Changes in Matter

Physical Changes - changes in state or appearance that do not result in a change in the chemical composition of the material

• Examples : Melting, Boiling, Crushing, Dissolving in Water

Chemical Changes - changes that involve the formation of a new substance or substances. These changes usually but not always result from interaction of one substance with a different substance.

• Examples : Combustion, Digestion, Decomposition, Corrosion

How can we tell if a chemical or physical change has occurred?

There usually are multiple signs that a chemical change has occurred. These sign may be:

• formation of a precipitate
• formation of a gas
• change in color
• heat released or absorbed.

In the case of a chemical change, a new substance with its own properties is produced. Physical changes can also release or absorb heat (freezing or melting), may form a precipitate (crystallization during freezing) or form a gas (boiling) so it is usually necessary to look for more than one indicator.

Chemical Equations

Chemical equations are a way we can represent a chemical change. Rather than using names of the elements and compounds that react, symbols and formulas are used.

ie.) The combustion of methane (CH₄) can be represented by the equation:

CH_{4 (g)}+ 2 O_2 (g)-- CO_2 (g) + 2 H₂0 _(g)

The symbols and formulas on the left of the arrow are the reactants. Those on the right are products. We make sure the equations obey the law of conservation of mass by placing coefficients in front of the symbols or formulas.

Symbols and formulas may be accompanied by state symbols in parenthesis immediately after the formula or symbol.

• (s) or (c) represents a solid
• (l) represents a liquid
• (g) represents a gas
• (aq) represents an aqueous  (water based) solution

Types of Equations

Equations fall into several categories. The major categories are:

1) Synthesis: 
A + B -- AB

2) Decomposition: 
AB -- A + B

3) Single Displacement : 
A + BC -- AC + B or   X + YZ -- YX + Z

4) Double Displacement : 
AB + CD -- AD + CB

5) Combustion of hydrocarbons 
HC + O₂ -- CO₂ + H₂O

The majority of reactions probably do not fall into these categories, but most of the equations we use in this course will fall into one of the above. While classifying equations just for the sake of classifying them may be futile, being able to recognize some types of equations can be helpful when you are predicting what might happen given two reactants.
 

Balancing Equations

As mentioned above, in order for an equation to obey the law of conservation of energy, it must be balanced. To balance an equation, one must adjust the coefficients, the numbers placed in front of the symbol or formula. One should never change the subscripts because this would change the compound. For example changing the subscript for oxygen in the water molecule from one to two would change the formula to hydrogen peroxide:

H₂O = Water

H₂O₂ = Hydrogen Peroxide

Balancing equations can be quite frustrating if you attack it in a trial and error manner. There are some "tricks" that you can use that make it more systematic and less trial and error. This can decrease the frustration factor and the time spent on these problems.

You can try your hand at balancing equations at the applet below.

http://www.dun.org/sulan/chembalancer/ques3.htm (you'll need to type a 1 in front of a symbol or formula if it doesn't have a coefficient, ie. 1 Na instead of just Na)
 

If a partial equation falls into a category listed above, we can use our knowledge of how elements combing and writing formulas and predict the products, write out their correct formula and balance the equation.

ie.)                                         Na  +  HCl -->  ?

If we look at this partial equation, we can see that it most closely resembles a single displacement equation. Since Na is a metal and in group IA, it will have a +1 oxidation state when it combines. This means that it will combine with the Cl ( with an oxidation number of -1) to form NaCl. The hydrogen is released as hydrogen gas (H₂). The unbalanced equation is:

Na  +  HCl --   NaCl  +   H₂

Inventorying both sides of the equation and adjusting the coefficients to balance, the equation becomes:

2 Na  + 2 HCl -- 2 NaCl  +  H₂
 

ie. 2)  AgNO₃  +  CaBr₂ -->  ?

Again, the partial equation seems to resemble one of the 5 types we discussed above. In this case, we see that it is a double displacement equation. We can determine the oxidation states of Ag (+1), NO3 (-1), Ca (+2) and Br (-1).The unbalanced equation is:

AgNO₃  +  CaBr₂ --  AgBr  + Ca(NO₃)₂

Inventorying both sides of the equation and adjusting the coefficients to balance, the equation becomes:

Energy and Chemical Changes

When we discussed physical changes like heating or cooling, melting or boiling we discussed 2 "accounts" in which energy was stored. One was the thermal (E_th) account and the other was the interaction (E_i) account. Chemical changes also involve energy transformations and the account that is involved with energy transactions is the chemical potential energy (E_ch) account.

 Reactions can either release or absorb energy. When energy is released, the temperature of the surroundings increases. This type of reaction is an exothermic reaction. Energy stored in the chemical account is transferred to the thermal account and then is released to the surroundings. Exothermic reactions result in an increase in the temperature of the surroundings.