2A2 Chemical Reactions

Question 1

What is a physical change?

Answer 1

A change in form or appearance without altering chemical composition.

Physical changes involve changes in a substance’s form or state, but the substance’s chemical structure remains intact.

Question 2

List three examples of physical changes.

Answer 2

1. Tearing paper
2. Dissolving sugar in water
3. Freezing water

Question 3

What is a chemical change?

Answer 3

A process where a substance transforms into a new substance.

Chemical changes result in the formation of new substances with different properties.

Question 4

List three signs of a chemical change.

Answer 4

1. Color change
2. Temperature change
3. Gas production (bubbling or odor)

Question 5

Why is boiling water considered a physical change?

Answer 5

It changes state but not composition.

Boiling involves a phase transition where the substance retains its chemical identity and can often be reversed.

Question 6

Why is burning a candle considered a chemical change?

Answer 6

The wax reacts with oxygen to form new substances.

The combustion of wax changes its chemical structure, producing carbon dioxide, water, and other byproducts.

Question 7

What is a chemical reaction?

Answer 7

A process that forms new substances.

Chemical reactions involve the breaking and forming of bonds, transforming reactants into products.

For example, combustion, synthesis, and decomposition reactions.

Question 8

Define:

Reactant

Answer 8

A substance that undergoes change in a chemical reaction.

Reactants are the initial substances in a chemical reaction that interact and transform into products.

Question 9

What are the substances formed in a chemical reaction?

Answer 9

Products

Products result from the transformation of reactants and have different properties from the original substances

Question 10

True or False:

Cutting a piece of paper is a chemical change.

Answer 10

False

Cutting paper only alters its size or shape, without affecting its chemical composition.

Question 11

Fill in the blank:

Melting ice is an example of a _______ change.

Answer 11

physical

Melting changes the state of matter from solid to liquid, but the chemical structure of water remains the same.

Question 12

Why is rusting of iron a chemical change?

Answer 12

Iron reacts with oxygen and water to form iron oxide (rust).

Rusting is an oxidation reaction where the iron’s chemical composition is permanently altered.

Question 13

Define:

phase change

Answer 13

A physical change in the state of matter.

A phase change occurs when a substance transitions between solid, liquid, or gas while retaining its chemical identity.

Question 14

What is an example of a chemical change involving a temperature change?

Answer 14

The combustion of wood.

Combustion produces heat, light, and new products like carbon dioxide, signifying a chemical change.

Question 15

Why is a physical change reversible?

Answer 15

The substance retains its original chemical composition.

Reversible physical changes, like melting or freezing, do not result in new substances.

Question 16

What is a chemical equation?

Answer 16

A representation of a chemical reaction using symbols and formulas.

Chemical equations show the reactants, products, and their relationships in terms of atomic and molecular formulas.

Question 17

Fill in the blanks:

In a chemical equation, the reactants are on the _____ side, and the products are on the _____ side.

Answer 17

left, right

Reactants are substances that undergo chemical changes, while products are the new substances formed after the reaction.

Question 18

True or False:

In a chemical equation, the number of atoms of each element is conserved.

Answer 18

True

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction, so the number of atoms of each element must remain the same before and after the reaction.

Question 19

What is the general format for writing a chemical equation?

Answer 19

Reactants → Products

Reactants are written on the left side of the equation, and products are written on the right side, separated by an arrow indicating the direction of the reaction.

Question 20

True or False:

The number of atoms can change when balancing a chemical equation.

Answer 20

False

Balancing an equation involves adjusting coefficients, not changing subscripts in chemical formulas, to ensure the same number of atoms on both sides.

Question 21

Fill in the blank:

In a balanced chemical equation, the total mass of the _______ equals the total mass of the ________.

Answer 21

reactants, products

This reflects the principle of the conservation of mass, which states that mass is neither created nor destroyed during a chemical reaction.

Question 22

What is a combustion reaction?

Answer 22

A reaction with oxygen that releases energy.

Combustion produces heat, light, carbon dioxide, and water. For example, the burning of methane: CH₄ + 2O₂ → CO₂ + 2H₂O.

Question 23

What is a single replacement reaction?

Answer 23

A reaction where one element replaces another element in a compound.

An example is Zn + 2HCl → ZnCl₂ + H₂, where zinc replaces hydrogen in hydrochloric acid.

Question 24

What is a coefficient in a chemical equation?

Answer 24

A number placed in front of a chemical formula to balance the equation.

