CHEMICAL REACTIONSChapter 21

• Chemical reactions are taking place all around you and even within you.

• A chemical reaction is a change in which one or more substances are converted into new substances.

Describing Chemical Reactions

A chemical reaction occurs when you bake a cake.

Physical Change

Change that alters the form of a substance but not the composition Tearing of paper Chopping wood Melting ice Change of state Dissolving

Chemical Changes

Result of a chemical reaction Breaking of bonds and formation of new

bonds Atoms are rearranged Energy is released or absorbed

Examples of Chemical Reactions

Combustion – substance and oxygen are combined and release heat or light

Metabolism – chemical change that releases energy through digestion

Fermentation – break down of carbohydrates by living things in the absence of oxygen

Corrosion- Rusting Electrolysis

Signs of a Chemical Reaction

Rapid release of energy– heat, light, sound

Production of a gas– bubbles Forming a precipitate– a solid that

doesn’t dissolve in liquid Substance getting colder Examples: Fire, antacid tablet in water,

rusting, silver tarnish

Exothermic/Endothermic More energy is

produced than it takes to break the bonds of the reactants

Energy is released from the reaction

Gives off heat and/or light

Fire MRE’s – wrapped in

special sleeve with Mg – addition of water starts reaction that heats food

Takes more energy to break bonds in the reactants than is released by forming products

Energy has to be put in for the reaction to occur

Gets colder Instant ice pack – water

and ammonium nitrate Rusting iron

Conservation of Mass

• For example, the mass of the candles and oxygen before burning is exactly equal to the mass of the remaining candle and gaseous products.

• The French chemist Antoine Lavoisier established that the total mass of the products always equals the total mass of the reactants.

• One of the questions that motivated Lavoisier was the mystery of exactly what happened when substances changed form.

Lavoisier's Contribution

• He began to answer this question by experimenting with mercury.

• Lavoisier placed a carefully measured mass of solid mercury (II) oxide, which he knew as mercury calx, into a sealed container.

Lavoisier's Contribution

• When he heated this container, he noted a dramatic change.

• The red powder had been transformed into a silvery liquid that he recognized as mercury metal, and a gas was produced.

• When he determined the mass of the liquid mercury and gas, their combined masses were exactly the same as the mass of the red powder he had started with.

Lavoisier's Contribution

• Lavoisier also established that the gas produced by heating mercury(II) oxide, which we call oxygen, was a component of air.

Conservation of mass

the mass of the reactants will always equal the mass of the products in a closed system(you have to “catch all of the products”

2H2 + O2 = 2H2O H – 1.01 amu O – 16 amu Reactants Products 4H – 4.04 2H2O – 36.04 2O – 32 amu Total – 36.04 amu

Conservation of atoms

the number of atoms in the reactants will always equal the number of atoms in products in a closed system

2H2 + O2 = 2H2O Reactants Products 4H 4H 2O 2O

• Scientists have developed a shorthand method to describe chemical reactions.

Writing Equations

• A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols.

• “Recipe” for chemical reactions • Bonds are breaking and reforming• CH4 + O2 → CO2 + H2O + heat + light

• Methane and oxygen yield carbon dioxide and water

• One or more substance (reactants) change into one or more new substances (products)

• Reactants to the left of the arrow

• Products to the right of the arrow

• Arrow is yields• Reactants yield the

products• Some of the symbols used

in chemical equations are listed in the table.

• Example: (aq) stands for aqueous, which means dissolved in water

Writing Equations

Balanced Equations

• Lavoisier's mercury(II) oxide reaction can be written as:

• Notice that the number of mercury atoms is the same on both sides of the equation but that the number of oxygen atoms is not the same.

Balanced Equations

Start with a skeleton equation like Lavoisier’s

Each side of the equation must have the same number of atoms of each element

We use coefficients to balance equations – (whole number in front of the chemical formula)

H2 + O2 → H2O 2H2 + O2 → 2H2O

Balancing

Two things you CANNOT do when balancing an equation

You cannot change a subscript

You can only write the coefficient in front of a chemical formula

Limiting and Excess Reactants

Limiting Reactant – the reactant that is used up first limits the amount of product

Excess Reactant – the reactant that is not completely used up because there is extra

If I have 1 cup flour and 2 eggs but only need 1 egg for my recipe – the flour is the limiting reactant and the egg is the excess reactant

Inhibitors and Catalysts

Catalysts speed up reactions. They do not change what goes in or comes out, but they make the reactions happen faster- example- enzymes

Catalysts are often affected by other factors such as pH and temperature

Inhibitors slow down reactions– such as food preservatives that help keep food from spoiling too quickly

Types of reactions

Combustion Synthesis (Addition) Decomposition Single Displacement (Single

Replacement) Double Displacement (Double

Replacement)

Combustion

A chemical reaction in which something is burned

Lavoisier was one of the first to observe this type of reactions

A combustion reaction occurs when a substance reacts with oxygen to produce heat and/or light

C6H12O6 + 6O2 6CO2 + 6H20 Products are always the same

Synthesis Reactions

Also called addition Two or more substances combine to form

another substance General Formula: A + B AB Example: 2H2 + O2 2H20

Decomposition

A substance breaks down, or decomposes into its components

General Formula: AB A + B Example: 2H20 + 2H2 + O2

Single Displacement

Also called single replacement One atom displaces, or replaces another

to form a new substance General Formula: A + BC AC + B Example:

Cu (s) + 2AgNO3 (aq) Cu (NO3)2 + 2Ag (s)

Double Displacement

Also called double replacement The positive ion of two compounds switch

with each other, making two entirely new compounds

General Formula: AB + CD AD + CB Example:

Ba (NO3)2 (aq) + K2SO4 (aq) BaSO4 (s) + 2KNO3 (aq)