unit 1 - chemistry...unit 1 - chemistry 22 / 24 #3. concentration 1 concentration is how much solute...
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Unit 1 - ChemistryChapter 2 - Word Equations and Balancing
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2.1 - Chemical Equations
Physical and Chemical Changes
Physical change: A substance changes its physical appearance, but notits composition. Example: All changes of state.
Chemical change: A substance is transformed into a chemically differentsubstance. Example: The burning of hydrogen in air.
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2.1 - Chemical Equations
Physical and Chemical Changes
Physical change: A substance changes its physical appearance, but notits composition. Example: All changes of state.
Chemical change: A substance is transformed into a chemically differentsubstance. Example: The burning of hydrogen in air.
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Indicators of a Chemical Reaction
Chemical change:
Heat and/or light is given off
A new colour appears
A gas is produced
A solid precipitate (ppt) is formed
Watch: Nitric acid + copper Watch: Lead nitrate + potassium iodide
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Form of a Chemical Reaction
How can we describe a wide range of reactions?
Chemical Equations:
Reactants −−→ Products
We further separate both the reactants and the products by a plus sign:
Reactant 1 + Reactant 2 −−→ Product 1 + Product 2
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Examples of word equations:
iron + oxygen −−→ iron(III) oxide
copper + silver nitrate −−→ silver + copper(II) nitrate
Examples of skeleton equations:
Zn + HCl −−→ H2 + ZnCl2
H2 + O2heat−−→ H2O or H2 + O2
4−→ H2O
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Examples: Write skeleton equations for the following reactions:
1 AgNO3 and NaCl react to form AgCl and NaNO3
2 potassium sulphate reacts with barium nitrate to form potassiumnitrate and barium sulphate
3 magnesium + oxygen −−→ magnesium oxide
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Practice: Write skeleton equations for the following reactions:
1 sodium + chlorine −−→ sodium chloride
2 carbon tetrahydride + oxygen −−→ carbon dioxide + water
3 lithium + iron (III) chloride −−→ iron + lithium chloride
4 barium chloride + potassium sulfate −−→ barium sulfate +potassium chloride
5 calcium chloride + potassium phosphate −−→ potassium chloride +calcium phosphate
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The Law of Conservation of Mass
In a chemical reaction, the total mass of the reactants is always equal tothe total mass of the products.
What does this imply?
In chemical reactions, the atoms of the reactants are simply rearranged to formthe products.
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The Law of Conservation of Mass
In a chemical reaction, the total mass of the reactants is always equal tothe total mass of the products.
What does this imply?
In chemical reactions, the atoms of the reactants are simply rearranged to formthe products.
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2.2 - Balancing Chemical Equations
What’s incorrect with the chemical equation below?
Fe + Cl2 −−→ FeCl3
Problem! - The Law of Conservation of Mass states that the mass of thereactants equals the mass of the products.
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2.2 - Balancing Chemical Equations
What’s incorrect with the chemical equation below?
Fe + Cl2 −−→ FeCl3
Problem! - The Law of Conservation of Mass states that the mass of thereactants equals the mass of the products.
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We cannot change the subscripts of the formulas to fix this imbalance, asthis completely changes the compounds. Instead, we must change thenumbers of molecules rather than their formulas.
Fe + Fe + Cl2 + Cl2 + Cl2 −−→ FeCl3 + FeCl3
2Fe + 3Cl2 −−→ 2FeCl3
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We cannot change the subscripts of the formulas to fix this imbalance, asthis completely changes the compounds. Instead, we must change thenumbers of molecules rather than their formulas.
Fe + Fe + Cl2 + Cl2 + Cl2 −−→ FeCl3 + FeCl3
2Fe + 3Cl2 −−→ 2FeCl3
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Steps to balancing chemical equations:
1 Balance metal elements
2 Balance non-metal elements excluding hydrogen and oxygen
3 Balance hydrogen
4 Balance oxygen
5 Reduce coefficients if possible
Tip! - If any polyatomics are present balance them as one unit. This onlyonly work if it occurs on both sides of the equation.
