Heat Energy and Chemical Equations

Part 1:

Changes in Matter & Energy

Balancing EquationsTypes of Reactions

Physical vs Chemical Changes in Matter & Energy

Matter is constantly undergoing changes. These changes can be identified as physical or chemicalPhysical Changes is any change that does not result in new substances formed. The original substance is just in a new form.

• For ExamplePhase Changes and DissolvingH2O(s) H2O(l)

NaCl(s) Na+1(aq) + Cl-1(aq)

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 ExampleBurning of paper & Rusting of a metals

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 ExampleH2O(s) + energy H2O(l)

CH4 + O2 CO2 + 2H2O + energy

Energy and Changes in Matter

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

Potential EnergyEnergy that is stored (i.e. in a chemical bond).Something has the “potential” to do some kind of workExample: the child at the top of the slide has potential energy

Kinetic Energy

Energy of motionExample: the child going down the slide now has kinetic energy

Law of Conservation of Energy

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

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.

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?

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.

Heat Energy and Changes in Matter

In virtually all changes in matter, energy is released or absorbed.System vs. SurroundingsThe 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.

Examples

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 –ΔH value because heat is leaving the system.The surrounding temperature increases because the system or reaction in this loses energyHeat is a product and will be written on the right side of the equation.

Endothermic Processes (Changes)

Endothermic processes ABSORB ENERGYA good way to remember this is to associate “ENDO” with “INSIDE”.Has a +ΔH 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.

Balancing Equations

Based on the Law of Conservation of MassIn 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

H2 + O2 H2O

2g + 32g = 18g

How do we get these masses to balance?

2H2 + O2 2H2O

4g + 32g = 36g

2H2O

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

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.

Examples:

K + Cl2 KCl

Al2 (SO4) 3 + CaCl2 AlCl3 + CaSO4

H2O + CO2 H2CO3

22

323

Try These:

H2 + I2 HI

Na + Cl2 NaCl

Al + O2 Al2O3

2

22

2324

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

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 productsEquations must be balanced so that the number of atoms of each element on each side of the equation are equal.

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.

Types of Chemical Reactions

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”

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”

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

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

Summary- There are 4 Basic Types of Reactions

SynthesisDecompositionSingle replacementDouble replacement

Chemical EquationsPart 2

Predicting Products of Reactions

Writing Equations

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?

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

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 >>>> “Zn” forms a positive ion so it will replace“H” the positive ion in HCl.

Br2 + HCl >>>>> “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.

SRR Example:Zn + 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.

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.

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

If a reaction occurs…

Switch and write the products.Balance.

Zn + HCl ZnCl2 + H22

What if the reaction does not occur?

Write “ no reaction”.No need to balance

Example: Mg + Li NO3 no reaction

Try these…

Mg + Zn(NO3)2

Mg + AgNO3

Cl2 + NaBr

Sn + NaNO3

Br2 + KF

Cu + AgNO3

MgCl2 + Na

Mg(NO3)2 + Ag

Mg(NO3)2 + Zn

NaCl + Br2

No reaction

No reactionCuNO3 + Ag

NaCl + Mg

2 22 2

2 2

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.

Ionic Compounds in Water

Break up into ionsExample 1: NaCl in water becomes Na+ + Cl-

Example 2: K3PO4 in water becomes K+ and PO4

-3

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.

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.

DDR Example:Na2CO3 (aq) + Ca(OH)2 (aq)

1) Write the ions above each reactant2) Switch the positive ions and write new ion pairs on product

side (don’t forget “+ ion” first3) Write the formulas for the products using the criss-cross rule.4) 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

Na2CO3 (aq) + Ca(OH)2 (aq)

Na+1 CO3-2 Ca+2

OH-1

Ca+2 CO3-2 Na+1 OH-

1

CaCO3 (s) + NaOH (aq)

2

Try these…Write products and predict if it “goes to completion” NH4

+1 CO3-2 Ca+2 Cl-1

(NH4)2CO3 (aq) + CaCl2(aq)

K+1 NO3-1 Ca+2 I-1

KNO3 (aq) + CaI2 (aq)

Na+1 OH-1 H+1 SO4-2

NaOH(aq) + H2SO4 (aq)

Cu+2 SO4-2 Na+1 OH-1

CuSO4 (aq) + NaOH (aq)

NH4+1 CO3

-2 Ca+2 Cl-1

(NH4)2CO3 (aq) + CaCl2(aq)

K+1 NO3-1 Ca+2 I-1

KNO3 (aq) + CaI2 (aq)

Na+1 OH-1 H+1 SO4-2

NaOH(aq) + H2SO4 (aq)

Cu+2 SO4-2 Na+1 OH-1

CuSO4 (aq) + NaOH (aq)

NH4Cl (aq) + CaCO3(s)

NH4+1 Cl-1 Ca+2 CO3

-

2

K+1 I-1 Ca+2 NO3-1

KI (aq) + Ca(NO3)2

(aq)

Writing the Products

Na+1 SO4-2 H+1 OH-

1Na2SO4 (aq) + H2O (l) Cu+2 OH-1 Na+1 SO4

-2

Cu(OH)2 (s) + Na2(SO4) (aq)

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.

Predicting Missing Reactant or Product

Based on law of conservation of mass matter can neither be created or destroyedWhen given a balanced equation you should be able to determine the formula of missing substanceCount the atoms on both sides, subtract the atoms on the missing formula side from the side with the known formulasAny missing element must be present in the unknown formula

2Na + 2H2O x + 2NaOH

Na = 2 Na = 2 H = 4 H = 2 O = 2 O = 2Whats missing?2 HydrogensX must be H2

Try These

Multiple Choice Questions2Na + 2H2O 2x + H2

1) NaO2) NaOH3) Na2O4) Na2OH

4Fe + 3O2 2X1) FeO2) Fe2O3

3) Fe3O2

4) Fe3O4