Unit 6 Section 1: Modeling Chemical Reactions

Unit 6 Section 1: Describing Chemical Reactions

Lesson Objectives

• Write Balanced Equations for chemical reactions

• Explain what causes chemical reactions

• Distinguish between types of reactions

​Introduction

• ​Chemical Reaction: The process by which one or more substances are changed into one or more different substances

  • ​Two components

    • ​Reactants: the original substances before the reaction takes place

      • Reactants must come together in order for a reaction to take place​

    • ​Products: the resulting substances after the reaction takes place

• ​According to the law of conservation of mass: the total mass of reactants must equal the total mass of the products

​Indications of a chemical reaction

• Two main categories:

  • Changes in Energy

    • Release of energy as heat and light: ​Usually shows a chemical reaction, but not always

    • Production of a sound​

​Indications of a chemical reaction

• Two main categories:

  • Formation of a new substance

    • ​Production of a gas

    • ​Formation of a precipitate

      • ​Precipitate: A solid substance that results from the mixing of two dissolved liquids

    • ​An odor change or A color change

      • Note: A color change only counts if it is not possible to revert to the original color of the reactants, otherwise it is a physical change

​Chemical Equations

• A Chemical Equation describes a chemical reaction by using symbols and formulas

  • Shows the relative molar amounts of both reactants and products

• A recipe for a reaction

  • Coefficients: Large numbers in front of the chemical formulas in a chemical equation

    • Tell the Number of Moles involved in the reaction

    • If no number, the number of moles is automatically one

• Example: Ammonium Dichromate Decomposition

Multiple Choice

How many Moles of water are involved in the decomposition of ammonium dichromate?

• 1

• 2

• 4

• 3

Multiple Choice

How many Moles of Nitrogen Gas are involved in the decomposition of ammonium dichromate?

• 1

• 2

• 4

• 3

​Characteristics of Chemical Equations

• ​​Required information

  • ​The equation must represent all known facts: all reactants and products must be known

  • ​The equation must contain the correct formulas for the reactants and products:

    • Make sure you keep track of diatomic molecules

  • ​The law of conservation of Mass must be satisfied: Atoms can not be created or destroyed, so the same number of each must appear on both sides of the equation

​Using Word Equations

• ​When writing chemical equations it helps to first write everything out using words instead of symbols

  • ​This only gives you a qualitative description, because no amounts are given

  • ​Example: Here is the word equation for the reaction of methane and oxygen 

• ​Once you have written everything using words, you can replace the words with the appropriate chemical formulas and symbols to write a formula equation

  • What are our chemical formulas?

    • ​Methane: CH₄

    • ​Oxygen: is one of the diatomic molecules: O₂

    • ​Carbon Dioxide: CO₂

    • ​Water: H₂O

  • We can show the state of matter each part of the equation is in using the first letter of that state of matter

​Using Formula Equations

​

• ​Our equation shows what's involved but not how much because it's not Balanced

  • ​Let's look at our reactants and our products and count how many of each atom we have on both sides

  • ​We need to add coefficients to the products to balance out the equation

    • ​Let's start by looking at Hydrogen since it is in the least number of places in the products

​Balancing Equations

Multiple Choice

How many more moles of hydrogen do we need on the products side of the equation to balance out the equation

• 1

• 2

• 3

• 4

• ​ ​The easiest way to balance our hydrogen is to add a 2 in front of the water

• ​Let's check our reactants and products again to see if it is balanced

  • ​Our equation is not balanced, so we need to add another coefficient, this time to the reactants

Multiple Choice

How many more moles of Oxygen do we need on the reactants side to have a balanced equation?

• 1

• 2

• 3

• 4

Balancing Equations Practice 1

• Balance the Equation: ___ AgNO₃ + ___ Cu → ___ Cu(NO₃)₂ + ___ Ag

  • Step 1: Identify what we have on each side of the equation

  • Step 2: Identify what we can balance first

    • In general, you should balance Oxygen and Hydrogen last, so we start with nitrogen

Dropdown

What number can we put in to balance the nitrogen in the following equation:

​

Select answer

AgNO₃ + Cu → Cu(NO₃)₂ + Ag

Balancing Equations Practice 1

• New Equation: 2 AgNO₃ + __Cu → _Cu(NO₃)₂ + _ Ag

  • Step 3: Check both sides of the equation for balance

  • Step 4: Fix any unbalanced parts of the equation

Dropdown

What number can we put in to balance the silver in the following equation:

​ 2 AgNO₃ + Cu → Cu(NO₃)₂ + ​

Select answer

Ag

Balancing Equations Practice 2

• Balance the following equation:

• In this equation, we have polyatomic ions that can be treated as one unit because they are on both sides of the equation.

• Step 1: Determine how many of each atom/polyatomic ion there are on each side of the equation.

