Chapter 4: Section 8

The universe is made up of a system and it’s surroundings. 

If the system is open (system) energy passes between the system and its surroundings. A closed system is isolated from the surrounding and heat neither enter nor leave.

When heat energy is released from the system the surrounding gain the energy. When the system loses heat energy there is also a lost in enthalpy 

The law of conservation of energy states that energy cannot be created nor destroyed, it can only be transferred.

When a system gains energy there is an increase in the amount of energy in the system so enthalpy becomes positive.

Whenever a change occurs the particles are rearranged and it can be either more or less organized. When it is mire disorganized the entropy has increased meaning the change in entropy is positive (i know very confusing). This can also occur when the particles are organized, so essentially its the opposite of the disorganization of particles.???

Two things that affect spontaneity are the changes in enthalpy and entropy. Exothermic changes can cause spontaneity because substances that are produced have lower energy than the reactants meaning the lower energy is favorable. Changes that result in an increase in entropy also cause spontaneity. Inserts picture below to make ease in explaining: 

One last thing temperature also affects if something is spontaneous or not. 

Gibbs free energy is the relation ship between delta h and s as well as temperature. It can be used to tell if the change will occur spontaneously. If delta g is negative then it will be spontaneous at a certain temperature meaning it is favorable. If delta g isn’t positive then a spontaneous reaction won’t occur. 

Polymers are molecules made of long strings of monomers that are attached to each other. 

Another polymer that is called, thermoplastic polymer is used to stitch the working parts of the mechanical heart to the heart itself. These are normally polyesters where the repeating monomer forms an ester linkage between an organic acid and an alcohol. 

Chem Talk: Chapter 4-Section 7

Thermodynamics

You can also control the speed of the reaction by how quickly heat is produced

For chemists and engineers being able to control chemical reactions, they must be able to predict when reactions can occur spontaneously which is defines if it continues to occur., and also what can be done to speed up or slow down reactions. One set of theories called thermodynamics is used to answer the question “Can a reaction occur spontaneously?” A second set of theories called kinetics is used to answer the question “How fast can a reaction occur?”

There are two factors that determine if a change can occur spontaneously. The first factor that affects spontaneity is if the change gives off or absorbs heat energy when it occurs.

The second factor that affects spontaneity is if the change results in particles becoming more or less disordered

Two commonsense rules apply to these two questions:

 First, lower energies are more stable than higher energies. So, in the same way that a ball tends to roll downhill, energy changes in chemical reactions tend to occur in ways that allow the substances to end up with lower energy. This means that changes that release energy tend to be favored. Second, everything tends to become more disorganized over time. So, changes in which particles become more disordered are favored over those that make particles become more ordered. The two factors that affect spontaneity can work together. If a change both releases energy and results in an increase in disorder, the change is definitely spontaneous. If a change both absorbs energy and results in

a decrease in disorder, the change is definitely not spontaneous.

However, if one factor is favorable and the other is not, whichever is the stronger tendency controls whether the change is spontaneous.

In this activity you explored a change that was spontaneous. Heat energy was given off as the system went from a higher energy to a lower one. This means at least one of the two factors affecting spontaneity is favorable for this reaction. This Chem Talk will focus on heat changes.

Heat Energy Changes: Endothermic and Exothermic Reactions

Consider the chemical reaction between magnesium and water that you observed in the activity.

Mg(s) 2HOH(l) → Mg(OH)2(s) H2(g) energy

magnesium water magnesium metal hydroxide

The main points here are that:

  • When chemical reactions happen, bonds the reactants (starting materials) break, and new bonds form to make products (ending materials).

  • Breaking bonds requires energy input, so bond-breaking is an endothermic change. On the other hand forming bonds releases energy, so bond-forming is an exothermic change.

  • The overall change can be endothermic or exothermic, depending on whether the total energy input or the total energy output is greater.

    In the chemical reaction between magnesium and water, the total energy input required to break the bonds in the reactants is 572 kJ.This number and similar numbers have been measured precisely and can be found in reference books.

    (Endothermic changes are positive, because the system gains energy.) When the new bonds form in the products (making magnesium hydroxide and hydrogen gas), the total energy output is –925 kJ. (Exothermic changes are negative, because the system loses energy.) The overall or net change is more exothermic than endothermic. More energy is produced than absorbed.

