Physical and Chemical Change

Physical and Chemical Change
Chapter 4 (page 102)

Essential Question: What role does energy play in chemical reactions, and how can they be identified?

Vocabulary: 1 Chemical change Irreversible change Interatomic forces
Intermolecular forces Nucleus Electron Electric charge Positive charge Negative charge Proton Neutral Chemical bond Covalent bond Molecule Enthalpy of formation reactivity

Is it still the same substance? Have the physical properties changed?
H2O(s) H2O(l) Yes, it is still the same substance, H2O. Yes, the physical properties have changed. Physical properties can be observed and measured. Here, they include density, state of matter (phase), etc. Is it still the same substance? Have the physical properties changed?

Is it still the same substance? Have the physical properties changed?
In the presence of a flame: 2C20H42(s) O2(g) H2O(g) CO2(g) No, the paraffin and oxygen have become water and carbon dioxide. Yes, the physical properties have changed. Paraffin is solid and “waxy” and oxygen is a gas molecule. After the reaction, they become two gases. Is it still the same substance? Have the physical properties changed?

Does the candle come back?
If the flame cools down: Does the candle come back? ? 2C20H42(s) O2(g) H2O(g) CO2(g) The obvious answer is “no.”

X The candle does not come back by itself.
If the flame cools down: The candle does not come back by itself. X 2C20H42(s) O2(g) H2O(g) CO2(g) In this chemical reaction, the atoms were rearranged. This change is irreversible.

Memory wire is an alloy of nickel and titanium that looks like ordinary wire
You can bend memory wire into complex shapes but when heated, it seems to return back to its original shape It seems that heat allows the wire to get energy needed to go back to its original form Physical changes can be reversed by physical means such as bending, changing temperature, cutting, shaping, filtering or drying

When a log is burned, heat is applied and the wood turns into ashes
If the ashes are allowed to cool, they do not spontaneously turn back into wood again!! This is a chemical change A chemical change occurs when one substance turns into one or more different substances Chemical changes are not reversible by physical means because the atoms rearrange atoms into different substances Some chemical changes can be reversed, but by more chemical changes

Physical and chemical changes are fundamentally different as said earlier, physical changes are reversible and chemical changes are not When a candle is burned there are two things happening, the solid wax turns into liquid, and if allow, it can become a solid again (physical change - reversible) However, as the wax burns it also disappears and cannot be found again (chemical change - irreversible) When wax burns it turns into carbon, water vapor and carbon dioxide (three very different substances) Candle wax is made from paraffin C20H42 (20 carbon and 42 hydrogen atoms) If melted and shaped and cut into little pieces, it would still contain C20H42 any physical change to paraffin would leave it with the same amount of carbon, and hydrogen

Energy is required to cause physical or chemical changes (the amount of energy determines what changes occur) If the energy is great enough then molecules may be broken and a chemical change can occur When molecules have a strong bond or attractive force, a lot of external energy is needed to cause a chemical change Intermolecular forces are the forces that keep molecules or atoms attracted and connected together For example because of the strong interatomic forces between hydrogen and oxygen, water takes a lot of energy to break the molecules apart Intermolecular forces are the forces that act between molecules (physical changes only involve intermolecular forces) Interatomic forces are the forces that bonds atoms together to form ions or molecules Interatomic forces are stronger than intermolecular forces To turn ice into water requires 100 times less energy that the energy needed to separate hydrogen from oxygen in water

Assignment Take a new sheet of paper and fold it into three sections
Write your name, the title of the chapter and the number On the first section from the sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

Any process that changes the shape of a substance is a physical change (including grinding in all forms) Sugar is still sugar even if it is made from large crystals, ground, granulated or powdered Adding food coloring to water does not change the properties of the water or the dye, just the color Mixing is also a physical change even though it may be vigorous

Mixing, drying, and whipping produces different looks, but the substances still have the same properties as before Drying is the opposite of dissolving Sometimes drying can be a chemical change as in the case of latex paints When latex or acrylic paints combine with oxygen to make a new substance that cannot be reversed back to the liquid state again

Physical vs. chemical change
In a physical change: - the molecules are rearranged intermolecular forces are broken interatomic forces are not broken In a chemical change: - the atoms are rearranged - interatomic forces are broken

Most matter exists as compounds and not as single atoms (because of stability)
Inside of atoms there are clues that tell how they will exist in nature and why they form compounds Imagine a football field with a small marble in the middle, that would represent the atom, nucleus and the electron cloud If your classroom was an atom, the nucleus would be the size of a grain of sand Most of an atom is made up of the electron cloud and % of the mass is made up of the nucleus

99.8% of the mass of the atom is in the nucleus!
Structure of the atom An atom is not a hard ball. 99.8% of the mass of the atom is in the nucleus!

