Science - Acids & Bases

What is an acid?

• An acid is a solution that releases H+ (cation) into an aqueous solution

• The more H+ ions, the more acidic the solution

• Acids are substances that give away H+ ions.

Common acids:

Acetic acid (vinegar), sulfuric acid (in paints), citric acid (oranges), and ascorbic acid (vitamin C)

Properties of acids

•Tastes Sour

•Conduct Electricity

•Corrosive, which means they break down certain substances.

•Some acids react strongly with metals

•Turns blue litmus paper red

•Have a pH less than 7

What is a base?

• A base is a solution that forms hydroxide ions (OH^-) ions when in water.

• Another word for base is alkali.

• Bases are substances that can accept H⁺ ions.

Common bases?

Soap, ammonia (cleaning products)

Properties of bases:

•Feel Slippery

•Taste Bitter

•Corrosive

•Can conduct electricity. (eg alkaline batteries.)

•Do not react with metals.

•Turns red litmus paper blue.

•Have a pH higher than 7 up to 14.

The pH scale:

• pH is a measure of how acidic or basic a solution is.

• •The pH scale ranges from 0 to 14.

• Acidic solutions have pH values below 7

• A solution with a pH of 0 is very acidic.

• A solution with a pH of 7 is neutral. (Pure water has a pH of 7. )

• Basic solutions have pH values above 7.

Indicators:

•Indicators give an approximate idea of the pH of a substance

Examples of Indicators:

methyl orange, phenolphthalein and universal indicator, and litmus paper

Strong acids:

•Strong acids dissociate (break apart into ions) completely in water.

Examples of strong acids:

•Nitric acid

•Hydrochloric acid

•Sulfuric acid

Weak acids:

•Weak acids do not dissociate completely in water

Examples weak acids:

•Acetic acid (vinegar)

•Citric acid

Acid-Base reactions

•A reaction between an acid and a base is called neutralization.

•When we react an acid and a base, you get a salt and water. (acid + base → a salt + water)

•An acid-base mixture is not as acidic or basic as the individual starting solutions.

Acid-Metal reactions

•When an acid reacts with a metal it produces a salt and hydrogen gas (metal + acid → a salt + hydrogen)

Acid-Carbonate reactions

• Acids will react with metal carbonate compounds to produce a salt (ionic compound), water and carbon dioxide gas (acid + metal carbonate → salt + water + carbon dioxide)

Covalent Bonding

• Covalent bonds are formed when non-metal atoms share electrons.

• They differ from ionic bonds bc there is no direct transfer of electrons when the bond is formed

• Non-metals have high electronegativity so they can attract easily but don't give up electrons easily

• when two non-metallic atoms come into contact they both want to stabilize their outer shell (complete octet rule)

• By sharing the single unpaired electrons, both atoms can stabilize their outer shells

Covalent Bonding (electron sharing)

• A covalent bond forms when two positive nuclei are simultaneously attracted to a shared pair of electrons.

• Covalent molecules tend to be discrete - they do not form repeating crystal lattice structures but exist as individual molecules, such as H₂O.

Properties of Covalent Molecules

• Low melting and boiling points (liquids or gases at room temperature), which means that the forces of attraction between their particles must be very weak.

• Poor conductors of electricity because there are no charged particles free to move through the structure.

Double & Triple Bonds

• Sometimes an element needs to share more than one electron to obtain a full valence shell.

• If an element shares two electrons with another element, it is called a double bond

• If an element shares three electrons with another element, it is called a triple bond

• Element can not share four electrons – the repulsion between the electrons is too great.

Metallic Bonding

• A metal is an element that loses its outer shell electrons to achieve stability.

• When this happens, the metal atom becomes a positively charged ion (cation) with a full outer shell.

• Metals typically have properties such as:

  • Malleability

  • Ductility

  • Electrical conductivity

  • Crystalline structures

What is Metallic Bonding

Metallic bonding is the electrostatic attraction between the positive ions and the delocalized electrons.

Properties of Metals

•Good conductors of heat & electricity

•Malleable (i.e. they can be hammered, bent or rolled into sheets or other shapes).

•Ductile: They can be drawn out into wires

•Lustrous / reflective when freshly cut or polished (i.e. have a shiny surface)

•Range of Melting points but most are quite high

•High densities

Determining the charge on the cations in the lattice

• The charge on metal cations depends on the type of metal (e.g. Na⁺ for sodium, Mg²⁺ for magnesium).

• The sea of delocalised electrons comes from the valence electrons of metal atoms.

• Magnesium forms Mg²⁺ ions, so its lattice has twice as many delocalised electrons as sodium, leading to stronger bonding and more free-moving electrons.

Modifying Metals

• Very few metals are used in their pure form

• Metals can also be modified by heat treatments or through the application of coatings.

Alloying

•Alloys are formed by mixing a metal with other metals.

•Sometimes non-metallic atoms are used to form an alloy.

•Alloys are prepared by melting the metals together and cooling the mixture to form the new solid material.

•Compared to their component metals alloys are generally

•Harder

•less malleable

•poorer conductors than their pure form 

Alternatives to Alloys

• The behaviour of a metal depends partly on the size of its atoms and how they are arranged.

• The metallic structure can be altered in different ways:

  • Work hardening – rearranges atoms into tighter groups.

  • Heat treatment – changes the size of atom groupings.

Work Hardening

• Hammering or cold working metals rearranges the atoms.

• Smaller grains are formed in the metal structure.

• The metal becomes harder but more brittle.

Heat treatment: Annealing

•Annealing is heating metal to a moderate temperature & then leaving it to cool slowly.

•This allows larger groups of atoms to form.

•The metal produced is softer and more ductile.

Heat treatment: Quenching

•Quenching is heating metal to a moderate temperature & then cooling it quickly (quenching it).

•Quenching allows tiny groups of atoms to form.

•The metal produced is harder but also brittle.

Heat treatment: Tempering

•In tempering, quenched metals are warmed again at a lower temperature and allowed to cool slowly.

•This reduces the brittleness of the material while retaining the hardness

Decomposition Reactions

•This is where a single substance is broken apart into 2 or more simpler substances

•There will be one reactant and 2 or more products

(for decomposition to occur, heating might take place)

Synthesis or Combination Reactions 

•Involve 2 or more substances combining to make a new substance

Single Displacement Reactions

•Where one element or group of elements is removed and replaced by another element or group of elements

Double Displacement Reactions

2 compounds completely swap ions with each other

•Swaps occur simultaneously, making 2 new compounds

•One product is often a solid (precipitate)

Precipitation Reactions

• Some double displacement reactions produce a solid called a precipitate.

• A precipitate forms when two soluble reactants combine to make an insoluble product.

• Soluble = dissolves; Insoluble = does not dissolve.

Combustion Reactions

•Occurs when a fuel (usually a hydrocarbon) combines with oxygen to form carbon dioxide and water