    The reaction in the MRE heater, like most changes (both chemical and physical) studied in chemistry, occurs at constant pressure. Chemists call an energy change that occurs at constant pressure “change in enthalpy” and represent it with the symbol ΔH.The Δ (the Greek letter “delta”)
    is frequently used to mean “change.” Hess’s Law states that, for a given reaction, enthalpy changes are additive.When the Standard Enthalpies
    of Formation (ΔHf °, see Tables) of the reactants are subtracted from
    the products, the result is the net change in enthalpy for the reaction. A negative ΔH tells that the reaction is exothermic; a positive ΔH tells that the reaction is endothermic.The enthalpy change (ΔH) for the reaction in the MRE heater can be calculated as shown below.

    ΔH = –925 kJ (for Mg(OH)2 − 2 (–286 kJ) (for 2H2O) = –353 kJ

    ΔH is negative, which tells you that the net change is exothermic (the system loses energy to the surroundings). Therefore, the temperature of the MRE heater increased.You could also compare the energy absorbed and the energy released graphically like we did in Activity 1.

    The enthalpy change that occurs when a reaction occurs is sometimes included in the chemical equation as shown below.

    Mg(s) 2H2O(l) → Mg(OH)2(s) H2(g) ΔH –353 kJ Energy Diagrams

    Another way to represent an enthalpy change is to draw an energy diagram which shows the progress of the reaction from reactants to products along the horizontal axis. Along the vertical
    axis, the diagram shows the change in potential energy that occurs as substances progress from reactants to products. An energy diagram provides the following two important pieces of information:

    Endothermic or exothermic: The relative locations of the reactant (starting) energy and product (ending) energy indicate whether the overall enthalpy change (ΔH) is endothermic or exothermic. If the starting point (reactants) is higher than the ending point (products), then the system releases heat energy to the surroundings (an exothermic change), and the overall enthalpy change is negative (ΔH 0). If the energy of the reactants is lower than that of the products, then the system absorbs heat energy from the surroundings (an endothermic change), and the overall enthalpy change is positive (ΔH 0).

Activation energy: Regardless of whether the change is endothermic
or exothermic, bonds must always be broken in the reactants before new bonds can form in the products. Therefore, some initial amount of energy must be supplied to the system in order for the reaction to begin.This is represented as an activation barrier, or an initial “bump” in the curve, to get from reactants to products.The height of this bump, measured from the reactant energy, is called the activation energy (Ea) and it is always positive. The in-between state (between reactants and products) is at the top of the barrier and is called the activated complex.

When you observed the reaction of magnesium with water and compared that reaction with the reaction of magnesium with NaCl and water, you could see that the reactions were very slow. Little heat was generated, although the salt solution reacted slightly faster than water alone.Tiny bubbles of hydrogen slowly formed. Clearly another substance was needed in the MRE heater in order to produce enough heat to warm a meal.

When you compared the reaction of magnesium in a salt solution with the reaction of magnesium, salt water, and powdered iron, the difference was very dramatic. After a few minutes, the sample containing iron produced much more heat and bubbled vigorously.This combination could indeed be used to heat a meal in an outdoors setting. Powdered iron is an important component of the MRE heater.

A comparison of the last two samples tested in the Investigate was simply a comparison of the same reaction and the same components, but the size of the magnesium particles was different.Test tube D contained a single large piece of magnesium strip while Test tube C used granulated magnesium. Granulated magnesium contains much more surface area than the single piece and the reaction proceeds much faster.This is because reactions can take place only when the reactants collide. When the reactant is a solid, these collisions can only occur on the surface. More surface area = faster reaction.

The iron is a key component for speeding up the reaction and it must not only be present, but physically imbedded in the magnesium in order for it to function.According to the scientists and engineers who developed this type of MRE heater, the iron is a catalyst for the reaction:

Mg(s) + 2H2O(l) Fe> Mg(OH)2(aq) + H2(g) + heat

A catalyst is a substance that speeds up a chemical reaction without being used up itself.A catalyst works by providing a lower-energy alternative pathway for the reaction to take. In essence, it provides a lower activation energy. As a result there is more starting materials at a given temperature that have enough energy to get over the energy barrier. This speeds up the reaction and generates heat quicker. As the temperature increases, the reaction speeds up even more.

