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# Chemical Reactions

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Have you ever tried to make soap at home? The art of soap-making is a type of chemical reaction! First, look at the structure of a soap molecule.

Figure 1. Soap molecule structure

Soap is a type of salt made up of fatty acids. The structure of soap consists of a nonpolar hydrocarbon chain containing a carboxylic acid group. The carboxylic acid group forms an ionic bond with a sodium or potassium ion. So, soap has a hydrophobic end that does not dissolve in water (hydrocarbon chain) and a hydrophilic end (ionic salt). When soap comes into contact with grease and water, the hydrophobic end of the soap interacts with the nonpolar molecules of grease and suspends the grease/oil, allowing it to be washed away by water!

Soap forms by the reaction between the triglycerides in fat/oil and an alkali such as aqueous sodium hydroxide or potassium hydroxide.

The chemical reaction for the making of soap is:

Figure 2. The saponification process

• First, we will discuss chemical reactions and how to balance them.
• Then, we will look at the different types of chemical reactions and give some examples.

## Chemical Reaction Definition

During a chemical reaction, different substances combine to form a new substance. When this happens, we can see changes in color, gas formation, temperature changes, or even the formation of a precipitate. Or, in some cases, the result looks the same as before.

Let's look at the definition of a chemical reaction.

A chemical reaction is a process that results in the interconversion of chemical species. In simpler terms, chemical reactions consist of making and/or breaking chemical bonds to form new products.

Chemists use chemical equations to show how what happens to the reactants and products involved in a chemical reaction.

• Reactants are substances that react together to form products.
• Products are the new substances formed after the chemical reaction takes place.

## Chemical Reaction Balance

A very important skill to have when it comes to chemical reactions is the ability to balance chemical equations.

A chemical equation is balanced when the number of atoms is the same on both sides.

To balance chemical equations, there are some steps we need to follow. As an example, take a look at the chemical equation below: $$\text{K + H}_{2}\text{O} \to \text{KOH + H}_{2}$$

Step 1: Count the number of atoms present on each side of the chemical equation.

First, we need to count the number of atoms for each type of atom present in the reactant and on the product side. To find out the number of atoms for each type, we can multiply the coefficient (which, in this case, is 1) by the number on the subscript.

To make it easier to remember, we can make a simple table below the chemical equation.

Figure 4. Counting the numbers of atoms present on each side of the equation

If the number of atoms on each side is the same, then it means that your equation is already balanced. Now, if the number of atoms on each side is different, then you need to go to step two.

Step 2: Determine how many atoms you must add in order to balance the chemical equation.

In order to balance this chemical equation, we have to look for multiples of the coefficient. In this example, the coefficients are 1.

The only way to get the same amount of H atoms on both sides you be to add a coefficient of 2 to the H2O molecules, and a coefficient of 2 to the KOH molecules. This way, we would get 4 H atoms on each side.

However, this would also change the number of K atoms on the product side, and the number of oxygen atoms on both sides. To overcome this, we can add a coefficient of 2 to the K on the reactant side. Now, the reaction should be balanced!

Figure 5. Adding coefficients to balance a chemical equation

## Types of Chemical Reactions

Now that we know what chemical reactions are and how to balance them, let's dive into the different types of chemical reactions. These are:

• Synthesis (composition) reactions

• Decomposition reactions

• Oxidation-reduction (REDOX) chemical reactions

• Combustion reactions

• Single replacement reactions

• Double replacement reactions

• Neutralization reactions

### Synthesis Reactions

Synthesis reactions are reactions in which two or more substances combine to form a new substance. The general formula for synthesis reactions is: $$A + B \to C$$.

For example, the formation of hydrogen bromide (HBr) is the result of a synthesis reaction between hydrogen and bromine.

$$H_{2}\text{ (g)} \text{ }+ \text{ }Br_{2} \text{ (l)}\to 2\text{ }HBr \text{ (g)}$$

### Decomposition Reactions

Next, we have decomposition reactions. Decomposition reactions are chemical reactions in which one substance gets converted into 2 or more substances, and the general formula for this is: $$AB \to A + B$$.

