CHEMISTRY: CHEMICAL VERSUS PHYSICAL CHANGES

Question 1

Chemical change

Answer 1

-substance changes into a new and different substance
-iron left exposed to the environment reacts with oxygen in the air
-this results in a new compound, iron oxide

Question 2

Physical change

Answer 2

-change in which the substance undergoing the change remains the same
-melting is a physical change
-ex. melting of ice in a cup of ice cubes—the result is a glass of water
-a change has occurred but the substance remains the same

Question 3

[chemical reactions shown w chemical equations]
Reactants

Answer 3

-the substances that react with each other
-are written on the left side of the equation

Question 4

[chemical reactions shown w chemical equations]
Products

Answer 4

-the substances that are the end products of the reaction
-written on the right side of the equation

Question 5

[chemical reactions shown w chemical equations]
Law of conservation of mass

Answer 5

-matter is neither created nor destroyed
-so the same number of atoms of each element must be present on each side of an equation
-equation is balanced using coefficients, the numbers placed in front of each substance
-ex. two hydrogen molecules are indicated as 2Hsub2

Question 6

[reactions]
Mole

Answer 6

-when we look at chemical reactions, we think of substances combining in quantities called moles
-a mole is equal to 6.02x10^23rd power particles
-you can think of it as a quantity like a dozen or gross

Question 7

[chemical reactions]
Combination or synthesis reactions

Answer 7

-involve two or more reactants that combine to create a new product
- 2H2+O2 —-> 2H2O
-in this reaction 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water
-there are 4 atoms of hydrogen and 2 atoms of oxygen on each side
-this is a balanced equation for synthesis reaction

Question 8

[chemical reactions]
Singe replacement reactions

Answer 8

-involve an element reacting with a compound
-during this reaction an atom of the single element replaces an element in the compound
-ex. Cl2 +2NaBr —-> 2NaCl + Br2
-chlorine replaces bromine in the compound

Question 9

[chemical reactions]
Double displacement reactions
1/2

Answer 9

-involve two ionic compounds
-the positive ions or metal in each compound switch positions
-a common type of double displacement reaction is an acid base reaction
-the products are always a salt and water
-since products are neutral (neither acidic nor basic) these specific types of displacement reactions are called neutralization reactions
- NaOH + HCl —> NaCl + H2O
-sodium hydroxide reacts with hydrochloric acid to form sodium chloride (a salt) and water
-in an aqueous solution an acid donates hydrogen ions (H+) and base donates hydroxide ions (OH-)
-another way of defining acids is to think of H+ as combining with water to form a hydronium ion (H3O+) and to think of the hydronium ion as being the positive ion produced by an acid

Question 10

[chemical reactions]
pH scale
1/2

Answer 10

-created to identify the strength of an acid or base (or alkaline) depending on the concentration of hydrogen and hydroxide ions
-the scale runs from 1 to 14, with a pH below 7 as acidic, a pH above 7 is basic, and a pH of 7 is neutral (neither acidic nor basic)
-a pH of 0 indicates an extremely strong acid, whereas a pH of 6 indicates the acid is fairly weak
-a pH of 14 indicates a very strong base, and a pH of 8 means the base is fairly weak
-substance that changes color in an acid or base is called an acid-base indicator, such as litmus (red in acid, blue in base), phenolphthalein (purple/red in a base, colorless in acid), or bromthymol blue (yellow in an acid, blue in base, green in neutral)

Question 11

[chemical reactions]
Decomposition reactions

Answer 11

-involve one reactant that is broken down into two or more simpler products
-often heat is used to drive this type of reaction
-H2CO3 —-> H2O + CO2
-heated carbonic acid decomposes into water and carbon dioxide

Question 12

Reaction depends on 2 things

Answer 12

-the two substances must come into contact
-enough energy has to be available
-if the appropriate amount of energy is available, then the reaction can proceed
-reaction rate is influenced by these two factors

Question 13

Reaction rate

Answer 13

-reaction rate is increased by raising the temp
-because an elevated temperature causes particles to move around in a quicker and more erratic manner, increased movement heightens the probability that two particles will come in contact
-increasing the surface area of a substance also boosts the reaction rate
-catalysts are substances that increase the rate of reaction by lowering the activation energy
-catalyst only affect the rate of reaction and are not used up in it

Question 14

Exothermic reaction

Answer 14

-reaction that releases energy
-its products contain less energy than its reactants

Question 15

Endothermic reaction

Answer 15

-reaction that absorbs energy
-its products contain more energy than its reactants