The sodium chloride also plays a role in the reaction by being an electrolyte. When dissolved in water, the salt provides a pathway for the electrons to move from the magnesium, which is thus oxidized, to the hydrogen atoms, which are reduced.The oxygen in water remains in the same oxidation state as it becomes a part of the hydroxide ion.

Chapter 4 Section 4 – Chem Talk

So today in our lab we played with circuits. Just the basic wires, metals and batteries etc, but we mostly focused on metals. Some of their properties include conducting electricity, conducting heat and since most metals can withstand high temperatures, making them malleable where they are then used to build strong structures. Most of the elements in the periodic table are metals. So pretty much if you draw a line from boron (B, #5) down to astatine (At, #85), the metals are all the elements to the left of the line, including elements 57–71 and 89–103 at the bottom of the table.

The Origin of the Activity Series

Much of chemistry came from the field of alchemy, which was to create gold out of other metals. You now know that it is not possible to do this by chemical means, because the identity of an atom is specified by how many protons are in its nucleus (the meaning of the atomic number on the periodic table).When an atom interacts with another, only the arrangement of electrons is changed.

In the process of experimenting with metals, however, alchemists discovered that some metals react more easily with most metal ion solutions than other metals do.They developed the activity series from their observations and since then, chemists have measured these more accurately.

Special Reactions between Metals and Metal Ions

The reactions that can occur between neutral metal atoms, such
as Zn, and metal ions, such as Cu2 , are part of a special class of reactions called oxidation-reduction reactions (“redox” for short). Oxidation is defined as giving up electrons, so all of the equations you listed in the activity series represented oxidations.

Zinc metal oxidizes: Zn → Zn2 2e–
Reduction is receiving electrons.An oxidation equation turned

backwards represents a reduction.
Copper ions reduce: Cu2 2e– → Cu

As you found out in the activity, when an oxidation happens, a reduction also occurs. Electrons that are given up have to be received by something. So, oxidation changes and reduction changes are often called half-reactions because you need both halves for a reaction to happen. You can use half-reactions to balance redox equations by making sure the number of electrons lost is equal to the number of electrons gained. You learned that although you can write the equation in two directions and you can test both equations, the reaction will occur naturally in only one direction.

When Is a Metal Not a Metal?

You may wonder why hydrogen is included in the Metal Section of the periodic table when it’s not a metal. Metals, in general, are solid, shiny, ductile, malleable, and conductive of heat and electricity which hydrogen is not. However, hydrogen takes on the positive nature of a metal in its role in strong acids, such as hydrochloric, sulfuric, and nitric acids.

Chapter 4 Section 2

The law of conservation of matter: one example of these are balanced equations. 

One definition of matter is that it cannot be created or destroyed but it can change form. When talking about chemicals this means the same amount of each element must enter and exit the equation and each side must have the same amount of product. 

From far back as 2500 years ago Democritus, Heraclitus, and Aristotle were among those who had discovered and questioned the existence of matter. With all valid points the inital statement that stayed was that matter cannot be created or destroyed.  

Chapter 4 Section 1

In this lab we used CO2 gas to blow up balloons and move a ruler. Each method required different materials and had different outcomes. Scientist also look at the generation of gas in different ways.

When matter change there is often a change in energy which means energy will move from one place to another. But the thing about energy is that the amount of it doesn’t change it will always stay constant. The only way energy changes is if it is gained or lost through mostly heat.
So if heat energy is released then the chemicals lose energy and vice versa. But to put it in more detail when heat is added the particles gain energy meaning they had more before the heat was gained. This additional energy has to fit somewhere so the only way to make it fit is for the particles to move faster. But realize that the bonds within the molecules hold the atoms together.

An energy change can be ‘positive’ or ‘negative’ but it depends on the perspective. When considering chemicals they gain energy during the change which is positive for the whole time. This is called endothermic change because the heat energy goes into the chemical. The opposite of this is called exothermic which essentially means that heat energy is leaving the chemical.

