Quiz
1. In an experiment 1.500 g of Mg was burnt to heat up
125 g of water, the Initial temperature for water is
25.0oC, the final temperature is 72.3oC. Calculate
the heat released by the magnesium
2. Would the reaction release or take in heat if the
energy in the products is less than the energy in the
reactants.
3. Would the reaction release or take in heat if the
energy in the products is more than the energy in the
reactants.
4. Define activation energy.
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 = mass x c x T
where
M= mass of the substance
T = final temp - initial temp
C= Specific heat capacity (for water is 4.18 J/goC)
Answers
In an experiment 1.500 g of Mg was burnt to heat
up 125 g of water, the Initial temperature for
water is 25.0oC, the final temperature is 72.3oC.
Calculate the heat released by the magnesium
72.3 – 25.0 = 47.3oC
125mL = 125g
E = 4.18 x 125 x 47.3
= 24714.25J
Or 25 kJ
Answers
1. Would the reaction release or take in heat if
the energy in the products is less than the
energy in the reactants.
Release energy
1. Would the reaction release or take in heat if
the energy in the products is more than the
energy in the reactants.
Take in energy
1. Define activation energy.
The energy required to start a reaction.
Learning Objective: Explain the significance of Avagadro’s number (N).
The Mole and Stoichiometry
A mole is defined as the number of atoms present in exactly 12.00 g of the 12C
isotope.
The number of atoms present in such a sample is 6.02 X 1023 and is called
Avogadro’s number (N).
The mole then is the measure of the amount of a substance. The mole has the
symbol n and units of mol.
Learning Objective: Define the term molar mass (M)
Molar Mass
The molar mass is simply the mass
of one mole of an element or
compound, it’s unit is g mol-1and it
has the same numerical value as the
relative molecular or atomic mass.
The molar mass, mass and moles are
related by the following equation.
m
n
M
Work out the molar mass of
the following.
a) CaSO4
b) Zn(NO3)2
c) Fe3(PO4)2
Work out M of the following.
a) CaSO4
136 g mol-1
b) Zn(NO3)2
127 g mol-1
c) Fe3(PO4)2
358 g mol-1
Learning Objective: Use n = m/M equation to inter-convert between
moles, mass and molar mass.
Molar Mass, mass and the mole
Molar mass, the mole and mass are related to each other by the
formula.
mass (g)
moles (mol)
m
n
M
molar mass
(g mol-1)
Example
How many moles of water (H2O) are there in 36 g of water? From the
periodic table we can work out the molar mass of water. M(H2O) = 18
g mol-1.
Therefore
n(H2O) = 36 g/18 g mol-1
= 2 mol
Learning Objective: Use n = m/M equation to inter-convert between
moles, mass and molar mass.
If we rearrange the formula we can work out the mass if we know
the moles of a substance.
mnxM
Example
What is the mass of 5 mol of water? From the periodic table we can work out
the molar mass of water. M(H2O) = 18 g mol-1.
Therefore
m(H2O) = 5 mol x 18 g mol-1
= 90 g
Enthalpy is measured in J per mole
The energy calculated must be divided by the
amount of reactant.
E.g.
- 2 moles of fuel gave out 1000KJ of energy, how much
energy did 1 mole of fuel give out?
- 5 moles of sugar gave out 500KJ of energy, how much
energy did 1 mole of sugar give out?
-1
Exercise 1: You have burnt 10g of sugar ( C6H12O6 ) to heat up
20mLs of water by 50 degrees. How much energy did 1 mole
of sugar give out? (enthalpy change of burning sugar). The
specific heat capacity for water is 4.18 J/gC.
1. Work out the total amount of energy that’s
absorbed by the water.
2. Work out the number of moles in 10g of sugar.
3. Work out how much energy did one mole of
sugar give out.
Exercise 1: You have burnt 10g of sugar ( C6H12O6 ) to heat up
20mLs of water by 50 degrees. How much energy did 1 mole
of sugar give out? (enthalpy change of burning sugar). The
specific heat capacity for water is 4.18 J/gC.
1. Work out the total amount of energy that’s
absorbed by the water.
20x4.18x50=4180J
2. Work out the number of moles in 10g of sugar.
10/180=0.05mol
3. Work out how much energy did one mole of sugar
give out.
4180 / 0.05mol = 83600 J/mol
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