The decomposition of calcium carbonate (CaCO3) into calcium oxide (CaO) and carbon dioxide (CO2) is an example of a decomposition reaction.

$$CaCO_{3}\text{ } (s)\to \text{ } CaO \text{ (s)} \text{ + } CO_{2} \text{ (g)}$$

### Oxidation-Reduction Reactions (REDOX)

In redox reactions, electrons are transferred from one species (oxidized) to another species (reduced). A change in oxidation state indicates that a redox reaction has taken place.

• Oxidation is the loss of electrons
• Reduction is the gain of electrons

For example, the reaction between zinc (Zn) and manganese oxide (MnO2) is a type of oxidation-reduction reaction.

$$Zn\text{ }( s)\text{ + }2\text{ }MnO_{2}\text{ }(s)\text{ }\to \text{ }ZnO \text{ }(s)\text{ + }Mn_{2}O_{3}\text{ }(s)$$

For an in-depth explanation of oxidation-reduction chemical reactions, check out "Redox Reactions"!

### Combustion Reactions

Combustion reactions are reactions involving a hydrocarbon reacting with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O). These reactions give out energy as heat and light.

A common combustion reaction is the reaction between the hydrocarbon methane (CH4) and oxygen (O2).

$$CH_{4}\text{ }(g) + O_{2}\text{ } (g) \text{ }\to\text{ } CO_{2} \text{ }(g)\text{ }+\text{ }H_{2}O \text{ }(g)$$

### Single Replacement Reactions

Single replacement reactions occur when one element in a compound is replaced by another element. The general formula for single displacement reactions is: $$\color{Teal} A \color{orchid} B +\color{Blue} C \color{black} \to \color{Teal} A \color{black} + \color{orchid} B \color{Blue}C$$

In single replacement reactions, the element that tends to form cations will replace the cation in a compound. Similarly, the element that tend to form anions will replace the anion in a compound.

For example, in a reaction between zinc chloride (ZnCl2) and copper (Cu), copper replaces the Zn to form cooper chloride (CuCl2).

$$ZnCl_{2}\text{ } (s)\text{ + } Cu \text{ } (s) \text{ } \to \text{ }CuCl_{2}\text{ } (s)\text{ } + \text{ } Zn\text{ } (s)$$

### Double Replacement Reactions

Double replacement reactions occur when two compounds reacts in aqueous solutions, causing the cations and anions of the two reactants to "trade partners or switch places" to form two new compounds. The general formula is this case is: $$\color{Teal} A\color{Orchid} B\color{black}\text{ } + \text{ }\color{Blue} C\color{Orange} D\color{black} \text{ }\to \text{ }\color{Teal} A\color{Orange} D\color{black}\text{ } + \text{ }\color{Orchid} B\color{Blue} C$$.

For example, the reaction between mercury (II) nitrate and diammonium sulfide in aqueous solutions to yield mercuric sulfide and ammonium nitrate is a type of double replacement reaction.

$$Hg(NO_{3})_{2} \text{ }(aq)\text{ + }(NH_{4})_{2}S\text{ }(aq) \text{ }\to HgS \text{ }(s) \text{ + } 2NH_{4}NO_{3} \text{ } (aq)$$

### Neutralization Reactions

Neutralization reactions are a special type of double replacement reactions in which one reactant is an acid and the other is a base, giving salt and water as products. The equation below is an example of a neutralization reaction.

$$HCl \text{ (acid) + }LiOH\text{ (base) } \to LiCl \text{ (salt)}+ H_{2}O \text{ (water)}$$

## Chemical Reaction of Photosynthesis

Now that you are aware of the different types of chemical reactions that can occur around you, let's take a look at the chemical reaction happens in photosynthesis.

Photosynthesis is the process of using solar energy to convert carbon dioxide (CO2) and H2O into carbohydrates (glucose) and oxygen (O2).

The process and chemical reaction of photosynthesis is shown in the figure below.