Coefficients show the relative amounts of reactants and products involved in the reaction.

For example, in 2H₂ + O₂ → 2H₂O, the coefficient 2 shows there are two molecules of hydrogen gas reacting to form water.

Question 25

# Fill in the blank: A combustion reaction always **involves** a/an \_\_\_\_\_\_ and oxygen.

Answer 25

fuel ## Footnote Fuels such as hydrocarbons, alcohols, or organic compounds undergo combustion with oxygen to release energy.

Question 26

# True or False: Acids **always** taste sour.

Answer 26

True ## Footnote **Acids** like lemon juice and vinegar have a sour taste due to the presence of *H⁺ ions*.

Question 27

What happens in an **acid-base** reaction?

Answer 27

The acid donates H⁺, and the base accepts H⁺ or releases OH⁻. ## Footnote This reaction results in the **neutralization** of the acid and base, producing new substances with different properties.

Question 28

What is a **neutralization** reaction?

Answer 28

A **reaction** between an acid and a base to produce water and a salt. ## Footnote For example, **HCl + NaOH → NaCl + H₂O**, where hydrochloric acid neutralizes sodium hydroxide.

Question 29

# True or False: In a neutralization reaction, the **products** formed are water and a salt.

Answer 29

True ## Footnote In neutralization, the hydrogen ions (H⁺) from the acid combine with hydroxide ions (OH⁻) from the base to form water.

Question 30

What is a **synthesis** reaction?

Answer 30

Two or more substances **combine** to form a more complex product. ## Footnote An example is 2H₂ + O₂ → 2H₂O, where hydrogen and oxygen combine to form water.

Question 31

# Fill in the blank: A **synthesis** reaction is also called a \_\_\_\_\_\_\_ reaction.

Answer 31

combination ## Footnote This is because two or more reactants **combine** to create a single product.

Question 32

# True or False: In a decomposition reaction, **energy** is needed to break bonds in the reactant.

Answer 32

True ## Footnote *Decomposition reactions* often require heat, light, or electricity to break the bonds of the reactant.

Question 33

What is the **general equation** for combustion?

Answer 33

Fuel + O₂ → CO₂ + H₂O + energy ## Footnote This equation *represents* the combustion of hydrocarbons, such as methane, that produce carbon dioxide, water, and energy when they react with oxygen.

Question 34

In the equation **2H₂ + O₂ → 2H₂O**, what is the coefficient of water (H₂O)?

Answer 34

2 ## Footnote The coefficient of 2 in front of H₂O indicates that two molecules of water are **produced** from the reaction of hydrogen and oxygen.

Question 35

What type of reaction is represented by the equation, **Fe + CuSO₄ → FeSO₄ + Cu**?

Answer 35

Single replacement reaction ## Footnote In this reaction, iron (Fe) replaces copper (Cu) in copper sulfate (CuSO₄), forming iron sulfate (FeSO₄) and copper.

Question 36

What does the **symbol** "**→**" in a chemical equation represent?

Answer 36

Produces or yields ## Footnote This arrow **separates** the reactants (on the left) from the products (on the right) in a chemical reaction.

Question 37

# Fill in the blank: In balancing equations, coefficients must be **written** as \_\_\_\_\_\_\_ numbers.

Answer 37

whole ## Footnote Coefficients must be **integers** (whole numbers) to maintain the proper ratio between reactants and products.

Question 38

In the equation **2H₂ + O₂ → 2H₂O**, what is the ratio of hydrogen (H₂) to oxygen (O₂)?

Answer 38

2:1 ## Footnote The coefficients show that two molecules of hydrogen react with one molecule of oxygen to form water.

Question 39

What does the **law of conservation of matter** state?

Answer 39

Matter **cannot be created or destroyed**; it can only be rearranged. ## Footnote This means every molecule in a reaction must be accounted for.

Question 40

# Define: balanced chemical equation

Answer 40

An equation with **equal atoms** on **both sides**. ## Footnote It represents the correct ratio of reactants and products, ensuring the *law of conservation of matter* is followed.

Question 41

What is a **word equation**?

Answer 41

An equation that uses **names of chemicals** instead of formulas. ## Footnote Example: glucose + oxygen → carbon dioxide + water. Converting a word equation to a formula equation is the first step in balancing.

Question 42

How do you **convert** a word chemical equation into a formula?

Answer 42

Use **chemical symbols** and **formulas**. ## Footnote This ensures each reactant and product is correctly represented using standard notation. For example, water is **H₂O**, and sodium chloride is **NaCl**.

