Chemical Change

Question 1

Define a Physical change:

Answer 1

A change in which:
1. No new substances are formed.
2. Energy changes are small in relation to chemical changes
3. Mass, number of atoms and molecules are being conserved

Question 2

Describe the rearrangement of molecules during physical changes:

Answer 2

Eg.
1. Molecules separate when water evaporates to form water vapour
2. When ice melts molecules become disorderly arranged due to weakening of intermolecular forces.

Question 3

Define a chemical change

Answer 3

A change in which:
1. New chemical substances are formed.
2. Energy changes are much larger than those of the physical change.
3. Mass and atoms are conserved, but the number of molecules is not
(Endothermic/exothermic)

Question 4

Endothermic reaction:

Answer 4

A reaction in which more energy is absorbed than released.
(Cold to touch)

Reactants + energy (∆H) → Products

Question 5

Exothermic reaction:

Answer 5

A reaction in which more energy is released than absorbed.
(Hot to touch)

Reactants → Products + energy (∆H)

Question 6

Describe examples of a chemical change that include the:

Answer 6

1. Decomposition of hydrogen peroxide to form water and oxygen
2. Synthesis reaction that occurs when hydrogen burns in oxygen to form water
3. Heating of iron and sulphur
4. Reaction of lead(ll) nitrate and potassium iodide.
5. Reaction of hydrochloric acid with sodium hydroxide to measure the change in temp.

Question 7

Daily examples of physical change:

Answer 7

• phase changes
• cutting into smaller pieces
• bending
• dissolving

Question 8

Examples of chemical changes

Answer 8

• all chemical reactions
• iron rusting
• burning substances
• electroplating
• metabolic reaction in cells
• combustion

Question 9

All changes will follow:

Answer 9

The law of conservation of mass

Question 10

Law of conservation of mass:

Answer 10

The total number of atoms or mass at the beginning of a physical or chemical change will be present at the end of the physical or chemical change.

Question 11

Summary of physical changes:

Answer 11

1.During a physical change the chemical identity of the substance does not change, changes include a change in shape or state of matter.
2. No new substances are formed
3. Small energy changes possible (melting, evaporating, freezing)
4. Number of atoms / mass before is equal to the atoms / mass after the
change.
5. Easily reversed (changes in state of matter)

Question 12

Chemical change:

Answer 12

1. During a chemical change the chemical composition of the substance changes. New substances with different characteristics are formed, chemical changes occur due to chemical reactions.
2. New substances are formed
3. Significant energy changes take place (endothermic, exothermic)
4. Number of atoms / mass before is equal to the atoms / mass after the change.
   (number of molecules can be different than before)
5. Not easily reversed

Question 13

Describe the Rearrangement of particles during physical change:

Answer 13

Particles will rearrange themselves, bonds between atoms will not be broken. Intermolecular forces between individual molecules will strengthen or weaken depending on the amount of energy due to the change in temperature experienced.

Question 14

Solids: (rearrangement of particles in physical change)

Answer 14

Solids are usually arranged in a fixed pattern (pulled together by strong intermolecular forces.)

Question 15

Liquids (rearrangement of particles during physical change)

Answer 15

Liquids have enough energy to break apart some of the intermolecular forces and molecules/atoms slide past each other loosely.

Question 16

Gas (arrangement of particles during physical change)

Answer 16

Gasses have enough energy to completely break free of intermolecular forces and move about freely.

Question 17

Gas (arrangement of particles during physical change)

Answer 17

Gasses have enough energy to completely break free of intermolecular forces and move about freely.

Question 18

Synthesis reactions:

Answer 18

Two or more compounds combine to form one compound
A + B → AB

Question 19

Combustion reactions:

Answer 19

Oxygen combines with a compound to form carbon dioxide and water
A + O2 → H2O + CO2

Question 20

Combustion reactions:

Answer 20

Oxygen combines with a compound to form carbon dioxide and water
A + O2 → H2O + CO2

Question 21

Displacement reaction

Answer 21

One element takes the place of another in a compound
A + BC → AC + B

Question 22

(Endothermic/exothermic)

Precipitation reactions

Answer 22

Two solutions (containing soluble salts) are mixed together, resulting in the formation of an insoluble solid (precipitate) forming
A + Soluble salt B → Precipitate + soluble salt C

Common Insoluble Compounds
Barium Sulfate (
BaSO
4
BaSO
4
​
):

Forms a white precipitate.
Insoluble in water and most acids.
Silver Chloride (
AgCl
AgCl):

Forms a white precipitate.
Insoluble in water.
Lead(II) Iodide (
PbI
2
PbI
2
​
):

Forms a yellow precipitate.
Insoluble in water.