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Balance the following equations:
1 Al + Br2 −−→ AlBr3
2 N2 + H2 −−→ NH3
3 N2H4 + O2 −−→ H2O + N2
4 Na + H2O −−→ NaOH + H2
5 Ca + HNO3 −−→ H2 + Ca(NO3)2
6 Fe(NO3)3 + (NH4)2CO3 −−→ Fe2(CO3)3 + NH4NO3
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2.3 - Types of Chemical Reactions
We shall investigate five types of chemical reactions:
1 Combustion
2 Synthesis
3 Decomposition
4 Single Displacement
5 Double Displacement
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Combustions Reactions
A combustion reaction is a reaction of the form:
hydrocarbon + oxygen −−→ carbon dioxide + water
For example:
2 C4H10 + 13 O2 −−→ 8 CO2 + 10 H2O
Demonstration: CH4 + 2 O2 −−→ CO2 + 2 H2O
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Synthesis (Combination) Reactions
Synthesis reactions involve the combination of smaller atoms and/ormolecules into larger molecules.
Simple synthesis reactions have the following general formula:
Examples:
Na + Cl2 −−→ NaCl
Mg + O2 −−→ MgO
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Decomposition Reactions
Decomposition reactions involve the splitting of a large molecule intoelements or smaller molecules.
Simple decomposition reactions have the following general formula:
Examples:
H2O −−→ H2 + O2
Li2O −−→ Li + O2
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Single Displacement Reactions
Single displacement reactions are chemical changes where one elementdisplaces or replaces another element from a compound. They have thefollowing general formula:
Note: Cations replace cations and anions replace anions!
Demonstration: CuCl2 + Al −−→ Cu + AlCl3
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Double Displacement Reactions
Double displacement reactions occur when elements in differentcompounds displace each other or exchange places. They have thefollowing general formula:
Note: Cations are always written first!
Demonstration: CuCl2 + Na3PO4 −−→ Cu3(PO4)2 + NaCl
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Question 1
1 Classify each of the following reactions as a combustion, synthesis,decomposition, single or double displacement reaction.
2 Balance the chemical equation.
1) Mg + S8 −−→ MgS
2) NaN3 −−→ Na + N2
3) AgNO3 + Ni −−→ Ni(NO3)2 + Ag
4) C2H6 + O2 −−→ CO2 + H2O
5) AgNO3 + NaCl −−→ NaNO3 + AgCl
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2.4 - Rates of Reaction
All reactions take place at different speeds. The speed at which a reactionoccurs is called its rate of reaction.
Collision Theory
The rate of a chemical reaction is affected by the number of collisions ofreactant molecules.
There are four factors that affect the rate of a reaction.
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#1. Temperature
1 As temperature increase the average speed of the molecules increases.
2 An increase in temperature makes the molecules collide more oftenand more effectively.
Example - Alka seltzer tablet in hot water versus in cold water. Whichreaction will finish first?
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#2. Surface Area
1 Surface area is the amount of area of a sample of matter that isvisible and able to react.
2 Increasing the surface area increases the number of collisions betweenmolecules and therefore the rate of reaction.
Example - A crushed up alka seltzer tablet versus a whole alka seltzertablet in water. Which reaction will finish first?
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#2. Surface Area
1 Surface area is the amount of area of a sample of matter that isvisible and able to react.
2 Increasing the surface area increases the number of collisions betweenmolecules and therefore the rate of reaction.
Example - A crushed up alka seltzer tablet versus a whole alka seltzertablet in water. Which reaction will finish first?
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#3. Concentration
1 Concentration is how much solute is in a solvent.
2 An increase in concentration increases the rate of a reaction.
3 More molecules are packed into a smaller space so they are morelikely to collide.
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#4. Catalysts
1 A catalyst is a substance that decreases the rate of a chemicalreaction without being consumed by the reaction. It “facilitates” thereaction, but doesn’t form a product.
2 A catalyst increases the fraction of collisions that are effective.
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