Matter has mass and volume, however matter also has electric charge
There are two basic types of electric charge positive and negative which attract each other The nucleus may have one or more protons and they have positive charges Surrounding the nucleus is an electron cloud that may have one or more electrons which have a negative charge Similar charges repel each other Also in the nucleus of the atom is one or more neutrons and they have no electrical charge

Electrons are strongly attracted to the protons but they do not fall into the nucleus because of their kinetic energy Think of the sun and the earth and how the sun pulls the earth into its gravitational field, but we do not get pulled into the sun because the speed at which earth travels around the sun The electrical attraction between protons and electrons are 1040 times stronger than earth’s gravitational pull The electric charge of an atom is always zero because the number of protons always equals the number of electrons

Assignment On the second section of that sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

Each atom has a different number of protons and electrons in them
The way the electrons are arranged is what creates each element’s unique chemical properties Hydrogen has one proton and one electron and it is the electron on the outside of the nucleus that makes it react with oxygen in a two - to – one ratio to make H2O Chemical bonds are created by electrons Two atoms that are sharing one or more electrons form a chemical bond and move together

One of the strongest bonds possible is a covalent bond which is two atoms sharing one electron
In a covalent bond electrons are shared not transferred and form molecules (nonmetals bonding) The water molecule has two covalent bonds (between each hydrogen and oxygen) Other examples of covalent bonds in action are O2, N2, NH3, CH4 (page 109 of text) Atoms can share one two or three electrons and that corresponds to single bonds, double bonds or triple bonds (all of these bonds are neutral)

Why chemical bonds form
It takes energy to separate atoms that are bonded together. The same energy is released when chemical bonds form. Atoms form bonds to reach a lower energy state.

Chemical bonds occur when the reactions are advantageous to their overall energy (energy is reduced)
When you put two magnets close together they pull closer and closer until they connect (their overall energy is less together than they were separate) To pull the magnets apart, it takes a lot of energy or force (like breaking bonds apart) If it took energy to pull them apart, then it is equivalent to the energy that was lost or released when they came together

Chemical Bonds and Electrons
A chemical bond forms when atoms transfer or share electrons. A covalent bond is formed when atoms share electrons. Covalent bond- a chemical bond resulting from the sharing of electrons between two atoms

Structure of the atom The number of electrons corresponds to the atomic number of the element, as shown in the periodic table.

If negative charges repel each other, how does an atom stay together?
Electrons have a negative electric charge. If negative charges repel each other, how does an atom stay together? Something with a positive charge has to keep the negative charges from “flying off.”

The general rule in chemistry is that atoms combine and arrange so that they can have the lowest possible energy (atoms have lower energy in compounds than when they are separated) Page 110 shows that two hydrogen atoms and one oxygen have way more energy separated that they do when they form water For every mole of water (18 grams) 285,000 joules of energy is released (called the enthalpy of formation ∆Hf) The ∆Hf is zero for pure elements

Chemical bonds form because there is an advantage in energy.
Lower energy = more stable Enthalpy of formation ∆Hf (kJ/mole)

Not all compounds are made of molecules, (two or more nonmetals sharing electrons), sometimes electrons are taken from one atom and used by another to make it perfect (like group 18 atoms - the noble gases) – called ionic bonding The loss or gain of electrons in a compound is called an ionic bond Ionic bonds form between metals and nonmetals (like sodium chloride, magnesium chloride, and aluminum chloride) in each of these cases one, two or three electrons are lost by the metal and gained by the nonmetal to form positive and negative ions Ions are atoms that have gained or lost one, two or three electrons to become more stable

Types of Bonds Ionic bond- a chemical bond resulting from electrostatic attraction between positive and negative ions. pure ionic bond - electrons are transferred.

