Unit 11: Chemical Changes

• A physical change is reversible and does not form new substances. This change is temporary. • A chemical change is a change in which a new substance is formed. • A chemical change is usually irreversible and forms new substances. This change is permanent. • E.g. iron rusting, baked cake • When a new substance is made from a chemical change, the process is called a chemical reaction. • In a chemical reaction, substances (called reactants) react with each other to form new substances (called products). Usually energy such as thermal, light and/or electricity is taken in or given out during a chemical reaction. • The starting substances are the reactants and the new substance formed is called the product. • Chemical reactions obey the law of conservation of energy. • In a chemical change, energy is neither created nor destroyed, they are either transformed or transferred from one form to another. • Chemical reactions obey the law of conservation of mass. • In a chemical change, matter is neither created or destroyed in a reaction; they are only rearranged. Mass is conserved. (unchanged)

• A chemical change is a change in which a new substance is formed.

• When a new substance is made from a chemical change, the process is called a chemical reaction.

• When a new substance is made from a chemical change, the process is called a chemical reaction. • In a chemical reaction, substances (called reactants) react with each other to form new substances (called products). Usually energy such as thermal, light and/or electricity is taken in or given out during a chemical reaction. • The starting substances are the reactants and the new substance formed is called the product.

• Chemical reactions obey the law of conservation of energy. • In a chemical change, energy is neither created nor destroyed, they are either transformed or transferred from one form to another.

• Chemical reactions obey the law of conservation of mass. • In a chemical change, matter is neither created or destroyed in a reaction; they are only rearranged. Mass is conserved. (unchanged)

• Bubbles of gas are formed (effervescence of odourless gas) • Change in appearance (state & colour) • A new odour may be noticed • The change is difficult or impossible to reverse • thermal or light energy is given off (or absorbed), Change in temperature • A sound is formed • Precipitation (solid formed from mixing two aqueous solutions)

there are 7 1. Combustion (burning) 2. Oxidation, Oxidation (Rusting) 3. Decomposition (Thermal / Electrolysis) 4. Precipitation 5. Neutralisation 6. Respiration 7. Photosynthesis

• Combustion (aka burning) * Involves oxygen as reactant and release of energy (usually thermal) - e.g. carbon + oxygen → carbon dioxide

• Oxidation * Oxidation is the addition of oxygen to a substance. - e.g. carbon + oxygen → carbon dioxide - e.g. magnesium + oxygen → magnesium oxide • Oxidation (Rusting) • Oxidation is the addition of oxygen to a substance. • e.g. iron + oxygen + water → hydrated iron (III) oxide (reddish-brown solid)

• Decomposition (Thermal) * Involves breakdown of a compound into simpler substances. * Involves strong heating - e.g. sugar → carbon + water vapour • Decomposition (Electroylsis) • Involves breakdown of a compound into simpler substances • Involves passing electricity • e.g. lead bromide → lead + bromine

• Precipitation * Involves the formation of a solid when two solutions are mixed.

• Neutralisation * The reaction of an acid and alkali to form a new substance. - e.g. acid + alkali → salt + water

• Respiration * process by which the cells in living organisms get energy. - e.g. glucose + oxygen → carbon dioxide + water

• Photosynthesis * Process by which green plants transform light energy into chemical energy. - carbon dioxide + water → glucose + oxygen (chlorophyll & light required)

• Solutions with pH values that are less than 7 are acidic. • The lower the pH value, the more acidic the solution. • For example, the pH of milk is approximately 6.5, while the pH of vinegar (ethanoic acid) is 2. • Solutions with pH values that are greater than 7 are alkaline. • The greater the pH value, the more alkaline the solution. • For example, the pH of baking soda is approximately 9.5, while the pH of oven cleaner is 13.5 • Substances which are very acidic or alkaline are usually corrosive and can burn our skin. • If a solution is neither acidic nor alkaline, we say that it has a neutral pH or a pH of 7 (Green universal indicator). • The pH of pure water is 7.

