regents chemistry chemical reactions: an introduction
TRANSCRIPT
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
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!
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
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)
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
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
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!