Regents Chemistry

Chemical Reactions: An Introduction

Physical vs. Chemical Properties

Substances are described by their physical and chemical propertiesPhysical properties describe how it looks, smells and feels. No change in composition occurs!

ex – color, odor, volume and state of matter

Chemical properties describe the substances ability to form new substances

ex – ability of wood to burn, metal to rust, food to digest

Examples of Properties

Classify each as a physical or chemical property

1. Boiling point of ethanol

2. The hardness of a diamond

3. Sugar ferments to form alcohol

4. A metal wire conducts electricity

Physical vs. Chemical Changes

A physical change is a change in one or more physical properties, but no change in the fundamental components that make up the substance. Most common are changes of state

A Chemical change is a change in fundamental components, a change into a new substance. Called reactions!

Examples

Classify each of the following as a physical or chemical change

1. Melting of ice into boiling of water

2. Electrolysis of water

3. Iron metal is melted

4. Tarnishing of silver

5. Breaking a rock into pieces

Worksheet

Regents Chemistry

Chemical Reactions

What’s a Chemical Reaction?

Chemical reactions are part of our everyday life

Natural gas is burned to heat houses

Steel on cars rusts

Plastic is made for all kinds of uses

Mini-hot packs are used to keepUs warm outdoors!

How Do We Know a Reaction Occurs?

Chemical reactions often give visible clues

Demossilver nitrate and potassium chloride

Methane gas and oxygen

Mini-heat packs

Evidence of a ReactionSome of the BIG clues.. 1. The color changes

Bunsen Burner 2. A solid forms

silver nitrate and potassium chloride 3. Bubbles form

zinc metal and HCl 4. Heat and/or a flame is produced, or heat

is absorbed Mini-heat packs

What’s involved in a reaction?

Chemical reactions have (2) parts The Reactants and the Products

Reactants are on the left side ofthe equation and shows what goesinto the reaction!

Products are on the right side ofthe equation and shows what comesout of the reaction!

K + H2O H2 + KOHExample

Reactants/Products Have Physical States!

We look at our previous example…

K + H2O H2 + KOH(s) (l) (g) (aq)

We place the abbreviation for the stateAfter each chemical formulaSymbol State

(s) solid(l) Liquid(g) gas(aq) aqueous (dissolved in water)

Writing Unbalanced Chemical Equations

We will start by writing basic chemical equations from word problems

Example: Solid carbon reacts with gaseous oxygen to form gaseous carbon dioxide

C(s) + O2(g) CO(g)

Reactants = solid carbon + gaseous oxygen

Products = gaseous carbon dioxide

Example #2

Solid zinc metal reacts with hydrochloric acid to produce bubbles of hydrogen gas and aqueous zinc chloride

Zn(s) + HCl(aq) H2(g) + ZnCl2(aq)

You must use the naming compound rulesTo figure out how to write the chemical formulas

Remember- some gases are always diatomicH2 , O2 , Cl2

worksheet

Exothermic and Endothermic Processes

Exothermic processes release energy Example: Burning of wood

Endothermic processes absorb energy Example: Cold packs!

Regents Chemistry

Balancing Chemical Equations

BALANCING CHEMICAL EQUATIONS

RECAPAll forward equations are written with reactants of the left an products on the right

Example: 2H2 + O2 2H2O

Coefficient means 2 moleculesof H2, 4 atomstotal

Subscript means2 atoms of H

4 atoms ofH in product

BALANCING CHEMICAL EQUATIONS

Atoms are neither created or destroyed… so all equations must be balanced!

RULES FOR BALANCING EQUATIONS

1. Cannot change subscripts

2. Can change coefficients

Examples – Balance this equation

Na(s) + H2O(l) NaOH(aq) + H2(g)

2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

Worksheet

BALANCING CHEMICAL EQUATIONS

Regents Chemistry

Writing and Balancing Equations

Regents Chemistry

Types of Reactions

Types of Reactions

Although we cannot classify all reactions into distinct categories…there are four major types of reactions you should know– Synthesis (combination) Reactions– Decomposition (analysis) Reactions– Single Replacement Reactions– Double Replacement Reactions

Synthesis (combination) Reactions

A reaction is classified as a synthesis reaction when two or more reactants combine and a single product is formed

B + G BG

4Fe(s) + 3O2(g) 2Fe2O3(s)

Decomposition (analysis) Reactions

Decomposition reactions are the reverse of combination reactions

Occur when one substance breaks down into simpler substances

BG B + G

CaCO3(s) CaO(s) + CO2(g)

Regents Chemistry

Single and Double ReplacementReactions

Single Replacement Reactions

Involves an element and a compound Occurs when a metal displaces the metal in

a compound

B2 + B1G B2G + B1

Not all metals will displace (react with) a metalin a compound..so how do we know if a reactionswill occur? …we use our Table J in our reference tables!