Question 43

# Fill in the blank: The **number written before** a chemical in an equation is called a \_\_\_\_\_\_\_.

Answer 43

coefficient ## Footnote Coefficients indicate how many molecules or moles of a substance participate in the reaction.

Question 44

How are the **coefficients used in balancing** chemical equations?

Answer 44

They **adjust** the number of molecules. ## Footnote Coefficients ensure each element has the *same number of atoms* on both sides of the equation by multiplying the number of atoms in a molecule without changing its identity.

Question 45

List *five* **steps to balance** a chemical equation.

Answer 45

1. Change a word equation into a formula. 1. Balance single-molecule atoms one at a time. 1. Balance non-H/O atoms. 1. Balance H and O. 1. Count atoms to verify balance. ## Footnote This sequence minimizes trial and error and ensures accuracy in balancing.

Question 46

How do you **check** if a chemical equation is balanced?

Answer 46

Ensure **equal atoms** on both sides. ## Footnote This is done by counting the number of atoms for each element in the reactants and products to confirm they are the same.

Question 47

Which atoms are typically **balanced last** in chemical equations?

Answer 47

Hydrogen and oxygen atoms. ## Footnote These atoms are often present in *multiple compounds*, making them easier to balance after other atoms.

Question 48

# Fill in the blank: In the reaction of **sodium and water**, the balanced equation is 2Na + 2H₂O → \_\_\_\_\_\_\_.

Answer 48

2NaOH + H₂ ## Footnote This reaction balances sodium, hydrogen, and oxygen atoms to comply with the conservation law.

Question 49

# Fill in the blank: In the **potassium and carbon dioxide** reaction, the balanced equation is 2K + CO₂ → \_\_\_\_\_\_\_.

Answer 49

K₂CO₃ ## Footnote This balances potassium, carbon, and oxygen atoms in the reaction.

Question 50

# True or False: A balanced chemical equation **always uses whole number** coefficients.

Answer 50

True ## Footnote Whole numbers ensure the reaction represents physical reality since partial molecules do not exist.

Question 51

What **reaction** breaks a compound into simpler substances?

Answer 51

Decomposition ## Footnote This reaction involves a single compound **splitting** into *multiple products* due to heat, light, or electricity. For example, *calcium carbonate (CaCO₃)* decomposes into *calcium oxide (CaO)* and *carbon dioxide (CO₂)* when heated.

Question 52

What **reaction** occurs when an element replaces another?

Answer 52

Displacement ## Footnote This reaction involves a more reactive element taking the place of a less reactive one, forming new products. For example, when *zinc (Zn)* reacts with *copper sulfate (CuSO₄)*, zinc displaces **copper**, forming *zinc sulfate (ZnSO₄)* and *copper (Cu)*.

Question 53

What happens in an **exothermic reaction**?

Answer 53

Energy is **released** as reactants form products. ## Footnote This often results in heat, light, or sound production, such as in *combustion* or *respiration*.

Question 54

List **three examples** of exothermic reactions.

Answer 54

1. Combustion 1. Condensation of steam 1. Neutralization (acid-base reaction)

Question 55

What happens in an **endothermic reaction**?

Answer 55

Energy is **absorbed** to break bonds and form products. ## Footnote These reactions require continuous energy input, like *evaporation* or *cooking*.

Question 56

List **three examples** of endothermic reactions.

Answer 56

1. Melting ice 1. Boiling water 1. Photosynthesis

Question 57

What is the **minimum energy required** to start a chemical reaction called?

Answer 57

Activation energy ## Footnote *Activation energy* helps break reactant bonds so the reaction can proceed.

Question 58

# True or False: **Boiling water** is an endothermic process.

Answer 58

True ## Footnote Boiling *requires energy* to convert liquid water into steam.

Question 59

# Fill in the blank: In an exothermic reaction, energy **transfers** from \_\_\_\_\_\_\_ to the surroundings.

Answer 59

system ## Footnote The **system** (*reactants and products*) releases energy, heating the surroundings.

Question 60

What **type of reaction** occurs when wood burns?

Answer 60

Exothermic ## Footnote **Combustion** releases heat and light, making it a classic exothermic reaction.

Question 61

# True or False: **Photosynthesis** is an exothermic reaction.

Answer 61

False ## Footnote Photosynthesis is an **endothermic** process that absorbs sunlight to produce energy-storing molecules.