1.2b - Rxn Questions

Chemical reaction – when atoms rearrange and substances change into other substances
Indications of a chemical Rx:

Colour changes
Heat
Gas produced
Formation of a precipitate

The Diatomics:

There are 7 elements that are always found as diatomics (a molecule with 2 atoms):hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine.

This is a list of the seven diatomic elements. The seven diatomic elements are:

Hydrogen (H2)
Nitrogen (N2)
Oxygen (O2)
Fluorine (F2)
Chlorine (Cl2)
Iodine (I2)
Bromine (Br2)

HS-PS1-2 Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Synthesis Reactions:

Synthesis Reactions Experimentation:

Iron (III), meaning it forms a Fe+3 ion, reacts with Oxygen gas (O2(g)) when heated forming Iron Oxide.

Predict the resulting product when the two chemicals react.
Write a balanced chemical equation for the reaction.
What evidence is present that indicates a chemical change is occurring?
How can we account for the changes in the mass of the steel wool?
1. Discuss how this reaction demonstrates a synthesis reaction.
What role did oxygen play in this reaction?

Experimentation:

Place some steel wool (Fe+3) in an evaporating dish.
Measure the mass of the steel wool and evaporating dish using the electronic balance.
Record the mass of the steel wool and evaporating dish combination below.
Leave the steel wool and evaporating dish on the electronic balance.
Using the small torch, burn the steel wool.
Move the flame slowly through the steel wool to ensure even burning.
Note the color changes, sparks, and other visual effects as the steel wool burns in the table below.
After the steel wool has cooled completely (about 2–3 minutes), measure its mass again using the electronic balance.
Dispose of the burnt steel wool in the designated waste container.

Synthesis Reaction Practice:

Two or more reactants (usually elements) join to form a compound

- A + B → AB where A and B represent elements

- Sodium metal and chlorine gas combine to form

sodium chloride

- Na(s) + Cl2(g) → NaCl(s)

How would you balance this eq?

2Na(s) + Cl2(g) → 2NaCl(s)

Balance these Common Synthesis Rxns:

N2(g) + O2(g) → N2O(g)

2N2(g) + O2(g) → 2N2O(g)

Fe(s) + S(s) → FeS(s)

H2(g) + O2(g) → H2O(l)

2H2(g) + O2(g) → 2H2O(l)

_H2 + _N2 → _NH3

3H2 + N2 → 2NH3

potassium and iodine

2K+I2(g) → 2KI

Lithium and Oxygen gas

4Li + O2 → 2Li2O

Calcium and Chlorine gas

Ca + Cl2 → CaCl2

Sample Problem:

Write the simple chemical reaction that takes place in the fuel cell, using hydrogen gas and oxygen gas to generate water and energy. Be sure to obey the laws of conservation of matter.

______ + ______ → ______ + Energy

Decomposition Reactions

Decomposition Reaction Experimentation

Sodium bicarbonate decomposes into sodium carbonate, water and carbon dioxide

_______ NaHCO3 → ____ Na2CO3 + _____ H2O + ______ CO2

Mass Change:
1. Calculate the mass difference before and after the reaction. This loss corresponds to the mass of the gas released (primarily CO2 ).
Temperature:
1. Record the temperature at which the reaction visibly began and compare it to the expected decomposition temperature (~80–200°C for NaHCO3).
Chemical Reaction:
1. Write the balanced equation for the decomposition reaction:
  NaHCO3 →Na2CO3 +H2O+CO2 .
Gas Test:
1. Discuss how the glowing splint test confirmed the presence of CO2 as a product.

1. Measuring Initial Mass

a. Place the Erlenmeyer flask on the electronic balance and record its mass.
b. Add approximately 20–30 grams of sodium bicarbonate to the flask and record the combined mass.

2. Heating the Sodium Bicarbonate

a. Place the flask on the hot plate and gradually heat it to around 200°C. Use the infrared temperature probe to monitor the temperature of the flask.
b. Observe the reaction as the sodium bicarbonate begins to decompose. Look for signs such as the release of gas (bubbling or fizzing) or color change.

3. Testing Gas Production

a. Light a wood splint with the lighter.
b. Carefully dip the burning splint into the mouth of the flask to test for the presence of carbon dioxide (CO2 ).
c. If the gas extinguishes the flame, this indicates the presence of CO2 .

4. Measuring Final Mass

a. Once the reaction is complete and no more visible gas or condensation is being released, remove the flask from the hot plate using tongs or gloves and allow it to cool.
b. Measure and record the mass of the cooled flask with its contents.

