SCIENCE CHEMISTRY (Chpt 11 - Chemical Changes) Flashcards
chemical reactions
involve the breaking and forming of chemical bons
physical changes
a process in which a substance undergoes changes without changing the composition or identity of the substance
–> no new substances formed
–> changes can usually be reversed/ temporary (constituents can be separated by physical separation methods: e.g. filtration)
–> properties of product(s) is/are the same as the reactant(s)
☆examples: melting of ice cubes, table salt mixed with water
chemical changes
a process in which a substance undergoes changes by changing the composition and identity of the substance (a result of chemical reactions)
–> one or more new substances formed
–> changes are permanent (products formed cannot be reversed back to the original reactants by physical methods) - large amounts of energy is required to reverse these changes
–> properties of the product(s) is/are different from the reactant(s)
☆examples:rusting of iron
- atoms lose, gain or share electrons → achieve electronic configuration of a noble gas + achieve greater stability
covalent bonding
formation of covalent bonds
formation of covalent bonds
- between two non-metals
- form diatomic molecules in an element
- sharing electrons
- use ‘dot and cross’ diagram
valence electrons (covalent bonding)
- electrons at outer most shell
- determine number of pairs of electrons to be shared
ionic bonding
formation of ionic bond
formation of ionic bond
- between atoms of metal and non-metal
- lose electrons (metal)
- gain electrons (non-metal)
- +ve ion (lose): neutral at first
- proton = electron
valence electrons (ionic bonding)
- fetermine electrical charger of ions formed
- electronic configuration of an ion (with atomic numbers 1 to 20)
- ‘dot and cross’ diagram to show ionic bonding
- derive formula of ionic compounds from its ions
law of conservation of mass
–> followed by chemical reactions
–> law: the form of matter can be changed BUT total amount of matter CANNOT BE created nor destroyed (total mass before and after a chemical change must remain the same)
☆ this is because atoms can neither be destroyed nor created in a chemical reaction. when reactants collide together, their respective atoms simply rearrange and combine chemically in a new way to form products
word equation
an equation in which the reactants and products in a chemical reaction are represented by words
–> e.g. A+B –> C+D
term/symbol in word equation
* reactant
reactant: a substance that is present at the start of the reaction (before chemical change occur)
term/symbol in word equation
* product
product: a new substance that is produced by the reaction
term/symbol in word equation
* ➡
➡ : produces/forms/yields/gives
balancing chemical equations
- count the atoms of each elements on the LHS and RHS of the equation
- identify unbalanced atoms and choose appropriate coefficients to balance them
- balance equations that appear once on each side of the equation first
- move on to the “most complex” compound or the one with the most atoms
- check and reassess the balance of all atoms
causes of different types of chemical changes
a chemical change can be triggered by:
1. heat or an increase in temperature
- e.g. combustion → burning of coal, thermal decomposition (heating of calcium carbonate)
- interaction with oxygen
→ oxygen is a highly reactive molecue: can react with several chemicals
- e.g. rusting, cellular respiration, reaction of copper and oxygen, rotting of food (apple turning brown)
- red and brown colour of tropical soils: result of chemical reactions with oxygen - using an electric current
- e.g. gold medal of Tokyo 2020 Olympic Games: made of recylced silver coated with gold by electroplating - exposure to light
- e.g. photosynthesis: a chemical reaction that occurs in green plants
- in presence of light, green plants use chlorophyll in their leaves, carbon dioxide and water to produce glucose and oxygen
- colours in clothes, photographs and books fade over time when exposed to light: due to chemical reaction caused by UV rays in sunlight - mixing substances
→ when two or more reactants are mixed together, they combine irreversibly to form one or more products
- e.g. mixing acids with metals, mixing acids with alkalis
How knowledge of chemical reaction help us in daily life
(1. cooking)
Process: cooking
Impact on our lives:
→ heat from cooking triggers chemical reactions: heating
- help soften food
- improve flavour
- enable our body to absorb nutrients from food easier
How knowledge of chemical reaction help us in daily life
(2. respiration)
Process: respiration
Impact on our lives:
→ extracts energy from food for our body cells to carry out vital activites in order to survive and reproduce
- glucose molecule:
‣ one of the simplest forms of nutrients
* when glucose molecules react with oxygen, energy is released in our body
* produces carbon dioxide and water as products
▫︎ word equation:
glucose + oxygen → carbon dioxide + water
How knowledge of chemical reaction help us in daily life
(3. decay)
Process: decay
Impact on our lives:
→ aka decomposition
* process of breaking down organic matter into simpler substances
- advantage
‣ plays a crucial role in ecosystems by returning nutrients to environemnt
‣ recycling of nutrients: essential for sustaining life and maintaining balance of ecosystems - disadvantage
‣ causes food to spoil
☞ method to slow down decay of food:
→ vacuum packaging of food - removes air from a package prior to sealing
- purpose: reduce amount of oxygen
◇ limit growth of bacteria and other microorganisms
