11Chp 7: CHEMICAL Chp 7: CHEMICAL REACTIONSREACTIONS

Reactants: Zn + IReactants: Zn + I22 Product: Zn IProduct: Zn I22

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Evidence of Reactions

• Just because the evidence is there DOES NOT mean a chemical reaction is taking place

•Have to look at everything that is going on

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Signs of a Reaction• Formation of a gas:

– Bubbles– Smoke– Odors/fumes

• Change in color (be careful, some changes in color are physical changes).

• Formation of a solid (precipitate – ppt)– Cloudy– Foggy– Solid at the bottom

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Signs of a Reaction

•Change in heat or light energy–Gets warm, hot, cold–Sparks or explodes–Glows

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Exothermic Reactions• Give off energy as heat or light

because–Energy is stored in chemical

bonds.– In EXOthermic reactions there

is MORE energy stored in bonds of reactants than needed to form products! So.. there is left over energy (which is released)!

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Endothermic Reactions• Absorb energy as heat or light

because –Because the energy stored in

the bonds of the reactants is NOT ENOUGH to hold together the products. MORE ENERGY IS NEEDED (so it must be absorbed – from the surroundings, a burner, or a hot plate)!

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• The way atoms are joined is changed• Atoms aren’t created or destroyed.

• Can be described several ways• In a sentence • Copper reacts with chlorine to form

copper (II) chloride.• In a word equation• Copper + chlorine copper (II)

chloride

In a chemical reaction

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Because of the principle of the Because of the principle of the

conservation of matterconservation of matter, ,

an an equation must be equation must be balancedbalanced..

It must have the same It must have the same number of atoms of the number of atoms of the same kind on both sides.same kind on both sides.

Lavoisier, 1788Lavoisier, 1788

Chemical EquationsChemical Equations

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Chemical Chemical EquationsEquations

Their Job: Depict the kind of Their Job: Depict the kind of reactantsreactants and and productsproducts and and their relative amounts in a reaction.their relative amounts in a reaction.

44 Al Al (s)(s) + + 33 OO2 (g)2 (g) ---> ---> 22 AlAl22OO3 (s)3 (s)

The The numbers in the frontnumbers in the front are called are called

stoichiometric coefficientsstoichiometric coefficients

The letters (s), (g), and (l) are the The letters (s), (g), and (l) are the physical states.physical states.

1010Symbols used in equations

• the arrow separates the reactants from the products

• Read “reacts to form” or “yields”• The plus sign means “and”

• (s) after the formula –solid (sometimes could say (cr)-crystal

• (g) after the formula -gas• (l) after the formula –liquid

1111Symbols used in equations(aq) after the formula means

dissolved in water, an aqueous solution.

used after a product indicates a gas (same as (g))

used after a product indicates a solid (same as (s))

1212Symbols used in equations

• indicates a reaction in, or going to, equilibrium (More later)

• or heat shows that heat is supplied to the reaction

• Pt is used to indicate a catalyst used supplied, in this case, platinum.

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What is a catalyst?

• A substance that speeds up a reaction without being changed by the reaction.

• Enzymes are biological or protein catalysts.

1414Rates of ReactionsIn order for a reaction to occur:• The particles must collide (touch)• They must collide with enough energy• They must collide in the right

orientation

Factors that affect reaction rate:• Temperature (particle energy)• Particle size• Surface area• Particle contact (stirring)

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Balanced Equation• Atoms can’t be created or

destroyed

• All the atoms we start with we must end up with

• A balanced equation has the same number of each element on both sides of the equation.

1616Balancing EquationsBalancing Equations

___ H___ H22(g) + ___ O(g) + ___ O22(g) ---> ___ H(g) ---> ___ H22O(l)O(l)2 2

What Happened to the Other Oxygen Atom?????

This equation is not balanced!

Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

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–When balancing, you may add coefficients in front of the compounds to balance the

reaction, but you may notnot change

the subscripts.

– Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (by balancing the charges for ionic, or shared electrons for covalent)

Balancing EquationsBalancing Equations

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Chemical EquationsChemical EquationsChemical EquationsChemical Equations4 Al(s) + 3 O4 Al(s) + 3 O22(g) (g)

---> 2 Al---> 2 Al22OO33(s)(s)

This equation meansThis equation means

4 Al atoms + 3 O4 Al atoms + 3 O22 molecules molecules

---produces--->---produces--->

2 molecules of Al2 molecules of Al22OO33

AND/ORAND/OR

4 moles of Al + 3 moles of O4 moles of Al + 3 moles of O22

---produces--->---produces--->

2 moles of Al2 moles of Al22OO33

1919Subscripts vs. Subscripts vs. CoefficientsCoefficients

• The subscripts The subscripts tell you how tell you how many atoms of many atoms of a particular a particular element are in a element are in a compound. The compound. The coefficient tells coefficient tells you about the you about the quantity, or quantity, or number, of number, of molecules of molecules of the compound.the compound.

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There are four basic steps to balancing a chemical equation.1. Write the correct formula for the reactants and the

products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, DO NOT CHANGE THE FORMULAS!

2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

4. Check your answer to see if:– The numbers of atoms on both sides of the

equation are now balanced.– The coefficients are in the lowest possible whole

number ratios. (reduced)

Steps to Balancing Steps to Balancing EquationsEquations

2121Some Suggestions to Help Some Suggestions to Help YouYou

Some Helpful Hints for balancing equations:• Take one element at a time, working left to right

except for H and O. Save H for next to last, and O until last.

• IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

• (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

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