Electrons are responsible for bond formation.
Electrons can be: - +

+1 –1 In ionic compounds, electrons are NOT SHARED but TRANSFERRED.
Sodium ion Chloride ion Cl –1 +1 Na Sodium atom Chlorine atom Ionic bond Cl Na

Covalent bonds A chemical bond is formed by sharing or transferring electrons. There are two covalent bonds in a water molecule. covalent bond: a chemical bond that consists of two shared electrons. molecule: a neutral group of atoms bonded together by covalent bonds.

Some elements can share multiple electrons with the same atom.
Multiple bonds Single bonds Double bond Triple bond Some elements can share multiple electrons with the same atom.

Comparison of Ionic and Molecular Compounds
high melting pt hard & brittle conductors in molten & aqueous state Molecules low melting pt vary in hardness nonconductors

Chemical bonds form because there is an advantage in energy.
Lower energy = more stable

All chemical reactions involve three key components:
Reactants Products Energy Reactants Products Energy (in or out)

Some elements are never found in pure form in nature:
Electrons are responsible for bonding among atoms. Some elements are never found in pure form in nature: Ex: Lithium (Li), sodium (Na) Some elements are always found in pure form in nature: Ex: Helium (He), argon (Ar) He

Some elements are more likely to react than others.
They are more reactive. reactivity: the tendency of elements to form chemical bonds.

Use the periodic table to help determine whether a compound is ionic or molecular.

Assignment 4-1 Write a three dollar summary paragraph about what you learned in this section along with answering the questions below. Turn to page 128 and answer # 1-15 on a separate sheet of paper. Honors Chemistry Homework: page 129 #

Vocabulary: 2 Chemical reaction Chemical change Reactant Product
Balanced equation Coefficient Conservation of mass Endothermic reaction Exothermic reaction Enthalpy System Surroundings Activation energy

Electrolysis ? Experimental setup: An electric current transfers a large amount of energy to water. ? Observations: The mass of water decreases over time. The volume of gas increases. Hypothesis: The gas produced is water vapor.

chemical change: a result of chemical reaction.
The energy from the electric current was high enough to cause a chemical change through a chemical reaction. + chemical reaction: a process that rearranges the atoms in any substance(s) to produce one or more different substances. chemical change: a result of chemical reaction.

How can we make both sides the same number of each atom?
The chemical equation ! 1 2 oxygen oxygen This equation is not balanced. 2 2 hydrogen hydrogen How can we make both sides the same number of each atom?

The chemical equation 2 2 coefficients oxygen oxygen 4 4 hydrogen

Octet Rule In many compounds, atoms of main group elements (p block) form bonds so that each atom has an octet of electrons.

Octet Rule Octet Rule- Chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.

Chemical changes result when one or more substances are converted into different ones with different properties The digestion of food is the process of braking food down from complex structures to more simple ones (with new properties) An example of a chemical change is electrolysis which is the breaking down of water from H2O into O2 and H2 by electrical means A chemical change is caused by a chemical reaction which is usually denoted with a balanced chemical equation Balanced chemical equations are important because they must uphold the law of conservation of matter

When dealing with chemical reactions, there are always reactants to the left of the arrow, and products to the right of the arrow There are also a combination of letters (that represent elements) and numbers (that represent the number of elements or that compound) The number to the front of the element is called a coefficient and tell how many of that element or compound The subscript is also a number that tells how much of that element is there in that compound subscript coefficient reactants → Products

The chemical equation 1. Determine if the equation is balanced.
Rules of balancing a chemical equation: 1. Determine if the equation is balanced. Count the number of each kind of atom on either side of the arrow. 2. If it is not balanced, use coefficients. Ex: H2O becomes 2H2O 3. Do not change the subscripts inside the molecular formulas. Ex: H2O cannot become H3O 4. Check that the equation is balanced. Count the number of each kind of atom on either side of the arrow with the new coefficients.

