AP e 4 Unit 4 - Chemical Reactions

AP Chemistry Unit 4

Chemical ReactionsISPS Chemistry Nov 2020 page 1

Unit 4 - Chemical Reactions

4.1 Introduction for Reactions 4.2 Net Ionic Equations 4.3 Representations of Reactions 4.4 Physical and Chemical Changes 4.5 Stoichiometry 4.6 Introduction to Titration 4.7 Types of Chemical Reactions 4.8 Introduction to Acid-Base Reactions 4.9 Oxidation-Reduction (Redox) Reactions

AP Chemistry Unit 4


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AP Chemistry Unit 4


4.1 Introduction for Reactions

Physical or Chemical ?

At times, the difference can be clear and obvious but there are also many times when it is far from clear-cut.

Heating of an iron bar: Rusting of an iron bar: Fe(s) → Fe(s) → Fe(s) Fe(s) → Fe2O3(s)

grey red grey grey reddy orange colour change colour change

AP Chemistry Unit 4


The key difference between these processes (best illustrated by the equations) is that there is no change in composition (metallic network) when an iron bar is heated/cooled wheras the iron has changed completely (metallic network → ionic network) during rusting

One definition of a chemical change is that existing bonds should be breaking and new bonds should be forming.

metallic bonds covalent bonds ionic bonds breaking breaking forming

However, we could argue that metallic bonds were breaking when we heated our iron bar and would have to break if we melted and thenboiled the iron.

What would prevent us from ever consideringthis as a chemical process would be the factthat there is no change in composition.

Fe(s) → Fe(l) → Fe(g)

In changes of state, we consider any bondsbeing broken as intermolecular attractions and,since no new bonds are forming, would confidently label this as a Physical change.

metallic bonds breaking during melting → Physical change metallic bonds breaking during rusting → Chemical change

Similarly, when water melts or boils, only intermolecular attractions are being broken.

Only the hydrogen bonds between water molecules are broken, the covalent bonds within the molecules remain intact.

H2O(s) → H2O(l) → H2O(g)

There is no change in the composition - Physical.

AP Chemistry Unit 4


By contrast, when water is electrolysed bubbles of gas form just like when water boils.

However, this time covalent bonds within the water molecules are breaking and new covalent bonds are forming in the oxygen and hydrogen molecules.

2 H2O(l) → 2 H2(g) + O2(g)

The composition has changed completely - Chemical change.

What about dissolving?

One can certainly argue that there are bondsbreaking (ionic bonds & hydrogen bonds) and new attractions (ion-dipole) taking their place.

We are also losing the ionic network so changein composition?

NaCl(s) → NaCl(aq)

Our equation makes clear that, in reality, therehas been no change in composition - our Na+

and Cl- ions remain - so Physical Change.

However, the formation of a solution can be spontaneously exothermic or endothermic, and in later units (particularly in the Equilibrium Unit) we will treat them in the same way as 'normal' reactions so, as I've said, not always clear-cut.

What about precipitation?

Can depend, if salt precipitates out of a saturated solution: NaCl(aq) → NaCl(s)

then there is no change in composition so it would be considered a Physical Change.

If we mix KI(aq) with Pb(NO3)2(aq) and precipitate out some yellow PbI2(s) :

2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq

then the change in composition means it would be considered a Chemical Change.

AP Chemistry Unit 4


Sometimes both changes are taking place.

The combustion of the candle wax is a Chemical change. It is an exothermic process. There is a change in composition.

C15H32(s) + 23 O2(g) → 15 CO2(g) + 16 H2O(l)

Meanwhile. all around the flame, some of the candle wax is melting in a Physical Change.

C15H32(s) → C15H32(l)

This is an endothermic process but there is no change in composition.

The melting of candle wax was endothermic because energy is needed to overcome the London Dispersion attractions between the wax molecules and to make them move faster in order to change from solid to liquid - intermolecular attractions only are broken.

The dehydration of copper(II) sulfateis considered a Physical change:

CuSO4. 5H2O(s) → CuSO4(s) + 5H2O(l)

since the fundamental composition ofthe copper(II) sulfate remains the same:one Cu2+ ion for every one SO4

2-.

