Heat Energy and Chemical Equations Part 1: Changes in Matter

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Heat Energy and Chemical EquationsPart
1Changes in Matter EnergyBalancing
EquationsTypes of Reactions
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Physical vs Chemical Changes in Matter Energy

• Matter is constantly undergoing changes. These
  changes can be identified as physical or chemical
• Physical Changes is any change that does not
  result in new substances formed. The original
  substance is just in a new form.
• For Example
• Phase Changes and Dissolving
• H2O(s) H2O(l)
• NaCl(s) Na1(aq) Cl-1(aq)

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Changes in Matter (continued)

• Chemical Changes occur when substances collide
  and change into new substances.
• This occurs because the bonds of the substances
  you start with break and new bonds are formed
  resulting in new substances.
• When a chemical change occurs it is called a
  chemical reaction.
• For Example
• Burning of paper Rusting of a metals

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Chemical Equations

• A chemical equation can be used to show the
  changes that take place during a chemical or
  physical change.
• The substances that enter the reaction are
  written on the left side of the equation and
  called reactants.
• If the reaction involves more than one substance
  they will be separated by a sign.
• The new substances that are formed from the
  reaction are known as products and they are
  written on the right side of the equation.
• Separating the reactants and the products is an
  arrow and is read as yields or produces.
• For Example
• H2O(s) energy H2O(l)
• CH4 O2 CO2 2H2O energy

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Energy and Changes in Matter

• Energy is defined as the ability to do work and
  it is NOT matter
• Energy is measured in the unit Joules (J) See
  Table D
• Can be POTENTIAL or KINETIC

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Potential Energy

• Energy that is stored (i.e. in a chemical bond).
• Something has the potential to do some kind of
  work
• Example the child at the top of the slide has
  potential energy

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Kinetic Energy

• Energy of motion
• Example the child going down the slide now has
  kinetic energy

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Law of Conservation of Energy

• Energy, like matter, is neither created nor
  destroyed, rather it is converted.

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Endothermic and Exothermic

• Chemical and Physical changes always involve the
  loss and gain of energy.
• This energy is most often expressed or described
  as heat.
• Based on whether energy is absorbed or released
  you can classify energy changes as either
  endothermic or exothermic.

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Understanding Heat Flow

• Heat is defined as the energy that transfers from
  one object to another.
• Heat will always flow from warm ? cool.
• What will happen if the two objects are touching?

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Heat Energy vs. Temperature

• We can measure heat flow by identifying changes
  in temperature.
• When an objects temperature goes down it is
  losing heat when it goes up it is absorbing heat.
• Temperature is a measure of the average kinetic
  energy or speed of the particles in matter.

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Heat Energy and Changes in Matter

• In virtually all changes in matter, energy is
  released or absorbed.
• System vs. Surroundings
• The system is what ever is being observed the
  surroundings are everything around the system.
• In a chemical change we can measure changes of
  the surroundings to identify whether or not the
  reaction (the system) gained or lost energy.

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Examples
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Exothermic Processes (Changes)

• Exothermic processes RELEASE ENERGY (i.e.
  explosions).
• A good way to remember this is to associate EXO
  with OUT.
• They have a ?H value because heat is leaving the
  system.

• The surrounding temperature increases because the
  system or reaction in this loses energy
• Heat is a product and will be written on the
  right side of the equation.

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Endothermic Processes (Changes)

• Endothermic processes ABSORB ENERGY
• A good way to remember this is to associate
  ENDO with INSIDE.
• Has a ?H value because heating is entering the
  system.
• The surrounding temperature decreases because the
  system or reaction absorbs heat from the
  surroundings make it feel cold.
• Heat is a reactant written on the left side of
  the equation.

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Balancing Equations

• Based on the Law of Conservation of Mass
• In a closed system, the mass of the reactants
  must equal the mass of the products.
• Example formation of water
• H2 O2 ? H2O
• mass must be conserved

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• H2 O2 ? H2O
• 2g 32g 18g
• How do we get these masses to balance?
• 2H2 O2 ? 2H2O
• 4g 32g 36g

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2H2O
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Counting atoms when balancing

• The number of atoms of each element must be equal
  on each side of the reaction. Therefore the
  masses will be equivalent.
• 2H2 O2 ? 2H2O
• 4H 2O 4H 2O

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Rules to Balance Equations

• 1. Start with the element that is only found once
  on both sides.
• 2. Keep polyatomic ions together. Count as a
  unit if not broken up.
• 3. Coefficients must be smallest possible whole
  number.