Reactants Products Iron (Fe) Oxygen (O)
Is it balanced? Reactants Products Iron (Fe) Oxygen (O)

Is it balanced? Reactants Products Iron (Fe) 1 2 Oxygen (O) 3

4 3 2 Reactants Products Iron (Fe) 4 Oxygen (O) 6 Balanced!

Steps for balancing Add coefficients to balance the equation.

Rules and Examples When balancing chemical equations: Mg + O2  MgO
Do not change the subscripts in the compound You can change coefficients to make the reactant side equal the product side The process is trial and error Balance the following equations: Mg + O2  MgO Na + H2O  NaOH + H2 Al + H2SO4  Al2(SO4)3 + H2 KClO3  KCl + O2

Energy and Reactions Enthalpy is the energy potential of a chemical reaction at standard temperature and pressure – measured in joules per mole (J/m) or kilojoules per mole If the enthalpy is positive, the reaction increases the total energy of the constituent atoms (endothermic) If the enthalpy is negative, the reaction decreases the total energy of the constituent atoms (exothermic) Energy is involved in chemical reactions in two ways: to break some (or all) bonds between atoms in the reactants so the atoms can form new bonds or when the atoms or products form new bonds to make new products.

Assignment On the third section of that sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

All chemical reactions involve three key components:
Reactants Products Energy Reactants Products Energy (in or out)

Two Types of Reactions We classify chemical reactions based on how the energy of the reactants compares to the energy of the products.

What is missing? Mixing water and CO2 doesn’t make
Photosynthesis Mixing water and CO2 doesn’t make sugar and O2 because energy input is needed. Respiration Energy input is not needed but just mixing sugar and O2 doesn’t make water and CO2.

Endothermic reactions
If forming new bonds in the products releases less energy than it took to break the original bonds, the reaction is endothermic. Photosynthesis and a cold pack are both endothermic reactions

Try to Balance this reaction.
Reactants Products Carbon (C) Oxygen (O) Hydrogen (H) Verify that the equation is balanced

Number of atoms or moles
Verify that the equation is balanced Reactants Products Carbon (C) 6 Oxygen (O) 18 Hydrogen (H) 12 Number of atoms or moles

Conservation of mass Total mass of reactants Total mass of products
108 g + 264 g 372 g 180 g + 192 g 372 g Conservation of mass

Exothermic reactions If forming new bonds releases more energy than it takes to break the old bonds, the reaction is exothermic. Heating or burning fuels as well as a heat pack are exothermic reactions

Energy “out” Energy as a product. reactants products
An exothermic reaction releases energy.

First law of thermodynamics:
Energy can neither be created nor destroyed. Energy is absorbed Are these violations of the first law? Energy is released

Systems and Activation Energy
The first law of thermodynamics states that energy cannot be created, so it is always conserved It is impossible to do experiments with the whole universe, so scientist clearly define what they are observing and they call it the system Everything around the system is called the surroundings Most often, the system will be the products and the reactants The total mass and energy of the system and the surroundings are always constant Any matter or energy gained by the system must be lost by the surroundings and visa versa Activation energy is the energy needed to begin a reaction and break chemical bonds in the reactants.

Activation energy C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J 2,800,000J
an energy barrier

Assignment 4-2 Write a three dollar summary paragraph about what you learned in this section along with answering the questions below. Turn to page 128 and answer # on a separate sheet of paper. Honors Chemistry Homework: page 129 #

Vocabulary: 3 Aqueous Precipitate Salt Oxidation Reduction Acid Base
pH scale

An aqueous solution is one where the chemicals are dissolved in water
Many reactions occur between chemicals that are dissolved in water (including those in the human body) Reactions occur more in water because molecules can move around better and interact with each other An aqueous solution is one where the chemicals are dissolved in water When writing balanced chemical equations there are some symbols that help indicate what is happening: (s) – indicates solid (l) – indicates liquid (g) – indicates gas (aq) – indicates aqueous A precipitate is an insoluble solid that forms when an aqueous chemical reaction produces a solid salts are made from ionic compounds (made from metal and non metal ions) that form ions when dissolved in water The most typically known salt ions are Cu2+ and (SO4-2) that are formed when copper sulfate dissolves in water

No reaction Chemical reaction DRY IN SOLUTION NaS(aq) CuSO4(aq)
Light blue powder NaS(aq) CuSO4(aq) No reaction Chemical reaction A precipitate is an insoluble compound.

