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EOG REVIEW 8 th grade SCIENCE. GOAL 4. Unit D Chapter 3 pages D66 – D100. “Chemical Reactions”. A physical change occurs when substances undergo a change that does not alter their chemical nature. A physical change involves a change in physical properties.

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EOG REVIEW

8th grade

SCIENCE



Unit D Chapter 3

pages D66 – D100

“Chemical Reactions”


A physical change occurs when substances

undergo a change that does not alter

their chemical nature.

A physical change involves a change

in physical properties.

Physical properties can be observed without changing the type of matter.


A chemical change occurs when substances

undergo a change that does alter

their chemical nature.

A chemical change involves a change

in chemical properties.


chemical reaction

  • the process by which chemical changes occur and new substances are formed

  • atoms are rearranged

  • chemical bonds are broken and new bonds formed between different atoms


reactant

a substance that is present

at the beginning

of a chemical reaction, takes part in the chemical reaction, and is changed into a new substance


product

a new substance formed

by a chemical reaction;

made by the rearrangement

of atoms and bonds in reactants


REACTANTS & PRODUCTS

  • can be elements or compounds

  • bonds between atoms

  • in the reactants are broken

  • and new bonds are formed

  • in the products


Identify the reactants & the products-

1) Zn + 2HCl ZnCl2 + H2

REACTANTS

PRODUCTS

Zn

ZnCl2

2HCl

H2

2) CH4 + 2O2 CO2 + 2H2O

REACTANTS

PRODUCTS

CH4

CO2

2O2

2H2O


EVIDENCE OF CHEMICAL REACTIONS

1) color change

2) formation of precipitate

3) formation of gas

4) temperature change


precipitate

a solid substance that forms

as a result of a chemical reaction between chemicals

in a solution


exothermic reaction

a chemical reaction that

releases energy and gives off heat


endothermic reaction

a chemical reaction that

absorbs energy


synthesis reaction

a reaction in which a

more complex substance

(a new compound)

is formed by the combination of

simpler reactants

(two or more elements or compounds)


decomposition reaction

a reaction in which a compound (reactant) breaks down into simpler products, which could be elements or other compounds


combustion reaction

a reaction in which one reactant

is always oxygen

and another reactant

often contains carbon and hydrogen

EXAMPLE: burning


CLASSIFICTIONS OF CHEMICAL REACTIONS

1) synthesis reaction

N2 +3H2 2NH3

2) decomposition reaction

2H2 O2H2 + O2

3) combustion

CH4 + 2O2 CO2 + 2H2 O


Law of Conservation of Mass

  • a law stating that atoms are not created or destroyed in a chemical reaction

  • a law stating that the total mass of reactants is EQUAL to the total mass of products in a chemical reaction


chemical formula

an expression that shows

the number and types of atoms joined in a compound


subscript

a number written slightly below and to the right of

a chemical symbol that shows how many atoms of an element are in a compound


coefficient

the number before a chemical formula that indicates how many molecules are involved in a chemical reaction


Balancing Equations

Zn + 2HCl ---> ZnCl2 + H2

REACTANTS

PRODUCTS

ZnCl2

Zn

2HCl

H2


BALANCED EQUATIONS:

Zn + 2HCl ---> ZnCl2 + H2

  • (subscript = 2)

  • (coefficient = 2)

1 Zn 2H 2Cl = 1Zn 2Cl 2H

WEBSITE: Balancing Act!


PHYSICAL FACTORS CAN VARY

RATE of CHEMICAL REACTIONS

1) concentration- number of particles present in a certain volume

high concentration = faster reaction

2) surface area- breaking large piece

of material into smaller pieces increases

surface area

more surface area = faster reaction

3) temperature- adding heat energy

high temperature = faster reaction

TEXTBOOK- (pg D75) Particles and Reaction Rates


CHEMICAL FACTORS CAN VARY

RATE of CHEMICAL REACTIONS

catalyst

a substance that

increases the rate

of a chemical reaction

but is not consumed in the reaction



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