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Examples

• K Cl2 ? KCl
• Al2 (SO4) 3 CaCl2 ? AlCl3 CaSO4
• H2O CO2 ? H2CO3

2
2
3
2
3
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Try These

• H2 I2 ? HI
• Na Cl2 ? NaCl
• Al O2 ? Al2O3

2
2
2
2
3
2
4
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Phases in equations

• You must write in the phase of each reactant and
  product.
• (s) Solid Mg (s) magnesium metal
• (l) Liquid Br2 (l) bromine liquid
• (g) Gas CH4 (g) methane gas
• (aq) aqueous NaBr (aq) sodium bromide dissolved
  in water

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Summary

• Chemical equations are written to show the
  chemical change from reactants to products.
• Endothermic Reactions absorb heat. Heat is a
  reactant.
• Exothermic reactions release heat. Heat is a
  product.
• Mass of reactants Mass of products
• Equations must be balanced so that the number of
  atoms of each element on each side of the
  equation are equal.

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Types of Chemical Reactions

• There are 4 main kinds of reactions that occur
  between elements and compounds.
• Each chemical reactions follow a specific pattern
  from reactants to products. If you recognize the
  pattern you will be able to identify the type of
  reaction.

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SYNTHESIS REACTIONS
Also called combination reactions. General
Equation Format A B ?
AB These reactions will only have ONE
product
Example 2H2 O2 ? 2H2O This reaction represents
the Synthesis of Water
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DECOMPOSITION REACTIONS
Also called analysis reactions. General Equation
Format AB ? A B You can
recognize this because it only has ONE reactant
Example CH4 ? C 2H2 This reaction is known as
the Decomposition of Methane
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SINGLE REPLACEMENT
Often yield such gases as H2 when it involves an
acid. AB C ? AC B Notice these
reactions have a compound and an element as a
reactant and product
Example Mg 2HCl ? MgCl2 H2
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DOUBLE REPLACEMENT

• Two elements in two different compounds switch or
  replace one another.
• Often occurs in aqueous solutions of ionic
  compounds.

AB CD ? AD CB These reactions will have
two compounds on the reactant and product
sides.
Example 3NaCl AlPO4 ? Na3PO4 AlCl3
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Summary- There are 4 Basic Types of Reactions

• Synthesis
• Decomposition
• Single replacement
• Double replacement

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Chemical EquationsPart 2

• Predicting Products of Reactions
• Writing Equations

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Predicting Products of Reactions

• How do we predict the products of a reaction?
• First you must know the types of reactions.
• What are the 4 types?

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Types of Reactions

• Synthesis 2 or more reactants 1 product
• Decomposition 1 reactant 2 or more
  products
• SRR element compound compound
  element
• DRR switch positive ions of reactants to form
  products

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Single Replacement Reactions

• In a single replacement reaction the lone element
  will replace the element in compound that has the
  same ionic charge.
• Zn HCl gtgtgtgt Zn forms a positive ion so it
  will replaceH the positive ion in HCl.
• Br2 HCl gtgtgtgtgt Br forms a negative ion so it
  will replace Cl the negative ion in HCl.
• Do they always occur?
• No!!!!!!!!!!!
• You can predict if a SRR occurs by studying the
  elements that are switching.

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SRR ExampleZn 2HCl ? ZnCl2 H2

• If the element lone element is more reactive
  (active) than the element it would replace then
  the reaction will occur (spontaneous reaction
  occurs) if the element is less reactive then the
  reaction will not occur (nonspontaneous does
  not occur)
• In the reaction above Zinc is more reactive so
  the Zinc will replace the Hydrogen in the
  compound to form Zinc Chloride.
• The Hydrogen would leave the reaction as a gas.

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How do we know if one element is more reactive
than another?

• Look at Table J on the reference tables.
• Table J is an Activity Series.
• This lists the reactivity of metals and
  non-metals.
• NOTE Ag and Au are the least reactive metals.
  F2 is the most reactive non-metal.

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How do we use the Activity Series?

• In a Single Replacement Reaction if the element
  by itself is more reactive than the element it
  would replace than the reaction is spontaneous.
• Ex Zn HCl ? ZnCl H will occur

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If a reaction occurs

• Switch and write the products.
• Balance.
• Zn HCl ? ZnCl2 H2

2
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What if the reaction does not occur?

