chemical equations revised.ppt

Heat Energy and Chemical

Equations

Part 1:

Changes in Matter & Energy

Balancing Equations

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) Na+1

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

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)

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

Potential Energy

Energy that is stored (i.e. in a chemical bond).

Something has the “potential” to do some kind of work

Kinetic Energy

Energy of motion

Law of Conservation of Energy

Energy, like matter, is neither

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

Understanding Heat Flow

Heat is defined as the energy that transfers from one object to

another.

Heat will always flow from warm 

cool.

Heat Energy vs. Temperature

We can measure heat flow by identifying changes in temperature.

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

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 energy

Endothermic Processes (Changes)

Endothermic processes ABSORB ENERGY

A 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.

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

H

+ O



H

O

2g + 32g = 18g

How do we get these masses to

balance?

2H

+ O



2H

O

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.

2H

+ O



2H

O

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.

Examples:

K + Cl₂  KCl

Al₂ (SO₄) ₃ + CaCl₂  AlCl₃ + CaSO₄

H₂O + CO₂  H₂CO₃

2 2

3 2

Try These:

H₂ + I₂  HI

Na + Cl₂  NaCl

Al + O₂  Al₂O₃

2

2 2

2 3

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

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

There are 4 main kinds of reactions that occur between elements and

compounds.

Each of these chemical reactions follow a specific format from reactants to

products. If you recognize the format you will be able identify the type of

reaction.

SYNTHESIS REACTIONS

Also called combination reactions.

General Equation Format

A + B  AB

***These reactions will only have ONE product***_{Example: 2H}

2 + O2  2H2O

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

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***

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.***

Summary- There are 4 Basic

Types of Reactions

Synthesis

Decomposition

Chemical Equations

Part 2

Predicting Products of Reactions

How do we predict the products of a reaction?

First you must know the types of reactions.

Types of Reactions

Synthesis – 2 or more reactants 1 product

Decomposition – 1 reactant 2 or more products

SRR – element + compound compound + element

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!!!!!!!!!!!

SRR Example:

Zn + 2HCl



ZnCl

+ H

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.

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

How do we use the Activity Series?

In a SRR, if the element by itself is more reactive than the element it would replace than the reaction will occur or is spontaneous.

If a reaction occurs…

Switch and write the products. Balance.

What if the reaction does not occur?

Write “ no reaction”. No need to balance

Example:

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 reaction

CuNO₃ + Ag NaCl + Mg

2 2

2 2

Double Replacement Reactions

All compounds are ionic.

Products are formed by switching the positive ions of the reactants.

Ionic Compounds in Water

Break up into ions

Example 1: NaCl in water becomes Na+

+ Cl

-Example 2: K₃PO₄ in water becomes K+

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.

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)

DDR Example:

Na

CO

+ Ca(OH)



1) Write the ions above each reactant

2) Switch the positive ions and write new ion pairs on product side

(don’t forget “+ ion” first

3) 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 CO

3-2 Ca+2 OH-1 Ca+2 CO3-2 Na+1 OH-1

Try these…

Write products and predict if it “goes

to completion

”

NH₄+1 CO

3-2 Ca+2 Cl-1

(NH₄)₂CO_{3 (aq)} + CaCl_2(aq) 

K+1 NO

3-1 Ca+2 I-1

KNO_{3 (aq)} + CaI_{2 (aq)} 

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

NaOH_(aq) + H₂SO_{4 (aq)} 

Cu+2 SO

4-2 Na+1 OH-1

NH₄+1 CO

3-2 Ca+2 Cl-1

(NH₄)₂CO₃ (aq) + CaCl₂(aq) 

K+1 NO

3-1 Ca+2 I-1

KNO₃ (aq) + CaI₂ (aq) 

Na+1 OH-1 H+1 SO 4-2

NaOH(aq) + H₂SO₄ (aq) 

Cu+2 SO

4-2 Na+1 OH-1

• CuSO4 (aq) + NaOH (aq) 

NH₄Cl (aq) + CaCO₃(s)

NH₄+1 Cl-1 Ca+2 CO 3-2

K+1 I-1 Ca+2 NO 3-1

KI (aq) + Ca(NO₃)₂ (aq)

Writing the Products

Na+1 SO

4-2 H+1 OH-1

Na₂SO₄ (aq) + H₂O (l)

Cu+2 OH-1 Na+1 SO 4-2

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

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

2Na + 2H

O



x + 2NaOH

Na = 2 Na = 2

H = 4 H = 2

O = 2 O = 2

Whats missing? 2 Hydrogens

Try These

Multiple Choice Questions 2Na + 2H₂O  2x + H₂

1) NaO 2) NaOH 3) Na₂O 4) Na₂OH

4Fe + 3O₂  2X