Drag and Drop

Balance the Ammonium in this equation:​

Placeholder

NH₄ +​ H₂SO₄ --> (NH₄)₂ + H₂O

Drag these tiles and drop them in the correct blank above

2

1

3

4

Balancing Equations Practice 2

• Balance the following equation:

• Step 3: Update the number of atoms/ions we have on both sides of the equation

• Step 4: Now we need to balance our oxygen and hydrogen

  • We need more on the products side, so we will add a coefficient to the water

Drag and Drop

Balance the Water in the Following Equation: 2 NH₄OH +H₂SO₄ --> (NH₄)SO₄ +​

Placeholder

H₂O

Drag these tiles and drop them in the correct blank above

2

1

3

4

Balancing Equations Practice 2

• Balance the following equation:

• Step 5: Update the number of atoms/ions we have on both sides of the equation

• Step 6: Double-check your work

Balancing Equations Practice 3

• Solid aluminum carbide, Al₄C₃, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.

• Step 1: Identify how many of each atom we have on each side of the Equation

Categorize

Options (8)

4 Aluminun

1 Aluminum

3 Carbon

1 Carbon

2 Hydrogen

1 Oxygen

7 Hydrogen

3 Oxygen

Organize these options into the right categories

Reactants

Products

Balancing Equations Practice 3

• Solid aluminum carbide, Al₄C₃, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.

• Step 2: Begin Balancing the equation with either the Aluminum or the Carbon Atoms

  • DO NOT TRY TO DO BOTH AT THE SAME TIME

    • Let's do Aluminum

Drag and Drop

Fill in the blank to balance Aluminum on both sides of the equation: Al₄C₃+H₂O-->CH₄+​

Placeholder

Al(OH)₃

Drag these tiles and drop them in the correct blank above

4

2

6

8

Balancing Equations Practice 3

• Solid aluminum carbide, Al₄C₃, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.

• Step 3: Update how many of each atom you have on both sides of the equation

• Step 4: Now balance the Carbon

Drag and Drop

Fill in the blank to balance Carbon on both sides of the equation: Al₄C₃+H₂O-->​

Placeholder

CH₄+​ 4 Al(OH)₃

Drag these tiles and drop them in the correct blank above

2

6

8

3

4

Balancing Equations Practice 3

• Solid aluminum carbide, Al₄C₃, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.

• Step 5: Update how many of each atom you have on both sides of the equation

• Step 6: Now we will choose between Oxygen and hydrogen to Balance

  • We should do Oxygen because it only appears in one spot on both sides of the equation

Drag and Drop

Fill in the blank to balance Oxygen on both sides of the equation: Al₄C₃+ ​

Placeholder

H₂O-->​ 3 CH₄+​ 4 Al(OH)₃

Drag these tiles and drop them in the correct blank above

2

6

8

4

12

10

Balancing Equations Practice 3

• Solid aluminum carbide, Al₄C₃, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.

• Step 6: Double-check your equation and see if it is balanced

Multiple Select

What two Categories indicate that a chemical reaction has taken place?

• Changes in energy

• Formation of a new Substance

• Changes in state of matter

• There is no way to tell if a chemical reaction has taken place

Multiple Choice

Coal contains carbon and other elements. Carbon dioxide forms when coal burns in the presence of oxygen. Which of these statements is the best evidence that a chemical reaction occurs when coal burns?

• The shape of the coal changes
• Oxygen is present
• A new substance is produced
• Coal is made up of more than one element

Multiple Choice

True or False: In a chemical reaction, no new atoms are created, and no atoms are destroyed. 

• true
• false

Multiple Choice

Is the following equation balanced: 4Fe + 3O₂ --> 2Fe₂O₃

• Yes

• No

Multiple Choice

True or False: The reactants are on the left side of the chemical equation.

• True
• False

Match

Match the following symbol to their meaning in a chemical reaction

• (s)

• Δ

• →

• (aq)

• ↔

Solid

A change

Yields

Aqueous

Reversible Reaction

• Solid

• A change

• Yields

• Aqueous

• Reversible Reaction

Multiple Choice

Is this equation balanced?

• Yes

• No

Multiple Choice

How many oxygens are in Al₂(SO₄)₃?

• 4
• 12
• 7
• 24

Multiple Choice

Is the equation balanced?: 2H₂O + O₂ --> 4MgO + 3Fe

• Yes
• No

Drag and Drop

Balance this equation
​

Placeholder

N₂ + ​

Placeholder

H₂ --> ​

Placeholder

NH₃

Drag these tiles and drop them in the correct blank above

5

6

4

1

3

2

7

8

9

Energy of Reactions

• Recall: All bonds are held together by stored potential energy

• During reactions, bonds are broken and formed

  • energy is needed to break bonds and is released when bonds are formed

• Law of Conservation of Energy: In any chemical reaction, energy is neither created nor destroyed

  • Any excess energy is absorbed by the surroundings

Endothermic and Exothermic

• All reactions either absorb or release energy.

• Exothermic: Reactions that Release energy to the surroundings

  • Products store less energy than reactants

  • Feel Hot

• Endothermic: Reactions that absorb energy from the surroundings

  • Products store more energy than reactants

  • Feel Cold

What causes reactions?