• Write no reaction.
• No need to balance
• Example
• Mg Li NO3 ? no reaction

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Try these

• Mg Zn(NO3)2 ?
• Mg AgNO3 ?
• Cl2 NaBr ?
• Sn NaNO3 ?
• Br2 KF ?
• Cu AgNO3 ?
• MgCl2 Na ?

Mg(NO3)2 Zn
Mg(NO3)2 Ag
2
2
2
NaCl Br2
2
No reaction
No reaction
CuNO3 Ag
NaCl Mg
2
2
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Double Replacement Reactions

• All compounds are ionic.
• Products are formed by switching the positive
  ions of the reactants.
• To write products write the ions and use the
  criss-cross rule.

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Ionic Compounds in Water

• Break up into ions
• Example 1 NaCl in water becomes Na Cl-
• Example 2 K3PO4 in water becomes K and PO4-3

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How do we know if a DDR occurs?

• DDR go to completion or occur if a solid,
  liquid, or gas forms
• (s) useTable F
• (l) H2O
• (g) H2, Cl2, etc.

If all products are (aq) then no reaction
happens.
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Using Table F to Predict a Solid

• DDR occurs if one product formed is insoluble
  (solid that precipitates out of solution).
• Check Table F for Solubility Guidelines.
• Soluble no precipitate, no rxn (aq)
• Insoluble precipitate, rxn occurs (s)
• If both products are soluble (aq) then reaction
  does not occur.

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DDR ExampleNa2CO3 (aq) Ca(OH)2 (aq) ?

• Write the ions above each reactant
• Switch the positive ions and write new ion pairs
  on product side (dont forget ion first
• Write the formulas for the products using the
  criss-cross rule.
• Check products with Table F guidelines.
• NaOH is soluble so it is labeled (aq) it
  dissolves. (see Group 1 ions)
• CaCO3 is insoluble (s) it will not dissolve
  forms a percipitate. (see carbonate)
• 5) Balance the equation

Ca2 CO3-2 Na1 OH-1
Na1 CO3-2 Ca2 OH-1
2
Na2CO3 (aq) Ca(OH)2 (aq) ?
CaCO3 (s) NaOH (aq)
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Try theseWrite products and predict if it goes
to completion

• NH41 CO3-2
  Ca2 Cl-1
• (NH4)2CO3 (aq) CaCl2(aq) ?
• K1 NO3-1 Ca2 I-1
• KNO3 (aq) CaI2 (aq) ?
• Na1 OH-1 H1 SO4-2
• NaOH(aq) H2SO4 (aq) ?
• Cu2 SO4-2 Na1 OH-1
• CuSO4 (aq) NaOH (aq) ?

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Writing the Products

• NH41 CO3-2 Ca2 Cl-1
• (NH4)2CO3 (aq) CaCl2(aq) ?
• K1 NO3-1 Ca2 I-1
• KNO3 (aq) CaI2 (aq) ?
• Na1 OH-1 H1 SO4-2
• NaOH(aq) H2SO4 (aq) ?
• Cu2 SO4-2 Na1 OH-1
• CuSO4 (aq) NaOH (aq) ?

NH41 Cl-1 Ca2 CO3-2
NH4Cl (aq) CaCO3(s)
K1 I-1 Ca2 NO3-1
KI (aq) Ca(NO3)2 (aq)
Na1 SO4-2 H1 OH-1
Na2SO4 (aq) H2O (l)
Cu2 OH-1 Na1 SO4-2
Cu(OH)2 (s) Na2(SO4) (aq)
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Summary

• Use Activity Series (Table J) to predict the
  products of single replacement reactions.
• Use Solubility Guidelines (Table F) to predict
  the products of double replacement reactions.

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Predicting Missing Reactant or Product

• Based on law of conservation of mass matter can
  neither be created or destroyed
• When given a balanced equation you should be able
  to determine the formula of missing substance
• Count the atoms on both sides, subtract the atoms
  on the missing formula side from the side with
  the known formulas
• Any missing element must be present in the
  unknown formula

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2Na 2H2O ? x 2NaOH

• Na 2 Na 2
• H 4 H 2
• O 2 O 2
• Whats missing?
• 2 Hydrogens
• X must be H2

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Try These

• Multiple Choice Questions
• 2Na 2H2O ? 2x H2
• 1) NaO
• 2) NaOH
• 3) Na2O
• 4) Na2OH
• 4Fe 3O2 ? 2X
• 1) FeO
• 2) Fe2O3
• 3) Fe3O2
• 4) Fe3O4