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Chapter 9Chapter 9Balancing EquationsBalancing Equations
Parts 1 & 2Parts 1 & 2
Terms• A chemical reaction is the process by which
one or more substances are changed into new substances.
• A chemical equation uses symbols to represent a chemical reaction.
• A reactant or reagent is a substance used to begin a chemical reaction.
• A product is a substance that forms during chemical reaction.
Potassium metal is added to water.
K + HK + H22OO
Water can also be written HOHWater can also be written HOH
K + HOHK + HOH
What are the products?
What gas might be produced?
What has happened to the liquid?
The litmus test
• Blue litmus paper turns red under acidic conditions.
• Red litmus paper turns blue under basic conditions.
• If both litmus tests result in no change then the solution is neutral.
The resulting liquid is an acid.
Common acids contain hydrogen ion (H+).
The resulting liquid is a base.
Most common bases contain hydroxide (OHˉ).
What are the products?
K + HOH → base + gasK + HOH → base + gas
→+
K + HK + H22O → KOH + HO → KOH + H
→+
What is wrong here?What is wrong here?
K + HK + H22O → KOH + HO → KOH + H
→+
H,O,N,Cl,F,I,Br mean H should be HH,O,N,Cl,F,I,Br mean H should be H22
K + HK + H22O → KOH + HO → KOH + H
→+
H,O,N,Cl,F,I,Br mean H should be HH,O,N,Cl,F,I,Br mean H should be H22
22
K + HK + H22O → KOH + HO → KOH + H22
→+
Now what is wrong?Now what is wrong?
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 and type of atoms number and type of atoms
on both sides of the equation.on both sides of the equation. Lavoisier, 1788Lavoisier, 1788
Chemical EquationsChemical Equations
– When balancing a chemical reaction 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.
Balancing EquationsBalancing Equations
Subscripts vs. Subscripts vs. CoefficientsCoefficients
• The subscripts tell you how many atoms of a The subscripts tell you how many atoms of a particular element are in a compound. The particular element are in a compound. The coefficient tells you about the quantity, or coefficient tells you about the quantity, or number, of molecules of the compound.number, of molecules of the compound.
2H2H22OO
• Two water molecules. Each of which consist of Two water molecules. Each of which consist of two atoms of hydrogen and one atom of oxygen.two atoms of hydrogen and one atom of oxygen.
K + HOH → KOH + HK + HOH → KOH + H22
→+
We must balance the equation.We must balance the equation.
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, once you write them correctly 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.
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
_K + _HOH → _KOH + _H_K + _HOH → _KOH + _H22
2K + 2HOH → 2KOH + H2K + 2HOH → 2KOH + H22
2K + 2H2K + 2H22O → 2KOH + HO → 2KOH + H22
→+
Consider the form of these substances.Consider the form of these substances.
2K + 2H2K + 2H22O → 2KOH + HO → 2KOH + H22
→+
solid
liquid
gas
2K2K(s)(s) + 2H + 2H22OO(l)(l) → 2KOH + H → 2KOH + H2(g)2(g)
→+
solid
liquid
gas
What about the KOH?What about the KOH?
potassium hydroxide (KOH) is a solid at room temperature.
• However KOH is soluble in water. Substances dissolved in water are said to be aqueous and are indicated as such by an (aq) in the equation.
2K2K(s)(s) + 2H + 2H22OO(l)(l) → 2KOH + H → 2KOH + H2(g)2(g)
→+
solid
liquid
gas
aqueous solutionaqueous solution
(aq)(aq)
• Precipitate (↓)
• “Heated”
• Escaping gas () • Catalyst H2SO4
• A word may be written above an arrow to indicate something is necessary for the reaction to occur.
Other Symbols Used in Other Symbols Used in EquationsEquations
electricity
∆
Balancing Equation Strategy
• Balance elements that occur in only one compound on each side first.
• Balance free elements last.
• Balance unchanged polyatomic ions as groups.
• Fractional coefficients are acceptable but must be cleared at the end using multiplication.
152 6 7C6H14O4 + O2 → CO2 + H2O 6
2. Balance H.
2 C6H14O4 + 15 O2 → 12 CO2 + 14 H2O
4. Multiply by two
Balancing Example
3. Balance O.
and check all elements.
Chemical Equation:
1. Balance C.
6 7
Ca3(PO4)2 + H2SO4 → CaSO4 + H3PO4
• In this equation, the polyatomic ions do not break up.
• Instead of counting individual atoms, polyatomic ion groups may be counted.
Ca3(PO4)2 + H2SO4 → CaSO4 + H3PO4
Cu + H2SO4 → CuSO4 + H2O + SO2
• The sulfate group breaks up. Each atom must be counted individually.
Cu + H2SO4 → CuSO4 + H2O + SO2
Balancing Equations
• This method of balancing equations is the inspection method.
• The method is trial and error.
• You need to Practice.
Homework
• Worksheet: “Balancing Equations Ch. 9”