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Chapter 7 Chemical Reactions.

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Presentation on theme: "Chapter 7 Chemical Reactions."— Presentation transcript:

1 Chapter 7 Chemical Reactions

2 Homework Assigned Problems (odd numbers only)
“Questions and Problems” to 7.31 (begins on page 200) “Additional Questions and Problems” to 7.49 (page 221) “Challenge Questions” (page 222)

3 Chemical Reactions Physical changes: Chemical changes:
Involves no changes in chemical identity of a substance No changes in physical properties (color, physical state, freezing point, boiling point) Chemical changes: A chemical reaction in which one or more substances changes to a different substance Properties that matter exhibits as it undergoes changes in chemical composition

4 Chemical Reactions Reactants  Products
Chemical properties determine whether or not a substance can be changed to another substance Reactions involve chemical changes in matter resulting in new substances Reactions involve rearrangement and exchange of atoms to produce new molecules Elements are not changed during a reaction Chemical properties determine whether or not a substance can be changed into another substance Reactants  Products

5 Changes During Chemical Reactions
A chemical change occurs when new substances are made Conversion of material(s) into one or more new substances These substances will have different properties from the original material New properties are visible (visual clues) Color change, precipitate formation, gas bubbles, flames, heat release

6 Changes During Chemical Reactions
Fe Fe2O3 Li LiOH, H2 HCO CO2 Lithium reacts with water to generate lithium hydroxide and hydrogen gas Bicarbonate ion reacting with water to generate carbon dioxide Na NaOH, H2

7 Changes During Chemical Reactions
In a chemical reaction: At least one new substance is produced Atoms are never created or destroyed Every atom present as a reactant has to be present as a product The atoms in reactants rearrange to form new products From the result of a chemical reaction (during the process) bonds in the reactants are broken, atoms rearrange and reform into products

8 Chemical Equations A chemical equation is a written statement that uses symbols and formulas (no words) to describe the changes during a chemical reaction It shows substances at the beginning of a reaction (reactants) It shows substances formed in the reaction (products) It is an abbreviated way of representing a chemical reaction on paper

9 Writing a Chemical Equation
Chemical reactions can be written as: Word equations Formula equations reactants products Word equations use the chemical names of the reactants and products Formula equations use symbols and formulas of the substances involved

10 Balancing Chemical Equations
A balanced chemical reaction has the same number of atoms of each element on both sides of the arrow Atoms are neither created nor destroyed Every atom must be accounted for Equations are balanced by placing a coefficient in front one or more of the substances in the equation A chemical reaction obeys the Law of Conservation of Mass If there are 10 carbon atoms in the reactants, there must be 10 carbon atoms in the reactants

11 Symbols Used in Equations
Symbols used after chemical formula to indicate physical state (g) = gas (l) = liquid (s) = solid (aq) = aqueous, dissolved in water

12 Writing Chemical Equations
When magnesium metal burns in air it produces a white, powdery compound magnesium oxide Burning in air means reacting with O2 Write the word equation The reactants are to the left of the arrow The products are to the right of the arrow Two or more reactants or products are separated by a plus sign A word description Reactants on the left and products on the right, separated by an arrow magnesium + oxygen magnesium oxide

13 Writing Chemical Equations
Indicate the physical state of each substance Use the correct chemical symbol to indicate liquids and solids Metals are solids, except for Hg which is liquid Use molecular form for gases (H2, O2, N2, all halogens) Identify polyatomic ions Elements in the solid and liquid states are represented in equations by the chemical symbols for the element magnesium(s) + oxygen(g) magnesium oxide(s)

14 Writing Chemical Equations
Convert the word equation into a formula equation Use the correct chemical symbol to indicate liquids and solids There must be the same number of each kind of atom on the reactant and product side of the equation Determine if the equation is balanced If not equal, must BALANCE Elements in the solid and liquid states are represented in equations by the chemical symbols for the element The arrow means “to produce” Always use molecular not empirical formulas Use the correct chemical symbol for the elements Elements that are gases at room temperature are identified by their correct molecular form ___Mg (s) +___O2 (g) ___MgO(s)

15 Balancing Chemical Equations
Balance equations by the use of a coefficient placed to the left of a substance NEVER change the subscripts of a compound to balance an element It changes the identity of the compound Can change coefficients but never subscript numbers Adding coefficients to the equation will adjust the number of reactant and or product molecules A coefficient will denote the amount of a substance

16 Balancing a Chemical Equation Example 1
2 2 Coefficient 2 1 Mg 2 1 Mg 2 O 2 1 O The coefficient affects both of the elements

17 Balancing a Chemical Equation: Example 2
When solid ammonium nitrite is heated it produces nitrogen gas and water vapor Write the formula equation

