a problem which I needed to solve but it became a problem that solved me. It made me realize that you don’t get anywhere with being inconsistent because it just leads to instability and an ominous downfall and that if I want to get positive outcomes now, I should stand back up and pursue consistency and motivation with everything I do. I may have lost the last question in first ever Chemistry quiz but undoubtedly, I gained something more crucial than a correct answer, a life realization.

To end this blog we would like to leave you a quote: Falling down is an accident. Staying down is a choice. Thank you for reading!

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Food Label

Written by: Isabel Chavez

Most food products have labels on them that I more commonly see as "Nutrition Facts". I have known its benefits to consumers ever since I could remember, but now more than ever, I notice it every time I buy something from the convenience store near our apartment. These labels display if not all, then at least most of the information that we need to know if we are conscious of what we eat/drink. Truly, we can understand more of what surrounds us. We just have to look closely and keep an open mind.

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Saturation of Mixtures

Written by: Isabel Chavez

Solutes in relation to solvents must be in moderation when we speak of mixing drinks. Too much water mixed with powdered chocolate would be too bland. Too much chocolate and not enough water would be too sweet. An amount of each that would fit suitably is the optimal for drinking.

In this short span of life given to us, we encounter so many kinds of emotions. Some are positive, some are negative. We may feel as if we're trapped in the feelings, but we should always remember that life goes on after all. Time does not wait for us, and so we have to make every second worth living. When we are sad, we should do something that makes us happy. When we fear, we should go back to our assurances that everything will be alright in the end. As people, we can do so much so that our problems wouldn't be too much of a burden. We just have to realize that we are truly capable.

#Chavez

Phase Changes

Written by: Isabel Chavez

Matter is not the only thing that changes. We do too. In this world we live in, we came as babies who are naturally blank slates. As we go on surviving, we are painted by different values and ideas coming from our environment. Just like liquids, gases, and solids could transform into one another, or be each other at the same time, there also are times when humans grow into a reflection of the world they live in. We could change every day, even be triggered by just a moment, and be morphed into another someone while still being ourselves. Amidst the confusion, difference in ideals and phases, the things we experience are crucial to our development as individuals. Change is constant, but so is our resilience.

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Stoichiometry

Written by: Isabel Chavez

Some things have to be balanced. Even chemical equations should be balanced before we start using them. But what do we do when we find out that we can't balance out everything? That there are episodes in our lifetime full of instability to the point that we can't see where everything leads up? We live for the spur of ignited passions, we live not for the equilibrium. We seek out things that are greater than ourselves and sometimes we fail to do so. We experience failure, we feel the pain, we let it burn us, and then we let ourselves heal. So even though there are imbalances in our life, we must learn to strive even harder, work a little smarter, and in the end, see brighter days for ourselves.

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Exhibition D: The Future

Written by: Emilyn Villareal

As someone who will take Civil Engineering, it would be a great help to find out how mixtures are created especially for cement and other solutions. I may go far from the topic of mixtures but I just want to say; to be able to build a better society, one must build a base for social change because change doesn’t always happen in one night, or change doesn’t happen from just thoughts. We need actions. As an engineering, I do not just want to build better and bigger infrastructures. I want to build something that can spark a catalyst within the people. Through the help of the people around me, and the knowledge I have gained from all my subjects in highschool (and my incoming subjects in college), I would like to do that. Imagine a world that doesn’t remain stagnant to what the system provides. Incredible, isn’t it?

#Villareal

Exhibition C: Respect

Written by: Emilyn Villareal

Chemistry is a fundamental subject for people who will take Chemical Engineering and most especially, Chemists. I admire people who stays in this for more than 5 years because we all know that chemistry is not an easy subject. It requires effort and perseverance to pass. To all the chemists and engineers out there, I salute you.

#Villareal

Exhibition B: NaCl

Written by: Emilyn Villareal

One of the most interesting things, I found out is that salt decreases a thing's freezing point. For example, in cooking, when you add salt to the food, the food will cook faster. Or in ice creams, vendors put salt in the ice to lower its freezing point, so it wouldn't easily melt.

#Villareal

Exhibition A: Time Management

Written by: Emilyn Villareal

Starting this 3rd quarter, I was perplexed. I didn't know how it would be possible to actually learn Physics, Chemistry and Biology at the same time that would totally be information overload. Not to mention we also have our Research and Capstone. Of all the difficulties I have encountered, this one will surely affect me the most. And it did, kind of but through this, I have learned to manage my time and provide time to each subject so I can focus on. I learned how to prioritize what was more important.

#Villareal

Future Engineer

Written by: Justine Ponte

As a future civil engineer, even though I am going to get a major in Civil Engineering. Chemistry has a big impact on my course because I can use my knowledge in Chemistry by knowing the amount of ingredients that I am going to use to have a strong and firm posts. The connection of Chemistry here is the solving of the solutions and converting units into another units.

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Boiling Point

Written by: Justine Ponte

Here in boiling point, it is just like the freezing point. If you want your boiling point to increase you need to add salt on the water so that the boiling point will increase. I was also amazed when I knew about this because every time I am watching my mother to boil something she is always adding salt, I think it is part of the process so that the meat will have a taste. When we discussed about this in our Chemistry class, I was very amazed that the reason of adding a salt to the water is to increase the boiling point of the water and with that the meat will be well-done.

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Stoichiometry

Written by: Justine Ponte

Stoichiometry is my favorite part in Chemistry because I am having fun of converting into something new. It also has a balancing equation and I love balancing because when you knew that you already balanced the equations it is a fulfillment and achievement. In addition, it was a big help knowing how to use the process of the stoichiometry because it helps you know how many moles of any substances you will needing to accomplish your required product.

