Transcript How to write up a practical: General review

Yr 10 Chemistry
Acids and Bases
Starter
 Calculate the new concentration if I ADD
200ml of water to 300ml of 2M HCl.
Objectives
 TO KNOW the term acid, base, pH,
titration, dissociate.
 TO BE ABLE to do calculations for
titrations
 TO UNDERSTAND how the pH
scale works.
What is an Acid and Base?
 Acids start with H in there formula.
 Bases usually have OH at the end of their
formula.
Neutralization
 Acid + Hydroxide  Salt + Water
HCl (aq) + NaOH (aq) è NaCl (aq) + H2O (l)
HNO3 (aq) + NaOH (aq) è NaNO3 (aq) + H2O (l)
CH3COOH (aq) + NaOH (aq)  NaCH3COO (aq) + H2O (l)
Acid + Reactive Metal  Salt + Hydrogen Gas
Na(s) + HCl(aq) → NaCl(aq) + H2 (g)
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2 (g)
Complete and Balance the
Following Equations
Note: The more unreactive metals
(e.g. Cu and Pb) do not react with
acids)
Bases in Solution
 Remember, just the soluble bases are
considered to be alkalis and when added to
water release an OH ion
 K2O (s) + H2O (l) ⇌ 2K+ (aq) + 2OH-(aq)
 NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
 CO32-(aq) + H2O (l) ⇌ HCO3- (aq) + OH-(aq)
 HCO3-(aq) ⇌ CO2 (g) + OH- (aq)
The pH Scale
 Stands for power of Hydrogen (it is the concentration of
hydrogen ions and equal to the power of ten with the sign
reversed)
Scale between 1 and 14
pH 7 is neutral
Acids are from 0 to 6.99
When acids are in solution they ‘dissociate’
to to become an anion and hydrogen ions
(H+). It is the amount of H+ that
determines how acidic a solution is.
pH
[H+]
[H+]
(mol dm-3) (mol dm-3)
0
1
2
100
10-1
10-2
1
0.1
0.01
3
4
5
6
10-3
10-4
10-5
10-6
0.001
0.0001
0.00001
0.000001
7
10-7
0.0000001
What is
happening to the
hydrogen ion
concentration
with each
decrease in pH
unit?
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The pH scale is a LOGARITHMIC SCALE.
One change in pH is a x10 jump in H+
Two changes in pH is a x100 jump in H+
Three changes in pH is a x1000 jump in H+
etc
pH
[H+]
[H+]
(mol dm-3) (mol dm-3)
8
9
10
10-8
10-9
10-10
0.0000001
0.00000001
0.000000001
11
12
13
14
10-11
10-12
10-13
10-14
0.0000000001
0.00000000001
0.000000000001
0.0000000000001
When we put a
base into
water, The OHsoaks up the H+
present (to
produce water)
reducing the
H+
concentration.
1. Black coffee has a pH of 5 and toothpaste has a
pH of 8. Identify which is more acidic and
deduce how many times the [H+] is greater in
the more acidic product.
(Total 2 marks)
Indicators
Indicator
Acid
Base
Methyl Orange
red
yellow
Phenophthalein clear
fuchsia
Universal
Indicator
red/orange/
yellow
yellow
blue/purple
Red  Blue
Blue  Red
Bromthymol
Blue
Litmus Paper
blue
Strong/Weak Acids
 Strong acids ‘dissociate’ more in water so have a
higher pH (as more H+ ions)
Common Strong Acids
Common Weak Acids
Strong/Weak Bases
 Strong Bases ‘dissociate’ more in water so have a
higher OH- ions.
 These reduce the H+ ions present so have higher
pH
Equations for Bases
Strong Bases
Weak Bases
Calculations and Titrations
 Titration: Determining the unknown concentration of a
solution.
 A type of titration is an acid/base titration.
 We can perform an experiment where we add a
base of known CONCENTRATION to a known
VOLUME of acid.
 Until all the acid has been neutralised (pH 7)
 If we now know the volume of base added we can
calculate the concentration of the acid.
Steps
 E.g We have added 10ml of 0.1M NaOH to 10ml HCl.
 Calculate the concentration of acid.
 Step 1: Write the BALANCED Formula.
 Step 2: Calculate the number of moles of based
added.
 Step 3 Determine the AMOUNT of acid present in
MOLES.
 Step 4 Calculate the concentration of acid.
Step 1
 NaOH + HCl  NaCl + H2O
Notice it takes 1 mole of base to react
with 1 mole of acid.
We have added 10ml of 0.1M NaOH to
10ml HCl.
Calculate the concentration of acid.
Step 2
 We need to calculate the amount of sodium
hydroxide added (in moles). Remember, in
quantitative chemistry we always need to work in
moles.
 C=n/V
 0.1 = n / 0.01 dm3
 n = 0.001 moles
We have added 10ml of 0.1M NaOH to
10ml HCl.
Calculate the concentration of acid.
Step 3
 If we used 0.001 moles of base with a 1:1 mole
ratio, the amount of acid present is 0.001
moles.
We have added 10ml of 0.1M NaOH to
10ml HCl.
Calculate the concentration of acid.
Step 4
 Using our quantitative formula again. Remember,
we use this formula whenever we are using
solutions.
 C=n/V
 C = 0.001 / 0.01 dm3
 C = 0.1 mol dm-3
We have added 10ml of 0.1M NaOH to
10ml HCl.
Calculate the concentration of acid.