We would consider the attractions broken between the ions and the water molecules to be intermolecular attractions.

If a few drops of water are then added to the colourless anhydeous CuSO4(s), the blue colour ofhydrated CuSO4. 5H2O(s) returns. A characteristic of Physical changes is that they are often easy to reverse.

The decomposition of copper(II) carbonate also involves a colour change (green → black) but is a Chemical change:

CuCO3(s) → CuO(s) + CO2(g)

There is a change in the composition and ionic and covalent bonds will be breaking and forming.

The chemical change can be reversed, but not easily.

Copper carbonate is also hydrated so water vapour will also be observed due to the Physical change:

H2O(s) → H2O(l)

AP Chemistry Unit 4


The decomposition of hydrogen peroxide is a Chemical change:

2 H2O2(aq) → 2 H2O(l) + O2(g)

involving the breaking and forming of covalent bonds anda clear change in composition.

However, it is a very slow reaction so is usually catalysed bythe addition of manganese (IV) oxide (MnO2 ).

When added, it is often in the form of lumps but changes to powder during the reaction. this would not be considered a change in composition.

MnO2(s) → MnO2(s) Fom lump to powder is a Physical change.

A chemical change occurs when substances are transformed into new substances, typically with different compositions.

Evidence for a 'new substance with different composition' can include: formation of a gas, formation of a solid precipitate, a colour change and release (exothermic) of energy which can be light, heat or electricity. Some of these can, however, also be seeen in Physical changes so it is important to look carefully at the context within which these changes are happening.

AP Chemistry Unit 4


4.1 Practice Problems1.

Equimolar samples of CH3OH(l) and C2H5OH(l) are placed in separate, previously evacuated, rigid 2.0 L vessels. Each vessel is attached to a pressure gauge, and the temperatures are kept at 300 K. In both vessels, liquid is observed to remain present at the bottom of the container at all times. The change in pressure inside the vessel containing CH3OH(l) is shown below.

Which of the following best describes the change that takes place immediately after the CH3OH(l) is introduced into the previously evacuated vessel?

A A chemical change takes place because covalent bonds are broken.

B A chemical change takes place because intermolecular attractions are overcome.

C A physical change takes place because covalent bonds are broken.

D A physical change takes place because intermolecular attractions are overcome.

2. 2 H2O2(l) → 2 H2O(l) + O2(g)

The exothermic process represented above is best classified as a

A physical change because a new phase appears in the products.

B physical change because O2(g) that was dissolved comes out of solution

C chemical change because entropy increases as the process proceeds.

D chemical change because covalent bonds are broken and new covalent bonds are formed.

O

O

AP Chemistry Unit 4


3. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

A student had two dilute, colorless solutions, HCl(aq) and NaOH(aq), which were at the same temperature. The student combined the solutions, and the reaction represented above occurred. Which of the following results would be evidence that a chemical reaction took place?

A The resulting solution is colorless.

B The temperature of the reaction mixture increases.

C The total volume of the mixture is approximately equal to the sum of the initial volumes.

D The resulting solution conducts electricity.

4. A student was asked to formulate a hypothesis about what would happen if 100.mL of 0.1MNaOH(aq) at 25°C was combined with 100.mL of 0.1MMgCl2(aq) at 25°C .

Which of the following hypotheses indicates that the student thought a chemical change would occur?

A The volume of the resulting solution will be equal to the sum of the volumes of the original solutions.

B The mass of the resulting solution will be equal to the sum of the masses of the original solutions.

C The resulting solution would contain a precipitate.

D The resulting solution will be clear.

5. A student was studying physical and chemical changes. The student carried out some procedures in the laboratory and recorded observations. For one of the procedures, the student concluded that a physical change took place, but not a chemical change.

Which of the following could have been the results of the procedure?

A A cube of metal was changed into a flat sheet of metal.

B When two liquids at room temperature were combined in a beaker, the beaker became hot.

C When two clear liquids were combined, the resulting mixture was cloudy.

D When a colorless liquid was added to a blue liquid, the resulting solution was yellow.

O

O

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AP e 4 Unit 4 - Chemical Reactions