Chapter 7

Reactions

Chemical Changes

• Substances react and form a new substance.• Reactants go in.• Products come out.• Reactants Products

• Represented by chemical equations

Chemical Equations

• For the process of burning:• Word equation:• Carbon + Oxygen → Carbon dioxide• Chemical equation:• C + O2 → CO2

Law of Conservation of Mass

• Mass is neither created nor destroyed in a chemical reaction.

• It may change form (solid to liquid or gas).• If 1 atom of Carbon goes into a reaction, 1

atom of carbon must come out. It can’t be lost or multiplied.

Chemical Reactions

• C + O2 → CO2+ →

1 atom of carbon goes in and 1 atom of carbon comes out

2 atoms of oxygen go in and 2 atoms of oxygen come outMass is neither created nor destroyed.

Chemical ReactionsN2H4 + O2 → N2 + H2O

+ → +

Why is this wrong?

Balancing EquationsN2H4 + O2 → N2 + H2O

+ → + +

Why is this wrong?2 atoms of nitrogen go in and 2 come out4 atoms of hydrogen go in but only 2 come out2 atoms of oxygen go in but only 1 comes outMass appears to be lost and that cannot happen!!

Balancing Equations

• You can balance equations by changing the coefficients.

• Coefficients are numbers that come in front of the formulas.

• Never!!!! Change the subscripts in a formula!!• Changing subscripts makes it a different

formula.

Balancing Equations

• N2H4 + O2 → N2 + H2O

• N2H4 + O2 → N2 + 2H2O• The 2 in front of water is a coefficient. • Now 2 atoms of nitrogen go in and 2 come out.• 4 atoms of hydrogen go in and (2x2=4) 4 come out.• 2 atoms of oxygen go in and (2x1+2) 2 come out.

Steps for Balancing Equations

• Count the atoms of each element on each side of the equation.

• Then change coefficients 1 at a time until the equation is balanced.

• REMEMBER: You cannot lose or gain atoms!!

Practice

• Balance the following equations:

• Cu + O2 → CuO

• N2 + H2 → NH3

• K + Br2 → KBr• Na + Cl → NaCl

Practice Answers

• Cu + O2 → CuO

• Cu + O2 → 2CuO

• 2Cu + O2 → 2CuO• I add a coefficient of 2 to get 2 oxygen atoms

but that also makes me have 2 copper atoms as products. I then have to add a 2 in front of copper. Now 2 copper in and out and 2 oxygen in and out.

Practice Answers

• N2 + H2 → NH3

• N2 + H2 → 2NH3

• First I add a 2 to the product to end with 2 nitrogen atoms, but now I am ending with 6 hydrogen atoms.

• N2 + 3H2 → 2NH3

• Adding a 3 in front of hydrogen gives me 6 atoms in the beginning as well.

Practice Answers

• K + Br2 → KBr

• K + Br2 → 2KBr• First I add a 2 to the product to end with 2

bromine atoms, but now I am also ending with 2 potassium atoms.

• 2K + Br2 → 2KBr• Adding a 2 in front of potassium fixes this.

Practice Answers

• Na + Cl → NaCl• 1 atom of each go in and 1 atom of each come

out• The equation is already balanced.

Moles

• Because chemical reactions often involve large numbers of small particles, chemists use a counting unit called the mole to measure amounts of a substance.

Moles

• 1 mole = 6.02 X 1023 particles• 1 mole of flowers = 6.02 X 1023 flowers or

602,000,000,000,000,000,000,000 flowers• 1 mole of apples = 6.02 X 1023 apples• 1 mole of oxygen = 6.02 X 1023 atoms of

oxygen

Molar Mass

• For an element, the molar mass is the same as the atomic mass expressed in grams.

• So for carbon, the atomic mass is 12.0 amu, and the molar mass is 12.0 grams.

• Element atomic mass molar mass• Oxygen 15.999 amu 15.999 grams• Nitrogen 14.007 amu 14.007 grams

Mole-Mass Conversions

• Once you know the molar mass of a substance, you can convert moles of that substance into mass (or mass into moles).

• Carbon has a molar mass of 12.0 grams.• How many moles are in 5 grams of carbon?• 22 g X 1 mol/12.0 g = 1.83 moles or 1.8 moles

in significant digits• mol is the abbreviation for moles

Mole-Mass Conversions

• How many moles are in 35 grams of oxygen (in significant digits)?

Mole-Mass Conversions

• How many moles are in 35.0 grams of oxygen (in significant digits)?

• 35.0 g X 1 mol/15.999 g = 2.19 mol O

Finding the mass of a compound

• To find the mass of a compound:• CO2

• Find the mass of each element in the compound.

• C = 12.011• O = 15.999

Finding the mass of a compound

• C = 12.011 g• O = 15.999 g• Then multiply the mass by the number of

atoms of each element. • There is only 1 atom of carbon so

12.001 x 1 = 12.001• There are 2 atoms of oxygen (O2) so

15.999 x 2 = 31.998

Finding the mass of a compound

• Then add the masses of each element together.

• 31.998 g O• +12.001 g C• 43.999 g CO2

Practice

• How many moles are in 55.0 grams of CO2?

• 55.0 g X 1 mol / 43.999 g = 1.25 mol CO2

• How many moles are in 25.0 grams of H2O?

• How many grams are in 2.3 moles of CO2?

Practice

• How many moles are in 25.0 grams of H2O?

• 1.0079 x 2 + 15.999 = 18.015 (mass of H2O)• 25.0 g x 1 mol/ 18.015 g = 1.39 mol

• How many grams are in 2.3 moles of CO2?

• 12.011 + 15.999 x 2 = 44.009 (mass of CO2)• 2.3 mol x 44.009 g/1 mol = 101.22 g =

1.0 x 102

Calculations

• In chemical reactions, the mass of a reactant or product can be calculated by using a balanced chemical equation and molar masses of the reactants and products.

Calculations

• How much oxygen is required to make 144 grams of water?

• 2H2 + O2 → 2H2O• First determine how many moles of water you are

trying to make.• Molar mass of water = 1.0079 (2) +15.999 =

18.015 grams• 144 g H2O x 1 mol H2O/ 18.015 g H2O = 7.99 mol

H2O

Calculations

• 144 g H2O = 7.99 mol H2O

• 2H2 + O2 → 2H2O• Now you can use the balanced equation to convert.• (7.99 mol H2O) x (1 mol O2 )/(2 mol H2O) = 4.00 mol

O2

• Finally convert moles back to grams.• (4.00 mol O2) x (15.999 g O2)/(1 mol O2) =

63.9 g O2

Practice Calculations

• How much magnesium do you need to make 34.5 grams of Magnesium Oxide?

• 2Mg + O2 → 2MgO• How much sodium chloride will 22.3 grams of

sodium make?• 2Na + Cl2 → 2NaCl

Practice Calculations

• How much magnesium do you need to make 34.5 grams of Magnesium Oxide?

• 2Mg + O2 → 2MgO• Molar mass of MgO = 24.305 + 15.999 =

40.304 g/ 1 mol MgO• 34.5 g MgO x 1 mol MgO/40.304 g MgO x 2

mol Mg/2 mol MgO x 24.305 g Mg/ 1 mol Mg= 20.8 g Mg

Practice Calculations

• How much sodium chloride will 22.3 grams of sodium make?

• 2Na + Cl2 → 2NaCl• 22.3 g Na x 1 mol Na/ 22.990 g Na x 2 mol

NaCl/ 2 mol Na x (22.990 + 35.453=58.443)g NaCl/ 1 mol NaCl = 56.7 g NaCl