## Chemical Reaction Examples

Lastly, let's look at some examples involving chemical reactions. Let's start with the following reaction: $$\text{Zn + AgNO}_{3}\to \text{Ag + Zn(NO}_{3}\text{)}_{2}$$.

Can you guess which type of chemical reaction it is? Well, if you guessed that it is a decomposition reaction, you are right!

Now, let's amplify the difficulty a bit and solve a problem similar to what you might see in your exam.

What type of chemical reaction occurs between CoCl3 and Ba(OH)2?

$$2\text{ }COCl_{3}\text{ + } 3\text{ }Ba(OH)_{2}\text{ }\to\text{ }3\text{ }BaCl_{2} \text{ + }2\text{ }Co(OH)_{3}$$

1. Combustion
2. Single replacement
3. Double replacement
4. Decomposition
5. Synthesis

The chemical reaction above is an example of a double replacement reaction because cations and anions of the two reactants "switched places" to form two new compounds.

Now, I hope that you feel more confident in your understanding of chemical reactions!

## Chemical Reactions - Key takeaways

• A chemical reaction is a process that results in the interconversion of chemical species.
• Synthesis reactions are reactions in which two or more substances combine to form a new substance, whereas decomposition reactions are chemical reactions in which one substance gets converted into 2 or more substances.
• Redox reactions are reactions in which electrons are transferred from one species (oxidized) to another species (reduced).
• Combustion reactions are reactions involving a hydrocarbon reacting with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O).
• Single replacement reactions occur when one element in a compound is replaced by another element.
• Double replacement reactions occur when two compounds reacts in aqueous solutions, causing the cations and anions of the two reactants to "trade partners or switch places" to form two new compounds.

## References

1. AP ® Chemistry COURSE AND EXAM DESCRIPTION Effective Fall 2020. (n.d.). https://apcentral.collegeboard.org/pdf/ap-chemistry-course-and-exam-description.pdf
2. David, M., Howe, E., & Scott, S. (2015). Head-Start to A-level Chemistry. Cordination Group Publications (Cgp) Ltd.
3. Jespersen, N. D., & Kerrigan, P. (2021). AP chemistry premium 2022-2023. Kaplan, Inc., D/B/A Barron’s Educational Series.
4. Moore, J. T., & Langley, R. (2021). McGraw Hill : AP chemistry, 2022. Mcgraw-Hill Education.
5. Princeton Review (Firm. (2020). Fast track chemistry : essential review for AP, honors, and other advanced study. The Princeton Review.
6. Zumdahl, S. S., Zumdahl, S. A., & Decoste, D. J. (2019). Chemistry. Cengage Learning Asia Pte Ltd.

During a chemical reaction, different substances combine to form a new substance. When this happens, we see changes in color, gas formation, temperature changes, or even the formation of a precipitate.

Increasing temperature, concentration, and surface area will increase the rate of a chemical reaction.

A chemical reaction is when different chemicals are combined to produce a new substance.

Enzymes are catalysts that work by increasing the rate of a reaction. They achieve this by lowering the activation energy needed to start a reaction.

In a chemical reaction, reactants are the chemicals on the left side of the equation. Reactants will react with each other to form the product.

## Final Chemical Reactions Quiz

Question

A _______ is a process where different chemicals are combined to produce a new substance.

Chemical reaction

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True or false: chemical reactions consist of reactants and products.

True

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____ are the chemicals on the left side of the chemical equation.

Reactants

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____ are the chemicals on the right side of the equation.

Products

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True or false: The law of conservation of mass states that the mass of products is inversely proportional to the mass of the reactants.

False

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Question

A ________ is an equation where the number of moles of each element on the reactant side is the same as the number of moles on the product side.

Balanced chemical equation

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Question

The steps for balancing chemical equations include the all of the following except:

Write down the unbalanced chemical equation.

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When balancing a chemical equation, how can we find the number of moles of each element?

You multiply the coefficient by the number of elements given by the subscript.

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True or false:  When the number of moles of each element on both sides is equal, then the reaction is balanced.