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

Predicting if a Reaction Will Occur

Table J (Activity Series) on p. 4 is arranged so that a metal listed on the table will react with the compound of a metal that is below it

For Example:Zn will react with a compound of copper

Zn(s) + Cu(NO3)2 Cu(s) + Zn(NO3)2(aq)

But because Cu is below Zn on the table, it will not react with compounds of Zn

Cu(s) + Zn(NO3)2 no reaction

So..

So..above it will bump it… below it will not!

The MORE ACTIVE will bump the LESS ACTIVE

There is one element on the table that is not a metal…Hydrogen ion (H+)!

All metals above hydrogen will react with acids to release hydrogen gas and produce a salt

Predicting if a Reaction Will Occur

Mg(s) + 2HCl(aq) H2(g) + MgCl2(aq)

In the 2nd column of Table J is a list of nonmetals A nonmetal will replace a less active nonmetal in a

compound according to the equation

Predicting if a Reaction Will Occur

G1 + BG2 BG1 + G2

For example..Fluorine is listed as the most active nonmetal, and it will replace chlorine, bromine and iodine from other binary compounds

F2(g) + 2NaCl(aq) Cl2(g) + 2NaF(aq)

Cl2(g) + NaF no reactionCl is below F so it will not react!

Practice Problems

Predict if reactions will occur and write the the products or NR for NO REACTION:

Ca(s) + AgNO3(aq)

Pb(s) + Al(NO3)3(aq)

Cr(s) + Pb(NO3)2(aq)

Co(s) + HCl(aq)

--> CaNO3(aq) + Ag(s)

--> NR

--> Cr(NO3)2(aq) + Pb(s)

--> CoCl2(aq) + H2(g)

Double Replacement Reactions

Double replacement reactions generally involve two soluble ionic compounds that react in solution to produce a precipitate, a gas, or a molecular compound such a water

Represented by the equation:

B1G1 + B2G2 B1G2 + B2G1

Predicting if a Reaction Will Occur in a

Double Replacement Reaction

Three general situations in which a double replacement reaction will occur

1. If a solid (precipitate) is formed– We look at Table F (p.2) in the reference tables

and check the solubility of the two compounds

Ex:AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

The reaction will occur because AgCl is insolublemeaning, it will not stay dissociated in solution

Soluble vs. Insoluble

Soluble means the substance will stay as ions in the solution (just floating around!)…no reaction will happen!

Insoluble means it will form a solid, gas or molecular compound (such as water) and not stay as ions…a reaction will occur!

Predicting a Reaction...

2. If a gas is formedNa2S(aq) + 2HCl(aq) H2S(g) + 2NaCl(aq)

If a molecular substance, such as H2O is formed

NaOH(aq) + HCl(aq) H2O(l) + NaCl(aq)

Practice Predicting..Will a Reaction Occur?

PbSO4(s) + NaNO3(aq)

Pb(NO3)2(aq) + Na2SO4(aq)

Ca(NO3)2(aq) + KCl(aq)

CaCl2(aq) + KNO3(aq) = NR

REGENTS CHEMISTRY

Determining Missing Mass in Equations

The Law of Conservation of Mass

The Law of Conservation of mass states that matter cannot be created nor destroyed

This is very useful when considering chemical reactions

WHY?

When given a balanced equation in which either the reactant or product is missing, we can determine the formula of the missing substance..this we have done!

Missing Formulas in Equations

K + ___________ = KCl

There must be a Cl in the reactantsbecause it appears in the products

Missing Mass in Equations

Just as the formula of a missing reactant or product can be determined, the mass of a missing substance can also be found!

TOTAL MASS BEFORE = TOTAL MASS AFTER

HOW???? USING SIMPLE MATH!

EXAMPLE

If 103.0 g of potassium chlorate is decomposed to form 62.7 g of potassium chloride and oxygen gas, how many grams of oxygen are formed?

2KClO3(aq) --> 2KCl(aq) + 3O2(g)

1. Find the total mass of the reactants2. The total mass of the reactants must equal the total mass of the products

EXAMPLE cont..

2KClO3(aq) --> 2KCl(aq) + 3O2(g)

mass of KCLO3 = mass of KCl + mass of O2

103.0g = 62.7g + mass O2

mass O2 = 103.0g - 62.7gmass O2 = 40.3g

Example #2

What mass of carbon dioxide will be produced if 144g of carbon react with 384 g oxygen gas?

C(s) + O2(g) --> CO2(g)

Mass C + mass O2 = mass CO2

144g C + 384g O2 --> mass CO2

528g = mass CO2worksheet

Unknown Reactants and Products

Just like we can find missing mass, we can also identify missing reactants or products in a formula

Remember 2 steps Whatever is on the left side must also be on the

right side..this tells you what us missing! Think about the type of formula..this will help

you complete the equation!