• Litmus Paper • pH Scale • Natural Indicators

• We can use the pH scale to measure how acidic or alkaline a solution is. The values of the pH scale range from 0 to 14. • How to write observation: “Green universal indicator turned (colour)/remained green”

• In the school laboratory, litmus paper is often used to test whether a solution is acidic or alkaline. • An acidic solution turns blue litmus paper red. It has no effect on red litmus paper. • An alkaline solution turns red litmus paper blue. It has no effect on blue litmus paper. • A neutral solution has no effect on red and blue litmus paper. • Red litmus paper = acidic, Blue litmus paper = alkaline (follows pH scale colours)

Natural Indicators * We can find pH indicators in nature too. They are called natural indicators. Many of these natural indicators can be prepared using plants.

• The number of valence electrons elements possess determines the type of chemical reactions the elements undergo, the type of bonding they formed and their reactivity. • Why only valence electrons involved in chemical reactions - Valence electrons are further away from the nucleus as compared to electrons from inner electron shells. Therefore, valence electrons experience lesser force of attraction from the protons in the nucleus. • In the modern Periodic Table, elements are arranged according to their proton number / atomic number. • Atoms of elements in the same group in the Periodic Table have the same number of valence electrons (except for helium). • Elements in the same group have similar chemical properties and will undergo the same type of chemical reactions. • Atoms of elements in the same period in the Periodic Table have the same number of electron shells. • Across the period, the properties of elements change from metallic to non-metallic. • Group = vertical row, Period = horizontal row • The number of electrons = number of protons (which means that atomic/proton number = no. of electrons) • Positively charged = net change >0 • Electrically neutral = net charge = 0 • Negatively charged = net charge <0 • Each electron shell can hold a certain maximum number of electrons. • 1st electron shell - 2 electrons • 2nd electron shell - 8 electrons • 3rd electron shell - 8 electrons (1st 20 elements only) • Atoms with a complete (fully occupied) valence shell are stable. They will not undergo chemical reaction under normal conditions. • E.g. noble gases. (18th group) • Note: Hydrogen, Nitrogen, Oxygen and group 17 elements exist as diatomic molecules! (i.e. they have 2 molecules)

• The noble gases are elements in Group 18 of the periodic table. • Elements in Group 18 are monatomic because they have full valence shells and are said to have a noble gas electronic configuration. • Each noble gas has eight valence electrons (octet electronic configuration) except helium, which has two (duplet electronic configuration). • Since noble gases are stable, other atoms will bond together to achieve a noble gas electronic configuration. • During the collision, the valence electrons of the colliding atoms interact with atoms of a different element to found compounds. • Ionic bond is the electrostatic force of attraction between oppositely charged ions. • It is formed when the metal atom transfer valence electron/s to the non-metal atom. • Metallic atoms usually have 1 to 3 valence electrons while non-metallic atoms usually have 4 to 8 valence electrons. • An ion is a charged particle that is formed when an atom gains or loses electrons. • Metal ions are called cations - Metal atoms tend to lose electrons and form positive ions. • Non-metal ions are called anions - Most non-metal atoms will gain electrons and form negative ions. • When number of protons = number of electrons → neutral (atomic number = number of protons and electrons) • When number of protons > number of electrons → positively charged → positive ion → cation (usually metals) • When number of electrons > number of protons → negatively charged → negative ion → anion (usually non-metals) • “(name) atom gain/lose electrons to form …” • Ionic bonding only happens between a metal and non-metal atom! • The metal electron should not have any valence electrons left so if you cant fit all the valence electrons in the non-metal ion, then add more metal/non-metal atom until both have full valence electron shell! (same for non-metal electron) • The substance usually changes name after ionic bonding (usually only the non-metal changes name). • To name the ions of Group 17 elements, we change the ending of the ion’s name from -ine to -ide. • e.g. Fluorine → Fluoride, Chlorine → Chloride, Oxygen → Oxide, Nitrogen → Nitride, Hydrogen → Hydride, Sulfur → Sulfide

• An ion is a charged particle that is formed when an atom gains or loses electrons. • Metal ions are called cations - Metal atoms tend to lose electrons and form positive ions. • Non-metal ions are called anions - Most non-metal atoms will gain electrons and form negative ions.

• Ionic bond is the electrostatic force of attraction between oppositely charged ions. • It is formed when the metal atom transfer valence electron/s to the non-metal atom. • Metallic atoms usually have 1 to 3 valence electrons while non-metallic atoms usually have 4 to 8 valence electrons.