14. CHEMICAL EQUATIONS

Question 1

1. What is a chemical equation?

Answer 1

• it is a shorthand way of representing a chemical change

Question 2

2. What must the chemical equation obey?

Answer 2

• it must obey the Law of Conservation of Mass
  (mass is neither created nor destroyed)

Question 3

3. Must the atoms be balanced in a chemical equation?

Answer 3

• yes
• there has to be the same number of atoms of each element on the left and the right side
• you must also include the physical states
  (s, l, g, aq)

Question 4

4. Give the name for the following symbols:

4.1: (g)
4.2: (l)
4.3: (s)
4.4: (aq)

Answer 4

4.1: substance is a gas

4.2: substance is a liquid

4.3: substance is a solid

4.4: substance exists in aqueous (water) solution

Question 5

5. What are the two necessary steps to balance out a chemical equation?

Answer 5

• you must introduce coefficients to balance the number of atoms of each element
• subscripts must never change

Question 6

6. How do we measure mass in the laboratory?

Answer 6

• we measure the mass of the reactants
• we measure it in grams

Question 7

7. With regards to chemical reactions, we need to relate the mass of the reacting atoms and molecules to the number of particles.
   How do we do this?

Answer 7

• We use the following unit:
  
  • 1 mole = Avogadro’s number
    = 6.022 x 10²³

Question 8

8. What is the molar mass?

Answer 8

• it is the sum of all the atomic masses of all the atoms

Question 9

9. How is Molar Mass worked out?

Answer 9

• you times the number of the atoms by their atomic mass number
• always round to two decimal figures

Question 10

10. Which equation do we use to calculate the number of moles in a gas, pure liquid or pure solid?

Answer 10

n = m/MM

• n= moles
• m = mass (grams)
• MM= molar mass (g/mols)

Question 11

11. What is an aqueous solution?

Answer 11

• it is a solute + solvent
• solute= substance being dissolved
• solvent= substance doing the dissolving

Question 12

12. What does solution concentration depend on?

Answer 12

• the amount of solute
• the amount of solvent

Question 13

13. What is Molarity?

Answer 13

• M= moles of the solute / litres of the solution

Question 14

14. What is another way of writing the molarity equation?

Answer 14

• M = n / v
• M= Molarity (mol/L)
• n = moles
• V= volume (in litres)

Question 15

15. When do we use the n= M x V equation?

Answer 15

• when we are dealing with aqueous (water) solutions

Question 16

16. What are Yields?

Answer 16

• they are a measure of how much product has been lost
• this is usually compared to the calculated product amount

Question 17

17. What is the formula for working out the percentage Yield?

Answer 17

• Percentage Yield = Actual yield of the product
  ——————————————- x 100
  Theoretical yield of the product

Question 18

18. What is the Theoretical Yield of the product?

Answer 18

• it is the amount of product expected to form
• this is on the basis of stoichiometric calculations
• it is what we calculate from balanced equations

Question 19

19. What is the Actual Yield?

Answer 19

• it is the amount of product that was actually formed in the experiment

Question 20

20. What is the Percentage Yield?

Answer 20

• it is usually less than 100%
• this is because we get losses in the lab
• there can also be side products formed
• it may also appear to be higher than 100%
• this happens when we have impurities in our product

Question 21

21. In the equation:
    C3H8 + 5 O2 —-> 3 CO2 + 4 H2O

Which is the excess reactant and why?

Answer 21

• oxygen
• it is readily available in the air
• we can use as much of it as we need

Question 22

22. In the equation:
    C3H8 + 5 O2 —-> 3 CO2 + 4 H2O

Which is the limiting reactant and why?

Answer 22

• propane (C3H8)
• it is the substance that runs out first

Question 23

23. What is the amount of product dependent on?

Answer 23

• it depends on the amount of limiting reactant
• this is because the reaction stops when the limiting reactant is used up

Question 24

24. How do you work out which one is the limiting reactant?

Answer 24

• you find the number of moles for the atom/molecule
• you divide this number by the coefficient
• the smallest number indicates the limiting reactant

Question 25

25. Does adding more solvent change the number of moles of a solution?

Answer 25

• no

Question 26

26. How is a solution diluted?

Answer 26

• we start off by taking a small amount of a concentrated solution
• this concentrated solution is placed in another container
• water is added to it
• this dilutes it

Question 27

27. Does water have an affect on the number of moles?

Answer 27

• no
• the number of moles of the concentrated solution do not change
• the number of moles in an aqueous solution is given by the equation: n=M x V

Question 28

28. What formula do we use to work out the dilution of a solution?

Answer 28

• M1 x V1 = M2 x V2
• the number 1 indicates the concentrated solution
• the number 2 indicates the diluted solution

Question 29

29. Read through this summary.
    Does everything make sense?

Answer 29

• yes

Question 30

30. Read through this summary.
    Does everything make sense?

Answer 30

• yes