True

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Types of Chemical Reactions

• Synthesis reactions (also called combination reactions)
• Decomposition reactions
• Single and double replacement reactions
• Combustion reactions

• Redox reactions
• Acid-base reactions (also called neutralization reactions)
• Precipitation reactions

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Question

Chemical reactions where two or more reactants produce a single product are called ______ reactions.

Synthesis

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______ reactions are chemical reactions where one reactant is broken down to form many products.

Decomposition

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______ are chemical reactions where the element by itself replaces one of the elements in the compound

Single replacement reactions

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_______ reactions are chemical reactions that occur in an aqueous solution. In this type of reaction, the cation (positive ions) and anion (negative ions) of one compound switch places with the cation and anion of the other compound to yield two different products.

Double replacement

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Question

Which type of reaction has the general formula:  A + O2 → CO2 + H2O

Combustion reaction

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True or false: combustion reactions are considered a type of redox reaction.

True

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In ______ reactions, there is a transfer of electrons that happens between the reactants involved in the reaction.

Redox

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OIL RIG means:

Oxidation Is Loss, Reduction Is Gain

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An element that loses electrons will be _____ , while an element that gains electrons will be _____.

Oxidized; Reduced

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Elements that are oxidized will have their oxidation numbers _____.

increased

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Elements that are reduced will have their oxidation numbers _____.

decreased

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What are oxidation numbers?

The specific charges on an element that tells whether an element is able to gain, lose or share electrons.

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The ______ is the element is reduced. The ______ is the element that is oxidized.

Oxidizing agent; Reducing agent

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True or false: When an acid and a base react together, they form salt and water as products: acid + base → water + salt.

True

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Precipitation reactions form a _____ as the product.

Precipitate (solid)

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A _____ is defined as an insoluble substance that separates itself from the solution.

Precipitate

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What is a physical property?

A physical property is a trait that can be seen and/or measured without changing the identity of the substance.

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What are intensive physical properties?

Physical properties that do not depend on the amount of substance

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What are extensive physical properties?

Physical properties that do depend on the amount of substance

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Which of the following are considered physical properties?

Shape, malleability, density, and mass

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What is a chemical property?

chemical property is the potential for that substance to undergo a chemical change. It describes how it would behave in certain scenarios.

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Which of the following is an example of a chemical property?

Iron can rust

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What is a physical change?

physical change is any change that does not change the identity of the substance. Physical changes can be reversed to regain some or all of the original material

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What is the difference between a homogeneous and a heterogeneous mixture?

A homogeneous mixture is uniform while a heterogeneous mixture is not. Both are a result of a physical change

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True or False: Salt dissolving in water is a physical change

True

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What is a chemical change?

chemical change is any change that does change the identity of the substance. This change may be reversible, but only through another chemical change, A chemical change can also be called a chemical reaction

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Which of the following are indicators of a chemical change?

All of the above

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True or false: A chemical change results in only chemical properties being changed

False

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True or False: Chemical changes are never reversible

False

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True or False: A substance bubbling always indicates a chemical change

False

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Which of the following is an example of a physical change?

Mixing oil and water

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What is Stoichiometry?

Stoichiometry is the ratio between products and reactants in a chemical reaction.

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What is yield?

The yield of a reaction is the amount of product made given a certain amount of reactant(s).

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What is a limiting reactant?

The limiting reactant is the reactant that is completely consumed in the reaction. Once this reactant is fully consumed, it stops the reaction and therefore limits the product made.

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What is the ideal gas equation and what does each variable represent?

PV=nRT

P=pressure

V=volume

n=moles

R=ideal gas constant

T=temperature

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True or False: Experimental yield is calculated using the balanced chemical equation

False

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What are Stoichiometric coefficients?

Stoichiometric coefficients are the numbers before an element/compound which indicate the number of moles present. They show the ratio between reactants and products. They are used to balance equations

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What is a precipitate reaction?

A special type of double replacement reaction where two aqueous compounds form a solid

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What must be true about a metal if it can participate in a single replacement reaction?

It is more reactive than the metal in the compound

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Question

True or False: Synthesis reactions produce one or more products

False

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