Decomposition Reactions Theory and Practice

A compounds breaks down into two or more products (often elements)

- AB → A + B where A and B represent elements

NaCl(s) → Na(s) + Cl2(s)

How would you balance this eq?

2NaCl(s) → 2Na(s) + Cl2(s)

Balance these Common Decomposition Rxns:

Al2O3 → Al + O2

2Al2O3 → 4Al + 3O2

KClO3 → KCl + O2

2KClO3 → 2KCl + 3O2

Ag2O → Ag + O2

2Ag2O → 4Ag + 1O2

sodium oxide (Na2O)

2Na2O -> 2Na + O2

Sample Problem:

In photography, silver bromide (AgBr) is used to develop pictures. The compound is 'photosensitive'. When the silver bromide is exposed to light it breaks down (decomposes), the silver metal remains developed into a photo.

Write a simple chemical equation for the decomposition of silver bromide.

2AgBr -> 2Ag + Br2

Single Displacement Reaction

Single Displacement Reaction Experimentation

Zinc metal will react with copper sulfate to form Zinc sulfate and copper metal.

___ Zn + ___ CuSO4 → ___ ZnSO4 + ____ Cu

Using a graduated cylinder, measure 20 ml of Copper sulfate add this to a test tube.
Using the time lapse (Hyperlapse) feature of your cell phone camera, record the reaction at 15x speed.
Add a single piece of Zinc (Zn) metal to the test tube.
Observe and record the changes happening within the test tube.

During the reaction, hydrogen gas (H2) may be produced as the CuSO4 transitions as H2SO4 .

Single Displacement Reaction Theory/Practice

Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. Both metals and non-metals take part in displacement reactions.

- - A + BC → B + AC where A is a metal
  - A + BC → C + BA where A is a non-metal

When A is a metal:

Aluminum foil in a solution of copper(II) chloride produces solid copper and aluminum chloride

Al(s) + CuCl2(aq) → Cu(s) + AlCl3(aq)

2Al(s) + 3CuCl2(aq) → 3Cu(s) + 2AlCl3(aq)

Balance these common single displacement reactions:

Zinc and Copper Chloride produces zinc chloride and copper ions.

Zn + CuCl2 -> ZnCl2 + Cu

Zinc and Hydrochloric acid produces Zinc Chloride and Hydrogen gas.

Zn + HCl -> ZnCl2 + H2

Zn + 2HCl -> ZnCl2 + H2

Chlorine gas plus sodium bromide produces sodium chloride (salt) and bromine gas.

Cl2 + NaBr -> NaCl + Br2

Cl2 + 2NaBr -> 2NaCl + Br2

When A is a non-metal:

- When fluorine is bubbled through a sodium iodide solution, iodine and sodium fluoride are produced

F2(g) + NaI(aq) → I2(g) + NaF(aq)

F2(g) + 2NaI(aq) → I2(g) + 2NaF(aq)

Sodium Fluoride is sold as tablets to prevent cavities in teeth.

Sample Question:

The cup shown below provides an example of tarnish, a chemical reaction caused when silver metal (Ag) reacts with hydrogen sulfide (H2S) gas produced by some industrial processes or as a result of decaying animal or plant materials.

Write simple balanced chemical equation to describe the reaction.

2Ag+H2S→Ag2S+H2

The tarnish (Ag2S) can be removed using a number of polishes, but the process also removes a small amount of silver along with the tarnish.

Source

Double Displacement Reactions

Double Displacement Reaction Experimentation

Silver Nitrate in solution reacts with Sodium Chloride to produce Sodium nitrate and Silver Chloride.

AgNO3 (aq) + NaCl(aq) -> NaNO3 (s) + AgCl(aq)

What did you observe when the silver nitrate solution was mixed with the sodium chloride solution?
Describe the appearance of the precipitate that formed.
What type of chemical reaction occurred when silver nitrate reacted with sodium chloride? (Hint: Consider whether there is a solid product formed.)
Explain why this reaction is classified as a double displacement reaction.
Write the balanced chemical equation for the reaction between silver nitrate and sodium chloride.
Identify the precipitate in this reaction and explain why it forms.
How can you confirm that the precipitate is silver chloride (AgCl\text{AgCl}AgCl)? (Hint: Consider its solubility or other physical properties.)
What happens to the sodium nitrate (NaNO3\text{NaNO}_3NaNO3) formed in this reaction?
How could you separate and collect the precipitate from the reaction mixture?
If you did not see a precipitate form, what could have gone wrong in the experiment?
Suggest ways to ensure that the reaction between silver nitrate and sodium chloride proceeds correctly.