How knowledge of chemical reaction help us in daily life
(4. combustion)
Process: combustion
Impact on our lives:
→ when fuels burn, release of air pollutants (e.g. carbon monoxide, sulfur dioxide and oxides of nitrogen) into the air
→ natural processes (lightning, volcanic eruptions) can also release air pollutants into the air
→ burning of substances containing carbon (e.g. fossil fuels, wood) releases carbon dioxide into air
→ using slash-and-burn method to clear forests or farmland generates massive smoke
‣ causes air pollution and affects our health when inhaled
→ deforestation can increase amount of carbon dioxide in air
‣ more heat trapped in atomosphere, causing global warming
‣ when more carbon dioxide dissolves in seawater, causes acidification of ocean
How knowledge of chemical reations help us in daily life
( 5. combustion - acidification of oceans )
⬆dissolved carbon dioxide in seawater = acidification of ocean
→ dissolved carbon dioxide in seawater : carbonic acid
→ ocean acidification - result of chemical reactions stemming from human activities
‣ ⬆carbon dioxide released from combustion (which dissolves in seawater), leads to formation of ⬆ carbonic acid
‣ seawater becomes more acidic over time (phenomena known as acidification of oceans)
→ affects lives of marine animals and plants
Examples:
⓵ shells and skeletons of small marine organisms (oysters, corals) contain calcium carbonate
- breaks down while reacting with acidic seawater
- makes these organisms more vulnerable
~ reaction bwt. acids and carbonates
✘ cannot build new shells or skeletons easily when carbonate compounds react readily with acidic seawater
✘ organisms like fish and plants cannot tolerate a lower pH value in seawater and may die
* in turn affects other organisms dependant on them for food or protection to survive
→ mangroves, marshes and sea grass meadows stor large amounts of carbon
‣ release carbon in form of carbon dioxide into atmosphere and oceans if destroyed
✓ important to protect them in order to stop or slow down process of ocean acidification
How knowledge of chemical reations help us in daily life
( 6. rusting - oxidation )
Process: rusting (oxidation)
→ causes iron to undergo a chemical change
‣ turns iron into a reddish-flaky substance: aka rust (easily crumbles into powder)
‣ weakens structures that contain iron
→ exposure of iron to oxygen in presence of moisture leads to formation of rust
‣ rusting occurs rapidly in humid climates
✓ ways to prevent iron from being exposed to water and oxygen to prevent rusting:
⓵ painting
⓶ oiling
⓷ drying
⓸ galvanishing (coating iron with a protective layer of zinc)
How knowledge of chemical reaction help us in daily life
(combustion - carbon monoxide)
Process: combustion
→ sources and effects of common air pollutants:
1. Carbon monoxide
* sources: burning of fuels in presence of insufficient oxygen in factories and motor vehicles (incomplete combustion)
✘ effects: a colourless, odourless, and highly toxic gas
✘ prevents red blood cell from picking up oxygen
✘ gives rise to headaches and eventually death: suffocation
How knowledge of chemical reaction help us in daily life
(combustion - sulfur dioxide)
Process: combustion
→ sources and effects of common air pollutants:
2. Sulfur dioxide
* sources: combustion of coal and fossil fuels containing sulfur in power stations and factories
* volcanic eruptions
✘ effects: irritates eyes, nose and lungs
✘ worsens symptons of asthms, heart disease and respiratory problems
How knowledge of chemical reaction help us in daily life
(combustion - oxides of nitrogen)
Process: combustion
→ sources and effects of common air pollutants:
3. Oxides of nitrogen
* sources: result of reaction between oxygen and nitrogen ✓nutrients to soil
→ durning lightning strikes
→ combustion of fuels in motor vehicles, power stations and factories at very high temperatures
✘ effects: dissolve in rainwater: forms acid rain (sulfur oxide + nitrogen oxide) - corrosive status
→ harms marine lives
→ damage buildings made of limestone (contains carbonate which can react with acids) particularly
☞ normal rain: wet depositon
☞ acid rain: dry deposition
☞ acid rain: pH 3
chemical change caused by: heat or increased in temperature
- combustion
- a scientific term for burning
- a chemical reaction in which a substance is heated in the presence of oxygen to form one or more new substances
- heat and/or light are usually given out during combustion
- an important chemical reaction (e.g. burning of fuels such as petrol, natural gas and coal provides more than 90% of energy needed for transport, industries and home)
☆examples: burning of hydrogen as a fuel for rockets [ hydrogen + oxygen → water ]
☆ burning methane for cooking [ methane + oxygen → carbon dioxide + water ]
☆ burning charcoal for grilling food [ carbon + oxygen → carbon dioxide ]
chemical change caused by: heat or increased in temperature
(complete combustion)
- complete combustion: takes place when a substance burns with a sufficient amount of oxygen (carbon dioxide is produced)
chemical change caused by: heat or increased in temperature
(incomplete combustion)
- incomplete combustion: takes place when a substance burns with an insufficient amount of oxygen (carbon monoxide and/or soot are produced)
chemical change caused by: heat or increased in temperature
- thermal decomposition
- a single compound breaks into two or more simpler substances when heated
☆ examples: decomposition of metal carbonates
calcium carbonate → calcium oxide + carbon dioxide (carbon: comes from carbonate)
☆ decomposition of metal hydroxides
magnesium hydroxide → magnesium oxide + water