Oxidation and Reduction
Oxidation is the loss of electrons Reduction is the gain of electrons Stronger nonmetals are reduced and weaker nonmetals become oxidized Generally: Metals become oxidized Nonmetals become reduced

Cu2+(aq) SO42-(aq) Cu(s) Zn(s) + and Zn(s) is disappearing Cu2+(aq) e– Cu(s) The copper ion gains 2 electrons to form solid copper. reduction: a chemical reaction that decreases the charge of an atom or ion by accepting electrons.

oxidation: a chemical reaction that increases the charge of an atom or ion by giving up electrons.
Zn(s) Zn2+(aq) e– Zinc gives up 2 electrons to become a zinc ion, dissolved in water.

To obtain the overall reaction add the reactants, then the products.
REACTION NAME Reactants Products Cu2+(aq) e– Cu(s) reduction Zn(s) Zn2+(aq) e– oxidation These 2 reactions are happening at the same time. To obtain the overall reaction add the reactants, then the products.

Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq) redox
REACTION NAME Reactants Products Cu2+(aq) e– Cu(s) reduction Zn(s) Zn2+(aq) e– oxidation Cu2+(aq) Zn(s) Cu(s) Zn2+(aq) redox

Acids and Bases When pure water is examined it is seen that one out of 550 million water molecules dissociated (separated) out the H+ and (OH-) ions The reaction is written with double arrows to show that it goes in both directions At equilibrium one out of every 550 million water molecules dissociates In acids, the ions dissociates to form more H+ ions than there are in water Create a sour taste in foods (like lemons) React with metals to form hydrogen gas Corrode metals and burn skin through a chemical reaction In bases, the ions dissociates to form more OH- than there are in water Create a bitter taste (like baking soda) Have a slippery feel, like soap Can also neutralize acids

Classify these items as acids or bases.
Bases: bathroom cleaner, baking soda Acids: lemon juice, gastric acid

Assignment On the fourth section of that sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

Acid /Base Reactions When acids and bases combine chemically, they form water and salt A salt can also e defined as what is formed when the positive ion from a base, and the negative ion from an acid combine NaOH + HCl  NaCl + HOH Sodium is the positive ion of the base and the chlorine is the negative ion from the acid Acids and bases are very important to chemistry since they are needed in the body for many reactions to occur (this importance grants them their own scale called the pH scale (shown on the next slide)

HCl(l) H+(aq) + Cl–(aq)
Acids give foods like lemons their sour taste. Acids react with metal to create hydrogen gas (H2). Acids can corrode metal and burn skin. Example: H2O(l) HCl(l) H+(aq) Cl–(aq) Hydrochloric acid acid: a chemical that dissolves in water to create more H+ ions than there are in neutral water.

NaOH(l) Na+(aq) + OH–(aq)
Bases create a bitter taste. Bases have a slippery feel, like soap. Bases can neutralize acids. Example: H2O(l) NaOH(l) Na+(aq) OH–(aq) Sodium hydroxide base: a chemical that dissolves in water to create fewer H+ ions (or more OH– ions)than there are in neutral water.

Higher H+ concentration Lower H+ concentration
A solution with water as the solvent is called an aqueous solution. Redox reactions (transfer of electrons) Acid-base reactions (transfer of protons, H+ ions) two types of chemical reactions that take place in an aqueous environment. Higher H+ concentration Lower H+ concentration

pH of Common Substances
Timberlake, Chemistry 7th Edition, page 335

O H S O Cu S O 2– 2+ Cu O H A salt is an ionic compound that forms ions when dissolved in water. O H O H

Assignment 4-3 Write a three dollar summary paragraph about what you learned in this section along with answering the questions below. Turn to page 129 and answer # on a separate sheet of paper. Honors Chemistry Homework: page 130 #

Test: - Next week Tuesday or Thursday depending on your class.
Homework requirement: Learn all terms and concepts covered on this topic. Make sure you have all assignments between page 128 and 131 completed and turned in by your test date.

Physical and Chemical Change

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