• Collision theory: Reactions only occur when reactants collide with enough energy at the right angle

• Reaction rate: the speed at which reactants become products

  • As reactants move faster, more collisions occur, making the reaction occur faster

    • Higher temperature=more movement=faster reaction rate

Drag and Drop

An ​

Placeholder

reaction releases energy into the surroundings and has lower energy in the products than in the reactants

Drag these tiles and drop them in the correct blank above

exothermic

endothermic

catalytical

synthesis

Drag and Drop

The Law of conservation of ​

Placeholder

says that energy in a reaction is neither created nor destroyed.​

Drag these tiles and drop them in the correct blank above

energy

matter

time

charge

temperature

Multiple Choice

What does collision theory state?

• Chemical reactions occur without any collision
• Reactant molecules collide with low kinetic energy
• Chemical reactions can occur only when reactant molecules collide with enough kinetic energy at the right angle

• Kinetic energy has no effect on chemical reactions
• Chemical reactions can occur only when reactant molecules collide with enough kinetic energy at any angle

Multiple Choice

How does temperature relate to reaction rate?

• Temperature has no effect on reaction rate.
• As temperature increases, the reaction rate generally decreases.
• As temperature decreases, the reaction rate generally increases.
• As temperature increases, the reaction rate generally increases.

Types of Chemical Reactions

• We classify reactions into different types based on what happens in them

  • The types we will discuss in this class

    • Synthesis

    • Decomposition

    • Single Displacement

    • Double Displacement

    • Combustion​

Synthesis Reactions

• In a Synthesis reaction, two or more substances combine to form a new compound

  • Represented by the following general equation: A + X --> AX

• One simple type of synthesis is the combination of an element with oxygen to form an oxide​

  • Almost all metals react to form oxides​

• Most metals react with the halogens (group 17) to form compounds​

Decomposition Reactions

• In Decomposition Reactions, a single compound undergoes a reaction that produces two or more simpler substances

  • Shown by the general equation AX --> A + X​

  • Most only take place when energy is added to the system

    • Electrolysis: the decomposition of a substance by an electric current​​

Single Displacement

• In a Single Displacement reaction, one element replaces a similar element in a compound

  • Shown by the general equation A + BX --> AX+B

• Activity Series: a list of elements organized according to the ease with which the elements undergo certain chemical reactions

  • tells how likely an element is to take part in a reaction

    • Elements can replace any element underneath them on the list, but not any element above them​

    • Halogens are special; they are in the exact order they are in on the Periodic table

Multiple Choice

Based on the activity series will the following reaction take place:

$$Zn_{\left(s\right)}+H_2O_{\left(l\right)}\ \frac{50\degree C}{\rightarrow}$$  

• Yes

• No

• Zinc only reacts with water when it is in Steam. Water boils at 100 Degrees Celsius, so this reaction will not take place

Multiple Choice

Based on the activity series will the following reaction take place?

$$Sn_{\left(s\right)}\ +O_{2\left(g\right)}\rightarrow$$  

• Yes

• No

Multiple Choice

Based on the activity series will the following reaction take place?

$$Cd_{\left(s\right)}+Pb\left(NO_3\right)_{2\left(aq\right)}\rightarrow$$  

• Yes

• No

Multiple Choice

Based on the activity series will the following reaction take place?

$$Cu_{\left(s\right)}+HCl_{\left(aq\right)}\rightarrow$$  

• Yes

• No

Multiple Choice

Based on the activity series will the following reaction take place?

$$MgCl_{2\left(aq\right)}+Zn_{\left(s\right)}\rightarrow$$  

• Yes

• No

Double Displacement Reactions

• In a Double Displacement reaction the ions of two compounds exchange places in an aqueous solution to form two new compounds

  • Represented by the equation: AX + BY --> AY + BX​

• ​Generally leads to the formation of a Precipitate

Combustion Reactions

• In a Combustion reaction, a substance combines with Oxygen, releasing a large amount of energy in the form of light and heat

  • The reactants tend to be hydrocarbons, Chemicals only containing Carbon and Hydrogen​

  • Products are usually Carbon Dioxide and Water, but carbon monoxide may also be a product

• Combustion reactions are often used to generate energy​

Multiple Choice

This type of reaction results in two or more reactants making one new substance

• Synthesis

• Decomposition

• Single Displacement

• Double Displacement

• Combustion

Multiple Choice

This type of reaction results in two Compounds switching or "trading" one of their ions.

• Synthesis

• Decomposition

• Single Displacement

• Double Displacement

• Combustion

Multiple Choice

Which metal is more reactive than calcium?

• magnesium

• potassium

• silver

• aluminum

Multiple Choice

This type of reaction results in one reactant undergoing a change that results in two or more products

• Synthesis

• Decomposition

• Single Displacement

• Double Displacement

• Combustion

Multiple Choice

This type of reaction results in one reactant replacing an ion on one of the other reactants

• Synthesis

• Decomposition

• Single Displacement

• Double Displacement

• Combustion

Multiple Choice

Will silver react with magnesium chloride solution?

• yes

• no

Multiple Choice

This type of reaction results in Heat, light, carbon dioxide and water as products

• Synthesis

• Decomposition

• Single Displacement

• Double Displacement

• Combustion