Chapter 4Chemical Reactions

4.1 Intro to Chemical Reactions

Chemical ReactionsAre the cause of chemical changes

Remember a chemical change causes one substance to change into another!

Physical vs. Chemical Change Review

Physical change changes things like shape & size

Also physical changes… Stirring Dissolving Drying Separating

Remember physical changes are REVERSIBLE

Chemical Change changes the actual structure of the molecule turning it into a completely different substance

Chemical Changes are IRREVERSIBLE- they cannot be undone

If they can be undone the substance has to go through another chemical change to do so

Physical vs. Chemical Change Review

Test your memory…What are the 4 signs of a chemical change?

Chemical Bonds and Intermolecular Forces

The molecules in the substance are held together by much weaker forces called Intermolecular Forces

The atoms in a molecule are held together by super strong forces called Chemical Bonds

Energy RequirementsBoth physical and chemical changes require energy

A Physical change only breaks intermolecular forces, while a chemical change has to break the chemical bonds

Which kind of change do you think requires more energy?

Chemical BondsWe just learned that molecules are created and held together by chemical bonds

Chemical bonds form when the electrons from one atom are either SHARED or TRANSFERRED with another atom

Reminder: Electrons are the negatively charged particles that orbit the nucleus

Chemical BondsThere are 2 types of chemical bonds: Ionic and Covalent

Ionic Bonds: Electrons are transferred from a metal to a non-metal

Covalent Bonds: Electrons are shared between two non-metals

Draw a “stairs” on your periodic tableThe elements to the left = metalsThe elements to the right = non-metals

PracticeDecide whether the element is a metal or non-metal

Na Au P

O Ne Cu

Mg H Pb

PracticeIs the molecule held together by an ionic bond or a covalent bond?

CH4 MgO

KBr CuCl2

HI NH3

ReactivityHave you noticed I use a lot of the same elements again and again? This is because certain elements are more reactive than others!

4.2 Understanding and Balancing Chemical Equations

Chemical EquationsChemical equations are written to show the chemical reaction

3 major parts to a chemical equation: Reactant Product Yield Arrow

Chemical Equations

Starting materials, always

on the left

Products

Yield Arrow shows the direction of the reaction, means “reacts to form”

NaCl + AgNO3 → NaNO3 + AgClReactants End results,

always on the right

Balancing EquationsWhen you have a chemical equation you need to make sure it is balanced

Law of Conservation of Mass: Mass cannot be created or destroyed

So the total mass and the number of atoms of each element must be the same on both sides of the equation!

Balancing EquationsWe balance equations by adding a coefficient in front of a molecule to change the number of atoms in the equation

We NEVER change the subscripts, this changes the molecule into something totally different! subscrip

t

coefficient

CoefficientsCoefficients get distributed throughout the entire molecule 4 H’s and 2 O’s

The coefficient also tells you the number of moles of the molecule it is in front of 2 moles of H2O

How to balance equationsThere is no “right” way to balance an equation but most people find this general process the easiest way to do so

Most of balancing equations is just trial and error

1) Start out listing the number of atoms of each element in the equation

2) Look for any unbalanced atoms and add a coefficient in front of the molecule that would balance that atom

3) Repeat step 2 until you have a balanced equation

Step One: Reactants Products 2 H 2

2 O 1 (x2)

Step Two: H2 + O2 2H2OReactants Products (x2) 2 H 4

2 O 2

Step Three: 2H2 + O2 2H2O

Balancing Equations Example: H2 + O2 H2O

Try it on your own!Zn + HCl → ZnCl2 + H2

Step One: Reactants Products Zn H

Cl

Step Two: Zn + HCl → ZnCl2 + H2Reactants Products

Zn H

Cl

One more…CH4 + O2 → CO2 + H2O

Conservation of EnergyWe already know that matter cannot be created or destroyed, the same is true for energy

Conservation of Energy: Energy cannot be created or destroyed, it can only be transferred or transformed

Energy and Chemical Reactions

In a reaction the chemical bonds are broken and then new bonds are formed.

Because of this reactions either give off or gain energy!

Energy usually = heat but can also mean light or sound

Endothermic &Exothermic ReactionsEndothermic Reactions: Need to absorb energy to go from reactants to products- Products have higher energy than the reactants- Takes more energy to form new bonds then to

break the original bonds

Exothermic Reactions: Releases energy to go from reactants to products- Products have lower energy than the reactants- Takes more energy to break the bonds then to

form new ones

Conservation of EnergyWhat happens to energy in a chemical reaction

The reaction = the “system” The room = the “surroundings”

1. The energy of the system + surroundings stays the same.2. Energy gained by the system must be lost by the surroundings.3. Energy lost by the system must be gained by the surroundings.

Exothermic ReactionsExothermic reactions give off energy, so why don’t they happen spontaneously?

Even though they give off a lot of energy, they require a small amount of energy to be added to the system so the reaction can start

After this small amount of energy is added then a HUGE amount of energy is releasedThis small amount of starter energy is called activation energy

4.3 Types of Chemical Reactions

SolutionsRemember that in a solution the solute is dissolved in the solvent

When the solute dissolves the molecule completely breaks down and freely floats around

Reactions can only occur if molecules can move around and touch each other so MANY reactions require the molecules to be dissolved in a solution

Aqueous SolutionsAny solute dissolved in water is called an aqueous solution

This is so important and so common that we consider being dissolved in water to be like a fourth phase (solid, liquid, gas, aqueous)

Writing phase of matter in chemical reactions

When we write a chemical equation we need to include the phase of each reactant/product in the equation

We use the symbols (s), (l), (g), and (aq) to showwhat phase the reactants and products are in:(s) = solid.(l) = liquid.(g) = gas.(aq) = dissolved in water.

Precipitate ReactionsBaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq) 

When two aqueous substances react and form a solid substance a precipitate reaction has formed

A precipitate forms when a molecule cannot dissolve in water

A precipitate reaction is a type of a double replacement reaction

Synthesis and DecompositionSynthesis- Two or more molecules coming together to form one new molecule

H2 + O2 H2O

Decomposition- One molecule breaks down to form two or more new molecules

H2CO3  →   H2O + CO

Replacement ReactionsSingle Replacement: One element trades places with another element in a molecule.

Example:  Zn + HCl → ZnCl2 + H2

Double Replacement: This is when the two parts of two molecules switch places, forming two completely new molecules

Example: NaCl + AgNO3 → NaNO3 + AgCl

Acid/Base Reaction

Acid/Base ReactionsAn acid/base reaction will ALWAYS produce water an a salt

Salt is NOT only the salt we use on food, it is any substance formed from the positive ion from a base and the negative ion from an acid

HBr + NaOH → NaBr + H2O

Combustion ReactionReactants: Hydrocarbon (only C’s and H’s) and Oxygen

react and always form

Products: Water (H2O) and Carbon Dioxide (CO2)CH4 + O2 → CO2 + H2O

PracticeMatch the chemical reaction to its type of reaction

a. Acid/Base NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)

b. Combustion Mg + 2H2O → Mg(OH)2 + H2

c. Decomposition 8 Fe + S8  → 8 FeSd. Double Replacement 2C2H6 + 7O2 → 4CO2 + 6H2Oe. Precipitation 2 H2O → 2 H2 + O2

f. Single Replacement MgO + 2KCl → K2O + MgCl2g. Synthesis HBr + NaOH → NaBr + H2O