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Name: KEY
Conjugate Pairs Practice
Key Skill: Learning to Draw Tie Lines
Look at each example drawn below. Sets of partners (called conjugates) are matched with “tie lines”.
HNO 3 + OH-^ NO 3 -^ +^ H 2 O
CH 3 NH 2 + H 2 O CH 3 NH 3 +^ + OH-
HCl + H 2 O H 3 O+^ + Cl-
Note that conjugates are the same compound that has either gained or lost an H+. One of the reactants is always matched with one of the products (reactants or products)
Draw tie lines for each of these reactions:
(a) OH-^ + HPO 4 -2^ PO 4 -3^ + H 2 O
(b) NH 3 + HBr NH 4 +^ + Br-
(c) HC 2 H 3 O 2 + H 2 O H 3 O+^ + C 2 H 3 O 2
Gained H+
Gained H+
Gained H+ Lost H+
Lost H+
Lost H+
Key Skill: Labeling the Acid and Base.
Tie lines allow us to figure out if the substance was an acid or a base. We can do this by determining if the reactant “gained” or “lost” H+. Look at these examples: Gained H+
HNO 3 + OH-^ NO 3 -^ +^ H 2 O
Lost H+
Since acids lose (or donate) H+, then HNO 3 is the acid. Since bases gain H+, then OH-^ is the base.
Lost H+
CH 3 NH 2 + H 2 O CH 3 NH 3 +^ + OH-
Gained H+
Since acids lose (or donate) H+, then H 2 O is the acid. Since bases gain H+, then CH 3 NH 2 is the base.
- Notice only the ___reactants____ (reactants or products) get labeled as an acid or a base.
- Draw tie lines and label the tie lines with ( Lost H+^ or Gained H+) as in the examples above. Then record the acid and base in the space provided.
(a) H 2 SO 4 + NH 3 HSO 4 ^ + NH 4 ^ __H 2 SO 4 _(acid) _NH 3 __ (base)
(b) NH 3 + HBr NH 4 +^ + Br-^ HBR__(acid) _NH 3 __ (base)
(c) HC 2 H 3 O 2 + H 2 O H 3 O+^ + C 2 H 3 O 2 _HC 2 H 3 O 2 __(acid) H 2 O (base)
Key Skill: Predicting the conjugate without an equation.
Example problem 1:
What is the conjugate base of H 2 O?
Solution:
When we draw tie lines, the conjugate base is always attached to an acid. (Check page 3 to confirm this.) So if the problem wants us to find a conjugate base of H 2 O, then water must be an acid.
H 2 O ?
Acid conjugate Base
Since acids donate (give away) H (see page 2 to confirm this) then the conjugate base must have one less H than H 2 O. Answer: OH-
Example problem 2:
What is the conjugate acid of H 2 PO 4 -?
Solution:
If the problem wants us to find a conjugate acid of H 2 PO 4 - , then H 2 PO 4 -^ must be an base.
H 2 PO 4 -^ ?
Base conjugate Acid
Since bases accept H (see page 2 to confirm this) then the conjugate acid must have one more H than H 2 PO 4 -. Answer: H 3 PO 4
- Try the same logic on this problem:
What is the conjugate acid of NH 2 -?
NH 2 -^ ?
Base conjugate acid_
The answer is a conjugate acid according to the problem.
Is NH 2 -^ going to gain or lose an H to become the conjugate acid? Gain
What is the formula of the conjugate acid? __NH 3 __
- Try the same logic on this problem:
What is the conjugate base of HCO 3 -?
HCO 3 -^ ?
Acid conjugate base
The answer is a conjugate acid according to the problem.
Is HCO 3 -^ going to gain or lose an H to become the conjugate base? lose
What is the formula of the conjugate base? _CO 3 2-
Fill in the blank: is NH 2 -^ an a cid or base?
Fill in the blank: is HCO 3 -^ an a cid or base?