Double Displacement Reactions Theory

Double displacement reactions may be defined as the chemical reactions in which one component each of both the reacting molecules is exchanged to form the products. During this reaction, the cations and anions of two different compounds switch places, forming two entirely different compounds.

Double replacement reactions swap elements between two compounds reacting together to form two new compounds

- Two solutions react to form a precipitate (ppt, solid) and another solution
  - Ionic solution + ionic solution → ionic solution + ionic solid
  - AB + CD → AD + CB

Pb(NO3)2(aq) + HCl(aq) → PbCl2(s) + HNO3(aq)

Pb(NO3)2(aq) + 2HCl(aq) → PbCl2(s) + 2HNO3(aq)

The Reason for the Rxn

During the Types of Chemical Reaction Lab, you were able to experiment with four different reactions and observe a fifth. One of the more dramatic reactions was reacting Zinc metal (Zn) with Hydrochloric Acid (HCl) to produce explosive Hydrogen gas (H2) and versatile zinc chloride (ZnCl2).

1. Balance the following equation and add subscripts to describe the state of the molecule:

2. Based on information provided in the periodic table, for each of the three elements, determine the charge of ion it would produce.

3. For each of the four molecules in the reaction, draw a complete Lewis Dot Diagram.

4. Referencing Figure 1: Bonding Strength v. Electronegativity, identify and justify the types of bonds between the following elements:

H + Cl
Zn + Cl

5. Within a CER format and making specific reference to the evidence provided in the Table 1: Some Properties of Some Elements, outline why the Chlorine molecules moved from the hydrogen atoms to the zinc atoms during the reaction.

Command Terms:

Draw - Represent by means of a labelled, accurate diagram or graph
Determine Obtain the only possible answer.
Identify - Provide an answer from a number of possibilities. Recognize and state briefly a distinguishing factor or feature.
Justify - Give valid reasons or evidence to support an answer or conclusion.
Outline Give a brief account or summary.

Table 1: Some Properties of Some Elements

Figure 1: Bonding Strength v. Difference in Electronegativity.

Creating and Balancing Rxns

Solid magnesium (Mg) reacts with fluorine gas (F2) to form a solid product.

Three students predicted the chemical formula for the solid product formed as a result of this reaction:

1. Sarah predicted MgF2
2. Lary predicted Mg2F2
3. Ahmed predicted Mg2F.

State whom you agree with and explain to the other two why they are incorrect?
Write a balanced chemical equation that shows the reaction between these elements. Include state symbols.

Solid lithium metal (Li) reacts with oxygen gas (O2) to form lithium oxide, an ionic compound.
1. Describe the process to determine the chemical formula for lithium oxide.
2. State the chemical formula for lithium oxide.
3. Write a balanced equation for this reaction.

Ammonia (NH3), a covalent compound, is formed when nitrogen gas (N2) and hydrogen gas (H2) react.
1. Draw the Lewis dot diagram for ammonia. (This is from the previous PE)

Write a balanced chemical equation that shows the formation of ammonia.

Explain how the electrons in a covalent bond behave differently from electrons in an ionic bond.

Methane reacts with oxygen to produce carbon dioxide and water.

CH4 + O2 → CO2 + H2O

State the Law of Conservation of Mass.
Balance the equation above.

Ana took the following measurements while conducting a simple experiment with methane:

Mass of methane: 100 g
Mass of oxygen gas: 200 g
Mass of water produced: 215 g

Determine the mass of the CO2 produced during this reaction.
Outline how you determined the mass of CO2 that was produced.

As a single atom, Calcium has 2 valence electrons. Calcium carbonate (CaCO3) breaks down into carbon dioxide (CO2) and a compound of calcium and oxygen.
1. Determine the chemical formula for the compound of calcium and oxygen.
2. Write the balanced chemical equation for this decomposition.
3. This reaction requires heat to be added to the system for the decomposition to occur. Why do you think that heat is needed for the decomposition to occur?

Oxygen is a gas. In its elemental form, oxygen is a diatomic molecule.

Based on the roots of the word diatomic, describe its meaning.

Name the six other elements that exist as diatomic molecules?

What kind of bond is formed between 2 oxygen atoms? Explain why this bond forms.

Chemical Rxn Formative Questions

HSPS1-2 Chem Rxn Review (formative)

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