Quiz
1.
2.
3.
4.
5.
A reaction that releases heat is called?
A reaction that takes in heat is called?
What’s the equation for calculating energy?
What does ΔH mean?
What is the sign of ΔH for endothermic
reaction?
6. What is the sign of ΔH for exothermic
reaction?
7. Where does the energy come from in chemical
reaction?
Calculating Enthalpy Changes
What you need
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Measure temperature change
Specific heat capacity of surroundings
Calculate energy change
Use mass to determine enthalpy change
Negative or positive
Measuring temperature change
• To measure the heat change
of a reaction experiments
are carried out in a
calorimeter
• The energy released (or
absorbed) by the reaction
results in a change in the
temperature of the water in
the surrounding container.
Specific heat capacity
• Specific heat capacity is the amount of heat
required to raise the temperature of 1g of a
substance 1 degree Celsius.
• The specific heat capacity for water is 4.18 J
I.E. it takes 4.18 J to raise 1g of water (1mL) 1
degree Celsius.
• Calorimeters measure the temp change in
water surrounding the reaction
Calculating energy change
• If the temperature change of a known mass of
water in the calorimeter is measured then the
E value for the water is given by
q = 4.18 x mass H2O x T
( where T = final temp - initial temp)
Specific heat capacity for water is 4.18 J/goC
For example
In an experiment 1.500 g of Mg(s) was combined with 125.0
mL of 1.0 M HCl. Initial temperature 25.0oC, final
temperature 72.3oC. Calculate the heat involved in the
reaction
72.3 – 25.0 = 47.3oC
Solution is mostly water so
125mL = 125g
E = 4.18 x 125 x 47.3
= 24714.25J
Or 25 kJ
Burning Chippies/magnesium
In pairs, collect:
2 pieces of chippies, tongs, 2cm of magnesium,
Bunsen from the cupboard, test tubes, test tube
holders and thermometers.
Put 10mL of water in the test tube and activate
the chippies. Place the chippies underneath the
test tube so all the heat is transferred to the test
tube, record the change in temperature.
Enthalpy is measured in J per mole
The energy calculated must be divided by the
amount of reactant.
1.500 g of Mg(s) was combined with 125.0 mL of 1.0 M HCl.
1.50g ÷ 24.31 = 0.0617 mol
24.7kJ ÷ 0.0617 = 400kJ mol-1
Positive or Negative
It the temperature of the surroundings increases
energy has been lost from the system.
-H
If the temperature of the surroundings decreases
energy has been absorbed by the system
+ H
Exercise 1: Calculate the value of rH in kJ mol -1 for the dissolving
of solid NaOH in water, if it is found that when 10 g of NaOH is
dissolved in 250 mL water the temperature of the water increases
from 25.0 oC to 29.8 oC.
First T
29.8 - 25.0 = 4.8
Then energy change
4.8 x 4.18 x 250 = 5016
Now work out amount of NaOH
10g ÷ 40g/mol = 0.25 mol
Enthalpy change
5016 ÷ .25 = 20064 Jmol-1 = 20.1kJmol-1
Exothermic so -20.1kJmol