18 Balancing a Chemical Equation Example 2
2 x N 2 x N 2 x O 2 1 x O 4 x H 4 2 x H

19 Balancing a Chemical Equation: Example 3
Nitrogen monoxide gas decomposes to produce dinitrogen monoxide gas and nitrogen dioxide gas Write the formula equation

20 Balancing a Chemical Equation Example 3
1 x N 3 x N 3 1 x O 3 x O

21 Balancing a Chemical Equation Example 4
Liquid nitric acid decomposes to reddish-brown nitrogen dioxide gas, liquid water and oxygen gas. Write the formula equation

22 Balancing a Chemical Equation Example 4
2 2 2 4 2 1 x N 4 2 1 x N 12 6 3 x O 12 7 5 x O 4 2 1 x H 4 2 x H

23 Types of Reactions Reactions are separated into groups of similar reactions Based on the form of the equation for the reaction Synthesis (combination) Decomposition Single replacement Double replacement Combustion

24 Types of Reactions Synthesis Reactions General form of equation:
Reactions in which two or more substances combine to form a third substance (one product forms) General form of equation: Can have Elements combine An element and a compound Two compounds A + B AB

25 Synthesis Reactions The combinations can include Examples Two elements
An element and a compound Two compounds Examples

26 Types of Reactions Decomposition Reactions General Form of Equation
Reactions in which one reactant breaks down into simpler (smaller) substances Generally initiated by addition of energy (electric current or heating substances to high temperature) Opposite of a Synthesis Reaction General Form of Equation AB A + B

27 Decomposition Reactions
Can be broken down to: Smaller compounds Elements Both Examples Symbol for heat is the delta triangle Light is symbolized by hV

28 Types of Reactions Single replacement reactions
One element replaces another element Forms a new compound which frees the replaced element Most reactions occur in an aqueous solution General Form of Equation A + BC AC + B

29 Single Replacement Reactions
Three types Metal replaces a metal Metal replaces hydrogen Nonmetal replaces nonmetal Examples metal replaces metal fi metal replaces hydrogen nonmetal replaces nonmetal

30 Type of Reactions Two compounds exchange ions or atoms to form new compounds Also called exchange reactions Shows the exchange of “associates” when comparing the reactants and products General Form of Equation AB + CD AD + BC

31 Double Replacement Reactions
Most of these reactions occur in aqueous solution Most involve acids, bases, and ionic compounds Products formed Precipitate (a solid that is insoluble) A gas Water

32 Double Replacement Reactions
Examples precipitate gas water

33 Summary of Reaction Types

34 Combustion Reactions Occurs when a hydrocarbon combines with oxygen which produces carbon dioxide, water and heat (flame) The reaction of oxygen with any substance If a combustion reaction is possible then the substance will burn Commonly known as burning, the reaction of oxygen with any substance

35 Combustion Reactions Examples The combustion of propane gas
Produces carbon dioxide and water Produces heat (flame) The combustion of sulfur Also a combination reaction Also produces heat (flame) hydrocarbon The combustion of sulfur also produces sulfur trioxide

36 Energy in Chemical Reactions
In a chemical reaction A change in energy occurs as bonds are broken (reactants) and new ones form (products) Nearly all chemical reactions absorb or produce heat Measured by the heat of reaction or enthalpy Enthalpy change is the amount of heat produced or consumed in a process (∆H )

37 Heat of Reaction Endothermic reactions absorb heat as they occur
If (∆H ) is positive, then heat is added to the reaction Exothermic reactions produce heat as they occur If (∆H ) is negative, then heat is evolved by the reaction Discuss definitions concerning the amount of heat produced or consumed

38 Heat of Reaction Photosynthesis reaction Cell metabolism
Carbon dioxide reacts with water to produce glucose and oxygen Cell metabolism Glucose reacts with oxygen to produce carbon dioxide and water ∆H = kJ The energy from the sun is absorbed to produce glucose from carbon dioxide and water The reverse of the reaction changes it from one that absorbs energy to one that produced energy. The quantity of the change in energy does not change, only the sign itself changes ∆H = kJ

39 Calculation of Heat in Reactions
The combustion of sulfur dioxide It reacts with oxygen to produce sulfur trioxide Calculate the heat produced when 75.2 g of sulfur trioxide is produced ∆H = kJ Reacts or combines Given 75.2 g SO3 Heat in kJ produced when SO3 is formed

40 Calculation of Heat in Reactions
Relation between g of SO3 and heat released Grams of SO3 Moles of SO3 Heat of rxn Molar mass kj Write the necessary conversion factors Set up the problem 46.5 kJ

41 End

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