When speaking about changes chemist often talk about they way the matter is organized. So for instance if u think of a solid the particles or closely packed and in a gas the particles are dispersed all over this is the same with disorder of matter. The more packed the particles are the more organized the structure becomes and vice versa. This dispersion of matter is called entropy which is symbolized by the letter ‘S’

Chem Talk – Section 5

A hydrate is a compound that has water bonded to it or can also be described as a substance that contains water
the solid that is left when the water is removed from a hydrate is called an anhydrate

One way that chemist established a way to count atoms is by using the unit measurement moles which is a quantity of something
one mole is equal to the number of carbon atoms in exactly 12g of pure carbon. the number 6.022 x 10^23 is call Avogadro’s number to honor him and his contributions to chemistry. The mass of one mole of two different substances will be different because the atom making up the substances have different atomic masses.

The mass of one mole is equal to an elements atomic mass in grams. For compounds the atomic mass of each element is added to get the formula weight. Essentially you just add the mass of all the elements present in a compound to determine the mass of one mole of that compound. Which is known as molar mass which means that anything less than that will only be part of a mole.

A percent composition is data used to calculate a formula for an unknown compound. The percent composition can often be gathered from analysis of the different elements the the substance contains. The empirical formula is the formula for the smallest possible ratio of the elements. Chemist also need to determine the molecular formula for more information. To determine the molecular formula of an unknown, the molecular mass is divided by the empirical mass which gives you the amount of units that are needed to obtain the the true molecular and the true molecular formula. Sometimes a chemist will know the molecular formula but will need to calculate the percent composition. to calculate the percent composition you have to add up the total mass of each element and then the total mass of the compound. Divide the total attic mass of each element by the total mass of the compound and multiply by 100 to make it a percent.

Gypsum is a natural mineral which is known by it chemical name as calcium sulfate dihydrate. When looking at its chemical formula which is written as CaSO4 * 2H20 it tells you that each molecule of calcium sulfate has two water molecules attached to it.

Normally cast are made from gypsum. The origin of gypsum is from gypsum quarries which are located near Paris. Which is also why they are sometimes called “plaster of Paris”. To make plaster of Paris you first start off with powdered gypsum by heating the hydrate calcium sulfate to about 160 degrees celsius. This drives off the water and creates a hemi-hydrate calcium sulfate. After you do this you can mix it with water which makes a paste that you can apply to the area that has a broken bone. This reaction takes about 30 minutes to set.

Chem Talk – Section 7

Dyes are defined as organic molecules that bond directly to a textile to produce a color. 

Chromophore is the reason why we see color in the dye. these are groups of atoms in the organic molecule that absorb only certain colors of visible light. the light of the colors not absorbed is reflected to our eyes. white light is the sum of all the colors of the spectrum. Sometimes an autochrome will be present which will modify the chromophore’s ability to absorb light energy 

The dye has to be a liquid so that it can absorb onto the textile. once it penetrates the fabric the mordant acts as a binding for the dye and textile. Mordants came from metallic salts that came from the word ‘to bite’.

A lake is a pigment formed by precipitation of coloring matter with a metal ion or also defined in a less complex way as how when you introduce a dye it forms an insoluble complex salt. This large complex molecule is less water soluble than the individual dye molecules. Thus causing the dyed material to take longer to loose its color.

Chem Talk – Section 6

The solid precipitates we saw in the investigation are insoluble compounds. If the pigment is insoluble in the liquid it will become suspension particles in the liquid. 

When certain cations and anions are combined, water-insoluble ionic compounds may form. When these ions are in separate aqueous solutions and then brought together, an insoluble solid, or precipitate forms. The precipitate is an ionic compound that forms because certain ions attract each other so strongly that they are removed from the water solution. A double-replacement reaction is one type of precipitation reaction where a precipitate forms when one of the products is insoluble. 

Ions that do not participate in the reaction are called spectator ions. Net ionic equation is a chemical equation for a reaction that list only those compounds participating in the reaction. 

To predict if a double-replacement reaction works you have to follow all six of the solubility rules. An ionic compound is said to be soluble if a large amount of it dissolves in water. An insoluble ionic compound is defines as one that does not dissolve in water.