Question 23

(Endothermic/ exothermic)
Decomposition reactions

Answer 23

Opposite of combination reaction, complex molecule breaks apart to form simpler compounds/atoms
AB → A + B

Question 24

Physical or chemical change & REASON:

H2O (l) → H2O (g)

Answer 24

Physical Change
No new substances are being formed. The intermolecular forces between molecules weaken which makes the molecules move further apart.
*Note: Mass, number of atoms and number of molecules are conserved in physical changes.

Question 25

Physical / chemical change with REASON: 2H2 (g) + O2 (g) → 2H2O (g)

Answer 25

Chemical Change Reason(s) A new substance is produced. *Note: Mass and number of atoms are conserved in chemical changes but the number of molecules may change.

Question 26

What will happen to the reading on a thermometer in an Exothermic experiment?

Answer 26

Surroundings will heat up, thermometer reading increases

Question 27

What will happen to the reading on a thermometer in an Endothermic experiment?

Answer 27

Surroundings will cool down, Thermometer reading decreases

Question 28

Define the law of conservation of mass:

Answer 28

The total mass of substances in a closed system remains constant, no matter what processes are acting inside the system.

Question 29

A reaction that obeys the law of conservation of mass:

Answer 29

mass(reactants) = mass(products)

Question 30

What happens during a synthesis reaction?

Answer 30

During a synthesis reaction, the number of molecules decreases as the reactants form new products.

Question 31

What happens in a decomposition reaction?

Answer 31

In a decomposition reaction a single compound breaks down to give two or more simpler substances.

Question 32

What is an aqueous solution?

Answer 32

Solute dissolved in water

Question 33

What is a pop test?

Answer 33

A burning splint explodes with a popping sound - the test is performed to detect a combustible gas HYDROGEN GAS

Question 34

Define a catalyst:

Answer 34

A substance that increases the rate of a chemical reaction without itself undergoing a permanent change.

Question 35

What is the glowing splint test?

Answer 35

*Light the wooden splint. After it has burnt for a while, blow out the flame and push the glowing splint into the gas that is produced in the reaction A glowing splint reignites : tests for OXYGEN GAS

Question 36

Chemical change involves colour changes of a substance.

Answer 36

Iron fillings : GREY Sulphur powder : YELLOW Iron (ll) sulphide: BLACK

Question 37

Colour observations after chemical change:

Answer 37

Lead(ll) nitrate: clear Sodium Iodide: clear Sodium nitrate: clear Lead(ll) Iodide: yellow

Question 38

Colour observations after indicator used

Answer 38

Sodium hydroxide: blue/ purple Hydrochloric acid: red Sodium chloride and water: green

Question 39

Describe the mole

Answer 39

The SI unit for the amount of a substance. (Measures the quantity of a substance)

Question 40

Define: One Mole

Answer 40

The amount of substance having the same number of particles as there are atoms in 12g of carbon-12

Question 41

Describe Avogadro’s number:

Answer 41

NA the number of particles (atoms, molecules, formula units or ions) present in one mole: 6.022 × 10²³ (particles per mole)

Question 42

Define: Molar Mass

Answer 42

The mass of one mole of a substance measured in g.mol^-1

Question 43

Define empirical formula

Answer 43

The simplest whole-number ration of atoms in a compound.

Question 44

Calculating: Mass Molar mass Number of Moles

Answer 44

n = m/M

Question 45

Define: Percentage composition

Answer 45

The mass of each element present in a compound expressed as percentage of the total mass of the compound

Question 46

Molecular mass=

Answer 46

Relative atomic mass

Question 47

Calculating: - moles (n) - number of particles (x)

Answer 47

n = x/ Na

Question 48

Particles can be classified as:

Answer 48

Atoms Ions Molecules Formula units

Question 49

What is avogadros constant? Described

Answer 49

The number of particles present in one mole of a substance.

Question 50

Define Concentration

Answer 50

The amount of solute per litre solution

Question 51

Calculating concentration in mol.dm^-3

Answer 51

C= n/v

Question 52

Solving problems involving dilution and change in concentration:

Answer 52

C1V1 = C2V2

Question 53

State Avogadro’s law:

Answer 53

One mole of any gas occupies the same volume at the same temperature and pressure

Question 54

STP:

Answer 54

Standard temp: 0 °C Standard pressure: 1 atmosphere (1 atm)

Question 55

Determine percentage yield of a product in chem reaction

Answer 55

% yield = actual yield / theoretical yield x 100

Question 56

Define: Actual Yield

Answer 56

The quantity physically obtained from a chemical reaction.

Question 57

What is a physical change

Answer 57

A change in the form or shape of a substance