Question

Chemical Reactions Types and Their Equations

Making Heat with Chemical Reactions

Have you ever wondered how an instant heat pack works? A disposable heat pack works by a chemical reaction that combines iron in the package with oxygen from the air when the outer packaging is removed producing iron oxide. You have probably seen the product of this reaction in what is commonly called rust. The reaction releases heat, which allows the pack to reach a sufficient temperature that is not uncomfortable. A disposable heat pack is pictured in Figure 1.

Balanced Chemical Equations

Every chemical equation represents a specific chemical reaction. Equations identify the reactants (the substances that interact together) and the products (new substances that are formed as a result of that interaction).

Reactants appear on the left side of an equation, and the products appear on the right. They are separated by an arrow (→), which indicates a reaction has taken place and the direction of the reaction. For an equation to be correct, it must be balanced: the same number of each kind of atom must appear on both sides of the equation. For example, when iron rusts, it combines with oxygen to form a new compound, iron (III) oxide (Fe₂O₃). The balanced chemical equation for this reaction is shown below.

4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

This equation indicates that four iron atoms combine with three molecules of diatomic oxygen (for a total of six oxygen atoms) to produce the two formula units of iron (III) oxide, each with two atoms of iron and three atoms of oxygen.

Signs are used to indicate what state the molecules are in:

(s): solid

(l): liquid

(g): gas

(aq): aqueous (compound dissolved in water)

Types of Chemical Reactions

A chemical reaction is the change of a substance into a new substance with a different chemical identity.

A chemical reaction is usually accompanied by easily observable changes, such as the emission of heat and light, the formation of a precipitate, the evolution of gas, or a color change. There are many different types of chemical reactions. Chemists have classified the reactions into general categories based on similarities.

Synthesis Reactions

In a synthesis reaction, two or more substances combine to form a new compound. For example, iron rusting is a synthesis reaction.This type of reaction is represented by the following equation.

A + B → AB

Where A and B represent the reacting elements or compounds and AB represents a compound as the product.

Decomposition Reactions

In a decomposition reaction, a single compound undergoes a reaction that produces two or more simpler substances. Decomposition reactions are often initiated with the addition of heat or electricity. A decomposition reaction can be represented by the following equation.

AB → A + B

Where AB represents a reacting compound and A and B represent elements or compounds as the products.

Single-Displacement Reactions

In a single–displacement reaction, one element replaces a similar element in the compound. Single-displacement reactions can be represented by the following equation .

AB + C → AC + B

Where AB is a reacting compound, C is a reacting element, AC is a product compound and B is the displaced element. This reaction will only happen if C is more chemically reactive than B.

Single–displacement reactions are usually the result of a redox reaction where one element gains electrons and another loses electrons as an ion in the reacting compound is displaced by the elemental form of the reacting element. The displacement of Ag⁺ by Cu is given by the reaction below.

2AgNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + 2Ag(s)

Double-Displacement Reactions

In a double-displacement reaction, the ions of two compounds exchange places in an aqueous solution to form two new compounds. A double-replacement reaction can be represented by the following equation.

AB + CD →AC + BD

Where A, B, C, and D are ions and AB, CD, AC, and BD are ionic compounds.

One of the products of a double-displacement reaction is often a precipitate, an insoluble gas, or a molecular compound such as water. The other product is usually soluble and remains dissolved in the solution. For example, AgCl is precipitated when solutions of AgNO₃ and NaCl are combined by the double-displacement reaction below.

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

About This Lab

In this lab, you will perform four classes of chemical reactions. In the synthesis and decomposition reactions, you will observe changes in the appearance of substances and changes in mass. In the single-displacement reaction, you will use a pressure gauge and thermometer to observe the changes. Finally, in the double-displacement reaction, you will observe the formation of a precipitate after combining solutions. After performing these reactions, you will identify the products formed and write a balanced chemical equation for each of those reactions.

Procedures

Experiment 1: Perform a Synthesis Reaction

Take a balance from the Instruments shelf and place it on the workbench.

Take a crucible from the Containers shelf and place it on the balance. Record the mass of the crucible in your Lab Notes. Remember to press Save Notes.

Remove the crucible from the balance. Add 5 g of magnesium (Mg) to the crucible from the Materials shelf.

Place the crucible on the balance. Record the total mass of the crucible plus magnesium (Mg) in your Lab Notes. Remove the crucible from the balance.

Take a Bunsen burner from the Instruments shelf and place it on the workbench.

Drag the crucible and place it on the Bunsen burner. Turn on the Bunsen burner by clicking the black knob at the bottom to the low flame.
(Clicking on the knob multiple times will increase the intensity of the flame until it is clicked off.)

Wait 1 – 2 minutes and then remove the crucible from the flame by placing it on the workbench and turning the burner off.

Wait 1 – 2 minutes for the crucible to cool, then place it on the balance. Measure and record the mass of the crucible and its contents in your Lab Notes.

Remove the crucible from the workbench by dragging it to the recycling bin beneath the workbench.

Experiment 2: Perform a Decomposition Reaction

Take a new clean crucible from the Containers shelf and place it on the balance. Record the mass of the crucible in your Lab Notes.

Remove the crucible from the balance. Add 5 g of copper (II) carbonate hydroxide hydrate (Cu₂CO₃(OH)₂ × H₂O) from the Materials shelf to the crucible.

Place the crucible on the balance. Record the total mass of the crucible and its contents in your Lab Notes.

Remove the crucible from the balance and place it on the Bunsen burner.

