Chemical Change Flashcards
Define a Physical change:
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
Describe the rearrangement of molecules during physical changes:
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.
Define a chemical change
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)
Endothermic reaction:
A reaction in which more energy is absorbed than released.
(Cold to touch)
Reactants + energy (∆H) → Products
Exothermic reaction:
A reaction in which more energy is released than absorbed.
(Hot to touch)
Reactants → Products + energy (∆H)
Describe examples of a chemical change that include the:
- Decomposition of hydrogen peroxide to form water and oxygen
- Synthesis reaction that occurs when hydrogen burns in oxygen to form water
- Heating of iron and sulphur
- Reaction of lead(ll) nitrate and potassium iodide.
- Reaction of hydrochloric acid with sodium hydroxide to measure the change in temp.
Daily examples of physical change:
- phase changes
- cutting into smaller pieces
- bending
- dissolving
Examples of chemical changes
- all chemical reactions
- iron rusting
- burning substances
- electroplating
- metabolic reaction in cells
- combustion
All changes will follow:
The law of conservation of mass
Law of conservation of mass:
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.
Summary of physical changes:
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)
Chemical change:
- 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.
- New substances are formed
- Significant energy changes take place (endothermic, exothermic)
- Number of atoms / mass before is equal to the atoms / mass after the change.
(number of molecules can be different than before) - Not easily reversed
Describe the Rearrangement of particles during physical change:
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.
Solids: (rearrangement of particles in physical change)
Solids are usually arranged in a fixed pattern (pulled together by strong intermolecular forces.)
Liquids (rearrangement of particles during physical change)
Liquids have enough energy to break apart some of the intermolecular forces and molecules/atoms slide past each other loosely.
Gas (arrangement of particles during physical change)
Gasses have enough energy to completely break free of intermolecular forces and move about freely.
Gas (arrangement of particles during physical change)
Gasses have enough energy to completely break free of intermolecular forces and move about freely.
Synthesis reactions:
Two or more compounds combine to form one compound
A + B → AB
Combustion reactions:
Oxygen combines with a compound to form carbon dioxide and water
A + O2 → H2O + CO2
Combustion reactions:
Oxygen combines with a compound to form carbon dioxide and water
A + O2 → H2O + CO2
Displacement reaction
One element takes the place of another in a compound
A + BC → AC + B
(Endothermic/exothermic)
Precipitation reactions
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.
(Endothermic/ exothermic)
Decomposition reactions
Opposite of combination reaction, complex molecule breaks apart to form simpler compounds/atoms
AB → A + B
Physical or chemical change & REASON:
H2O (l) → H2O (g)
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.
Physical / chemical change with REASON:
2H2 (g) + O2 (g) → 2H2O (g)
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.
What will happen to the reading on a thermometer in an Exothermic experiment?
Surroundings will heat up, thermometer reading increases
What will happen to the reading on a thermometer in an Endothermic experiment?
Surroundings will cool down, Thermometer reading decreases
Define the law of conservation of mass:
The total mass of substances in a closed system remains constant, no matter what processes are acting inside the system.
A reaction that obeys the law of conservation of mass:
mass(reactants) = mass(products)
What happens during a synthesis reaction?
During a synthesis reaction, the number of molecules decreases as the reactants form new products.
What happens in a decomposition reaction?
In a decomposition reaction a single compound breaks down to give two or more simpler substances.
What is an aqueous solution?
Solute dissolved in water
What is a pop test?
A burning splint explodes with a popping sound
- the test is performed to detect a combustible gas HYDROGEN GAS
Define a catalyst:
A substance that increases the rate of a chemical reaction without itself undergoing a permanent change.
What is the glowing splint test?
*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
Chemical change involves colour changes of a substance.
Iron fillings : GREY
Sulphur powder : YELLOW
Iron (ll) sulphide: BLACK
Colour observations after chemical change:
Lead(ll) nitrate: clear
Sodium Iodide: clear
Sodium nitrate: clear
Lead(ll) Iodide: yellow
Colour observations after indicator used
Sodium hydroxide: blue/ purple
Hydrochloric acid: red
Sodium chloride and water: green
Describe the mole
The SI unit for the amount of a substance.
(Measures the quantity of a substance)
Define: One Mole
The amount of substance having the same number of particles as there are atoms in 12g of carbon-12
Describe Avogadro’s number:
NA
the number of particles (atoms, molecules, formula units or ions) present in one mole:
6.022 × 10²³ (particles per mole)
Define: Molar Mass
The mass of one mole of a substance measured in g.mol^-1
Define empirical formula
The simplest whole-number ration of atoms in a compound.
Calculating:
Mass
Molar mass
Number of Moles
n = m/M
Define: Percentage composition
The mass of each element present in a compound expressed as percentage of the total mass of the compound
Molecular mass=
Relative atomic mass
Calculating:
- moles (n)
- number of particles (x)
n = x/ Na
Particles can be classified as:
Atoms
Ions
Molecules
Formula units
What is avogadros constant? Described
The number of particles present in one mole of a substance.
Define Concentration
The amount of solute per litre solution
Calculating concentration in mol.dm^-3
C= n/v
Solving problems involving dilution and change in concentration:
C1V1 = C2V2
State Avogadro’s law:
One mole of any gas occupies the same volume at the same temperature and pressure
STP:
Standard temp: 0 °C
Standard pressure: 1 atmosphere (1 atm)
Determine percentage yield of a product in chem reaction
% yield = actual yield / theoretical yield x 100
Define: Actual Yield
The quantity physically obtained from a chemical reaction.
What is a physical change
A change in the form or shape of a substance
