Chemical Reactions Physical and Chemical Change Law of Conservation of Mass Balancing Equations

Chemical Reactions

Physical and Chemical Change

Law of Conservation of MassBalancing Equations

Chemical Reactions

Chemical reactions are used in many ways in daily life.

A chemical reaction is the process by which a chemical change happens.

All chemical reactions are also accompanied by changes in energy

Chemical Reactions

Some chemical reactions absorb energy, such as in the chemical reactions that cook food.

Chemical Reactions

Other chemical reactions release energy in the form of heat, light and/or sound, such as the burning of wood in a campfire

Chemical reactions happen at different rates. Some chemical reactions are fast, such as when rocket fuel burns.

Chemical Reactions

Chemical Reactions

Other chemical reactions happen slowly, such as the formation of rust on a corroding bicycle chain.

The chemical reactions in your own body, which are keeping you alive, are among the fastest chemical reactions known.

Chemical Reactions

Chemical Reactions

Scientists are constantly working to find new kinds of chemical reactions in order to produce new substances with useful properties.

All chemical reactions involve the conversion of starting materials, called reactants, into new substances, called products. The products have different properties than the reactants.

Chemical Reactions

Physical Properties

A description of a substance that does not involve forming a new substance.

Examples: Colour Texture State Density Solubility Melting point

Chemical Properties

A description of what a substance does as it changes into one or more new substances.

Examples:CombustibilityCorrosionReaction with acidBleaching ability

Properties & Change

Properties are descriptions similar to an adjective: describes what the substance is like.

Change are descriptions similar to a verb: describes what the substance is doing

A physical change is the change in the state or form of a substance that does not change the original substance.

A physical change can result in new physical properties but not new chemical properties.

Physical Change

Classes of physical change:1. Change in state (includes

dissolving)2. Change in form

Example: Evaporation Cutting paper in half

Physical Change

A chemical change is the transformation of one or more substances into new substances with new properties

Chemical change

Presence of a new colourFormation of a precipitate

Visual Clues to a Chemical Change

Release of heat or light


Production of gas or bubbles

ExampleReactants: Solid magnesium metal placed into a solution of hydrochloric acidClue: bubbles / gases Product: hydrogen gas and magnesium chloride


A chemical reaction is often described by writing a chemical equation

Chemical Equations

A chemical equation uses either words or symbols and formulas to describe the changes that occur during a chemical reaction.

Examples

Word equation:Hydrogen gas + oxygen gas water

Formula equation:H2 + O2 H2O

Chemical Equations

Chemical Equations

Every chemical equation must have:One or more reactantsOne or more productsAn arrow directing reactant to

product If there are more than one reactants

or products, the chemical names/formulas are separated by a ‘+’ sign

Chemical Equations

For example, the chemical reaction between solid magnesium metal and hydrochloric acid is:

Notice that hydrogen is expressed in the formula equation as H2. Recall that pure hydrogen exists as a diatomic molecule.

You will need to know which elements exist as molecules when writing formula equations

word equation:magnesium + hydrochloric acid magnesium chloride + hydrogenword equation:magnesium + hydrochloric acid magnesium chloride + hydrogen

formula equation:Mg + HCl MgCl2 + H2

formula equation:Mg + HCl MgCl2 + H2

States of Matter in Chemical EquationsThe chemical formulas in a chemical equation will often include the state of matter of each substance (s) = solid(l) = liquid (e.g. water and oils)(g) = gas (aq) = aqueous (substance is dissolved in water, e.g. most ionic compounds)

Examples: H2 (g) + O2 (g) H2O (l)Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

Coefficients in Chemical Equations

A coefficient is a whole number that is placed in front of the symbol of an element to show the ratios of the different substances that are present in the chemical reaction

Example: Mg + HCl MgCl2 + H2Mg + 2HCl MgCl2 + H2

A coefficient of 2 is in front of the formula HCl This means that Mg and HCl combine in a ratio of 1:2

In a chemical reaction, the mass of the products always equals the mass of the reactants.

In other words, the mass is conserved.

Law of Conservation of Mass

No atoms are destroyed and no new atoms are produced during a chemical reaction.

Instead, the atoms in the reactants are simply rearranged to form the products

Chemical bonds between atoms are broken and new ones are formed, and the atom simply reconnect in new ways

Conservation of Mass

The rearrangement of atoms that occurs during a chemical reaction can be illustrated using models or diagrams.

In this equation, there are equal numbers of hydrogen atoms (4) and equal numbers of oxygen atoms (2) on both the reactants side and the products side.

word equation: hydrogen + oxygen waterformula equation: H2 + O2 H2O


When the number of each kind of atom is the same in the reactants and products, the equation is said to be balanced.

balanced equation: 2H2 + O2 2H2O



A chemical equation that is complete except for coefficients is called an unbalanced equation or skeleton equation.

Example:Skeleton equation: H2 + O2 H2OBalanced equation: 2H2 + O2 2H2O

Skeleton Chemical Equations

Balancing Chemical Equations

To balance a chemical equation, begin by counting the number of atoms of each element in the skeleton equation.

Balance by placing coefficients in front of the chemical formulas until the number of atoms in the reactants equals to the products.

Rules for using coefficients

Use only whole numbers.Check that the coefficients in the

equation are the lowest common factor.

Never change a subscript in a formula to help make atoms balance!

Balance atoms of elements in any complicated looking formulas first and balance atoms of pure elements last.

H2H2

Hints to help balance equations


Hydrogen atoms and/or oxygen atoms will often appear in many or all of the formulas of the reactants and products.

When this is the case, balance other elements first, balance hydrogen second last and oxygen last.

You may be able to treat polyatomic ions as a unit.

Example: If NO3- appears in the reactants

and products of a skeleton equation, count the number of NO3

- groups rather than the number of N and O atoms separately.


Example 1:Balance the following chemical equation:

AlBr3(s) + Cl2(g) AlCl3(s) + Br2(g)1.) Count the number of atoms in the reactants

and products:


Example 1:2AlBr3(s) + Cl2(g) AlCl3(s) + 3Br2(g)

Balance the number of bromine atoms by adding a coefficient of 2 in front of AlBr3 and a coefficient of 3 in front of Br2. Count the atoms again:


Example 1:2AlBr3(s) + Cl2(g) AlCl3(s) + 3Br2(g)

The number of aluminum atoms is no longer equal.


Example 1:2AlBr3(s) + Cl2(g) 2AlCl3(s) + 3Br2(g)

Balance the number of aluminum atoms by adding a coefficient of 2 in front of AlCl3. Count the atoms again:


Example 1:2AlBr3(s) + Cl2(g) 2AlCl3(s) + 3Br2(g)

The number of chlorine atoms is no longer balanced.


Example 1:2AlBr3(s) + 3Cl2(g) 2AlCl3(s) + 3Br2(g)

Balance the number of chlorine atoms by adding a coefficient of 3 in front of Cl2. Count the atoms again:


Example 1:2AlBr3(s) + 3Cl2(g) 2AlCl3(s) + 3Br2(g)

The equation is balanced!


Try it!Balance the following chemical

equations:

1. Al + F2 AlF3

2. Ca + H2O Ca(OH)2 + H2

3. CaCl2 + Na3PO4 Ca3(PO4)2 + NaCl


Try it!Balance the following chemical

equations:

1. 2Al + 3F2 2AlF3

2. Ca + 2H2O Ca(OH)2 + H2

3. 3CaCl2 + 2Na3PO4 Ca3(PO4)2 +

6NaCl


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Chemical Reactions Physical and Chemical Change Law of Conservation of Mass Balancing Equations