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Freezing Point

Written by: Justine Ponte

In our Chemistry class we talked about freezing point and we discussed about how the freezing point gets lower. As we go deeper, our teacher told us how the freezing point gets lower and it is because of the sugar or salt added to the ice. I was so amazed when I knew about this because I am a fan of dirty ice creams and since I was a kid I was wondering how the ice cream stays longer even though it is summer. When I knew about this strategy or technique I would love to make my own ice cream and I know what to do to preserve the ice cream. If you would also like to do this, I would encourage you to use salt, but you can also use sugar, but salt is cheaper.

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Step 1: In the Mole's Shoes

Written by: Erica Cuizon

Converting values to mole was one of the main concepts I very much remember with Chemistry as we have done conversion countless of times. We needed it with mole fraction, molality, molarity, and with colligative properties to help us solve for the answer. In fact, converting values to moles is one of the processes in commencing the solving. In our chemistry lecture, our teacher told us that in step 1, one must convert substances to moles to make it easier. Only then can we solve the problem. I guess in life, we pretty much need the step 1. Living in a world with different people and of different backgrounds and personalities, often times, we find ourselves judging them without paying attention to their reasons. We continuously judge and judge but it is crucial to understand, even in terms of chemistry concepts, that I may be a kilogram and she may be a mole. And in solving problems in our relationship with friends or strangers, we need to learn to set aside our differences and learn to see each other from each other’s perspective. Like how the old saying goes, "Put yourself in other people’s shoes. But to do that, you have to get out of your own." Put yourself in their shoes to allow one to see or experience something from someone else's point of view and to see how it feels. To understand a mole, I need to become one and possibly, understanding this can alleviate arguments that ruin relationships which also add negativity in this world. In the near future, as I enter the professional world with different types of people, I think it is great that I inculcated this concept and principle in my morality because it will help me interact and induce good workmanship with others. With unity and understanding as colleagues in the future, I believe beautiful outputs can blossom. For instance, as an engineer, I do not work solo in creating buildings. I think the building can only soar high if I learn how to understand my team mates and I figure out how to deal with arguments with candor and respect. Truly, this realization of mine will be of use to me today and in the future as I learn how to communicate with people and I am grateful that Chemistry, one of the subjects I least expect to teach me morally, influenced my philosophical thinking. In chemistry, our teacher told us that step 1: one must convert substances to moles to make it easier. Only then can we solve the problem. In life, the world told me that step 1: one must put oneself on others' shoes to make it easier. Only then can we solve the problem.

#Cuizon

Immiscible

Written by: Erica Cuizon

During our lecture with solutions, factors which influence the solubility of solute and solvent was tackled. Factors I refer to which affects solubility include the nature of the solvent and solute, the temperature, the size, and agitation. Fortunately, I was able to grasp the concept well and understood the Chemistry behind it but similarly with other lecture times in school, I found myself drifting into my own world again realizing things about myself. I was appalled, really, how this concept with solubility became a glaring reflection of what and who I may be in the future. When our teacher was discussing the solute and solvent part of the lecture, I saw myself in the solute in some way. My success and/or failure in the future may be caused by agitation, my nature as a solute, temperature, and size. Metaphorically speaking, the more the world shakes me (agitation), the more heated the arguments and problems I encounter, and the greater the intimidation and inferiority or smallness I feel given my introverted nature, it is likely that I become dissolved in the flawed ideals and norms of society with no choice but to be miscible and hide myself in a monochrome crowd. It is probable that I may find myself giving in and trying to blend in and give in to what the world wants no matter how flawed or fallacious the system is. However, we don’t know the possibilities. I may be someone meant to go differently, an immiscible substance in a world that cannot dissolve me. Maybe, I am not compatible too much with the expectations of the world and that's okay. It just gives me a better opportunity to float and make myself stand out. Maybe, we are all immiscible in this world, living with different purposes. In the future, I may become a writer as it is one of the fields I feel passionate about and the world might not agree with some of what I believe in but I wish to stand my ground because being open-minded and not letting the world dissolve me and my character is a good thing as I, together with hopeful others, can help bring change for those who are dissipated with the faulty ideals of this world. This principle is undoubtedly a vital realization in whatever I do in the future because having one's own stand with things and avoiding being brain washed by this doubtful and prejudiced world is a crucial thing not only as a future professional in my chosen field but also as a human being in a community who is a puzzle piece in bringing good change for others.

#Cuizon

Henry’s Law & Reality

Written by: Erica Cuizon

Henry's Law was one of the concepts discussed this Chemistry period. To briefly explain, Henry's law states that the amount of a given gas dissolved in a given type and volume of liquid (solubility) is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Taking down notes and listening, I was able to cope and follow with the discussion especially in the solving part of the lesson. The gist I got from the concept was that the greater the pressure, the solubility also increases since both variables are directly proportional. Given this statement, I reflected deeply and became anxious in the future in my career. The real world outside the four walls of our classroom may be immensely harder and challenging than the problems in our books. I felt uneasy with the immense pressure of my future profession. But then, I realized that variables even in the real world are directly proportional. The greater the pressure the world puts on my shoulder, the more I should give in to my work. The harder things get, the more I should love and be passionate with my career. With giving all I've got even under great pressure may result to something worth every sweat. I guess Henry's Law is not just a law in chemistry but a principle in facing challenges in reality and I believe this is something that could strengthen me in the future.

#Cuizon

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