Double-click the crucible to open its Item Properties. Check the option box and click OK to give you a cut-away view of the crucible's contents. Record the observations in your Lab Notes.

Turn on the Bunsen burner to the low flame.

Watch the crucible until the green compound changes color. Record the observations in your Lab Notes.

Remove the crucible from the Bunsen burner and turn the burner off.

Drag the crucible and place it on the balance. Record the mass of the crucible and its contents in your lab notes.

Clear the workbench by dragging all of the Instruments and Containers to the recycling bin.

Experiment 3: Perform a Single Displacement Reaction

Take a clean 150 mL Erlenmeyer flask from the Containers shelf and place it on the workbench.

Add 15 mL of 6 M hydrochloric acid (HCl) from the Materials shelf to the Erlenmeyer flask.

Double-click on the Erlenmeyer flask and select the Close to put an airtight stopper on it.

Take a thermometer from the Instruments shelf and connect it to the Erlenmeyer flask.

Take a pressure gauge and connect it to the Erlenmeyer flask.

Add 0.25 g of Zinc (Zn) from the Materials shelf to the Erlenmeyer flask. You can try the same experiment with 25 g of zinc and see what happens.

Observe and record any indication that a reaction has occurred in your Lab Notes.  
Remember to press Save Notes.

Wait for the solution in the Erlenmeyer flask to cool down within half a degree of room temperature (21.5°C). Remove the thermometer and pressure gauge.

Take a syringe from the Instruments shelf and connect it to the Erlenmeyer flask. This allows gas to escape from the Erlenmeyer flask into the syringe.

Pass the mouse cursor over the syringe and a gray tool tip will briefly display the total volume. Record the volume of the gas collected in the syringe in your Lab Notes.

Clear the workbench by dragging all of the containers and instruments to the recycling bin.

Experiment 4: Perform a Double Displacement Reaction

Take two 50 mL beakers from the Containers shelf and place them on the workbench.

Add 15 mL of 1 M sodium hydroxide (NaOH) solution from the Materials shelf to one of the beakers.

Add 6 mL of nickel (II) chloride solution (NiCl₂) from the Materials shelf to the other, empty, beaker. You may want to label your beakers by double-clicking on each to get the Item Properties window.

Observe the contents of each beaker. Record the observations in your Lab Notes.

Pour the entire contents of one beaker into the other. Record your observations of the reaction that occurs in your Lab Notes.  

Clear the workbench by dragging all of the containers and instruments to the recycling bin.  

Lab Notes:

Front number below refers to line number in above experiments

Experiment # 1

2. empty crucible 88g

3.      5 g of Mg to crucible

4. crucible plus Mg = 93 g

8. mass after heating = 96.289 g

Experiment #2

1. 88g

2. add 5 g copper (II) carabonate hydroxide hydrate

3. crucible plus contents = 93 g

9. crucible plus contents after heating = 91.327 g

Experiment #3

2. added 15 mL of 6M hydrochloric acid

6. added .25 g zinc

7. When zinc added to copper(II) carbonate hydroxide hydrate pressure and temp both increased pressure from 1 to 1.70    and   temp from 21.5 to 26.5 C

8. when cooled down pressure went down slightly 1.70 to 1.69   and temp went down 26.50 to 21.50 C

10. 92.2 mL

Experiment 4

2. clear

3. green

5. clear liquid and green solid

End of Lab Notes

Multiply Choice

Answer each part for maximum points.

Question 7

a. In the decomposition reaction, 1 mole of water (MW = 18.015 g/mol) was produced for every mole of CuO (MW = 79.545 g/mol) produced. Given that 3.327 g of CuO was produced during the reaction, how many grams of water were released as water vapor? (number of moles = mass (g) / molar mass (g/mol)). Choose the closest answer.

1.80 g        3.98 g       0.92 g                  0.75 g

b. A yellow precipitate is formed when combining two colorless solutions. What type of reaction have you likely observed?

A. double displacement

B. decomposition

C. synthesis

D. single displacement

c. After heating a solid, you observe that the mass has decreased and there has been a change in color. What type of reaction have you likely observed?

A. synthesis

B. single displacement

C. decomposition

D. double displacement

d. A solid sample of metal is dropped into an acidic solution. Bubbles form as the metal is dissolved. What kind of reaction have you likely observed?

A. double displacement

B. decomposition

C. Synthesis

D. single displacement

e. When hydrochloric acid and zinc were combined, the flask was sealed and thus no gas could escape from the flask. Why did the pressure decrease as the temperature decreased?

A. The gas entered the liquid, similar to a carbonated beverage.

B. The gas was converted to a solid.

C. The produced gas behaved as an ideal gas where pressure is proportional to temperature.

D. The temperature probe was not sensitive to this change.

Answer 1

Q7. (a.) Correct answer is (D.) 0.75 g

(b.) Correct answer is (A.) double displacement

(c.) Correct answer is (C.) decomposition

(d.) Correct answer is (D.) single displacement

(e.) Correct answer is (C.) The produced gas behaved as an ideal gas where pressure is proportional to temperature.

Explanation

(a.) mass CuO = 3.327 g

moles CuO = (mass CuO) / (molar mass CuO)

moles CuO = (3.327 g) / (79.545 g/mol)

moles CuO = 0.0418 mol

moles water vapor = moles CuO

moles water vapor = 0.0418 mol

mass water vapor = (moles water vapor) * (molar mass H₂O)

mass water vapor = (0.0418 mol) * (18.015 g/mol)

mass water vapor = 0.75 g