Chemical Reactions

Chemical Change How do you know if a chemical change has taken place? How do you know if a chemical change has taken place? All chemical changes are results of chemical reactions. All chemical changes are results of chemical reactions.

Chemical Reactions Chemical Reactions: occur when bonds are broken and a new product is formed. (Chemical Change) Chemical Reactions: occur when bonds are broken and a new product is formed. (Chemical Change) Involve the rearrangement of atoms Involve the rearrangement of atoms

Chemical reactions Chemical formulas are used to write chemical equations. Chemical formulas are used to write chemical equations. The chemical compounds used to start the reaction are Reactants. The chemical compounds used to start the reaction are Reactants. The chemical compounds at the end of the reaction are Products. The chemical compounds at the end of the reaction are Products.

Reaction Symbols There are several types of Reaction symbols used. There are several types of Reaction symbols used. + plus (added to) + plus (added to)  forms (produces or yields)  forms (produces or yields) g – gas g – gas s – solid s – solid l – liquid l – liquid aq—aqueous solution aq—aqueous solution

Law of Conservation of Mass Matter cannot be created or destroyed. It is always conserved in a reaction. Matter cannot be created or destroyed. It is always conserved in a reaction. Atoms cannot simply appear or disappear. Atoms cannot simply appear or disappear. The amount used at the start of the reaction is equal to the amount at the end of the reaction. The amount used at the start of the reaction is equal to the amount at the end of the reaction. # of atoms of REACTANTS = # of atoms of PRODUCTS # of atoms of REACTANTS = # of atoms of PRODUCTS

Take a look at this… Does it look balanced? Does it look balanced?

We must the get the O 2 to balance.. Is this balanced now? Is this balanced now?

And now….Is it balanced? YES! YES!

Practice Counting Atoms MgCl 2 Mg= 1 atom of magnesium Cl= 2 atoms of chlorine H 2 SO 4 S= 1 O= 4 H= 2

More counting! NH 4 Cl N= 1 H= 4 Mg(NO 3 ) 2 Mg= 1 N= 2 O= 6 Cl= 1 (NH 4 ) 3 PO 4 N= 3 H= 12 P= 1 O= 4

Balanced Equations In order to conserve atoms, we must balance the number and type of atoms. In order to conserve atoms, we must balance the number and type of atoms. CAN’T CHANGE SUBSCRIPTS! CAN’T CHANGE SUBSCRIPTS! Can only change the numbers of atoms on each side of the equation by adding COEFFICIENTS! Can only change the numbers of atoms on each side of the equation by adding COEFFICIENTS! Coefficients: the numbers that come in front of a chemical formula. Coefficients: the numbers that come in front of a chemical formula.

Let’s Try to Figure out Coefficients How many do you have of each? How many do you have of each? 3 H 2 02 H 2 O 3 3 H 2 02 H 2 O 3 4 Fe 2 O 3 6 NaCl 4 Fe 2 O 3 6 NaCl NO 2 NO 2

Steps to Balancing First, count the number and type of each atom on both sides First, count the number and type of each atom on both sides If there are different numbers of atoms for an element on both sides, you must add coefficients to compounds to change the number of atoms If there are different numbers of atoms for an element on both sides, you must add coefficients to compounds to change the number of atoms

Second, figure out what coefficient is needed in order to make the numbers of atoms equal. Second, figure out what coefficient is needed in order to make the numbers of atoms equal. REMEMBER: IF YOU CHANGE A COEFFICIENT, ALL ELEMENTS IN THAT COMPOUND ARE AFFECTED. REMEMBER: IF YOU CHANGE A COEFFICIENT, ALL ELEMENTS IN THAT COMPOUND ARE AFFECTED.

Magnesium + Oxygen (g) Magnesium Oxide Magnesium + Oxygen (g) Magnesium Oxide Mg MgO

Now try on your own… C + Cl 2 CCl 4 C + Cl 2 CCl 4 Al 2 O 3 Al + O 2 Al 2 O 3 Al + O 2

Helpful Hints to Balancing If an element appears in more than one compound on the same side of the equation, leave that element until LAST to balance If an element appears in more than one compound on the same side of the equation, leave that element until LAST to balance

Other Helpful Hints If there is an even number of atoms of an element on one side of the equation and an odd number on the other, place a coefficient of 2 in front of the ODD compound If there is an even number of atoms of an element on one side of the equation and an odd number on the other, place a coefficient of 2 in front of the ODD compound

Energy in Reactions Energy is either released or absorbed Energy is either released or absorbed Exothermic or endothermic Exothermic or endothermic

Exothermic v. Endothermic Exothermic rxns produce energy (RELEASE!) so the temperature rises! Exothermic rxns produce energy (RELEASE!) so the temperature rises! Endothermic rxns require a lot of energy (ABSORBS!) so the temperature drops! Endothermic rxns require a lot of energy (ABSORBS!) so the temperature drops!

Exothermic Reactions LESS energy is required to break the bonds in the reactants than is released when bonds are formed to make new products LESS energy is required to break the bonds in the reactants than is released when bonds are formed to make new products More energy is released when new products form More energy is released when new products form

Endothermic MORE energy is required to break the bonds of the reactants than is released from the formation of new bonds MORE energy is required to break the bonds of the reactants than is released from the formation of new bonds Energy is being pulled into the reaction from outside Energy is being pulled into the reaction from outside

THE END! :0)