Transcript Heat in Chemical Reactions

Heat in Chemical
Reactions

Heat: the transfer of energy to a
substance causing an increase in that
substance’s average kinetic energy

Temperature: a measure of the average
kinetic energy of a material’s particles.

Number of Particles vs. Average Speed
(Temperature):
K = C + 273
 F = 1.8C + 32
 C = (5/9)(F – 32)

Temperature Conversions

Reactions that release heat are
EXOTHERMIC
C3H8 + 5O2  3CO2 + 4H20 +
2043kJ
How many grams of propane are
necessary to react with excess
oxygen to produce 18,000 kJ of
heat?
Heat in chemical reactions

Reactions that absorb heat are
ENDOTHERMIC
C + H20 + 113kJ  CO + H2
How much heat energy is required
to react with excess carbon and
water to produce 45 L of hydrogen
gas at STP?
Heat in chemical reactions
q = mCpΔT
q = Heat energy (J)
m = mass of substance (g)
Cp = specific heat capacity
ΔT = Tf – Ti
(in Celsius)
Specific Heat Capacity

Cp (ice) = 2.1 J/g

Cp (water) = 4.18 J/g

Cp (steam) = 1.7 J/g
Specific Heat Capacity

How much energy is required to heat 43
grams of water from 22 degrees Celsius
to 85 degrees Celsius?

The combustion of glucose in cellular
respiration produces 2603 kJ/mol glucose
of energy. This reaction is being used to
heat a container of 74 grams of water
from 12 degrees to 93 degrees. How
many grams of oxygen are being reacted
with the glucose to cause this change in
temperature?

A 47 gram chunk of hot copper is placed
into 200 mL of water originally at 33
degrees Celsius. Afterwards, the water
now has a temperature of 35.4 degrees
Celsius. Assuming all heat lost by the
copper was gained by the water, how hot
was the copper before putting it into the
water? The specific heat of copper is
0.38.
What happens to temperature as
heat is added?

The amount of heat absorbed by 1 mole
of a substance to melt from a solid to a
liquid.
◦ For water, ΔHfus = 6.01 kJ/mol

How much heat would be required to melt
30 grams of water at 0˚Celsius?
Molar Heat of Fusion

The amount of heat required to cause 1
mole of a liquid to vaporize
◦ For water, ΔHvap = 40.7 kJ/mol

How much heat would be required to
vaporize 30 grams of water at 100˚C?
Molar Heat of Vaporization
I have 67 grams of ice at -10˚C. How
much heat will be required to convert
this amount of water to steam at a
temperature of 125 ˚C?

The enthalpy of a substance is equal to its
total energy plus a small added term that
takes into account volume and pressure.

It’s a measure of how much energy is
stored in a substance.
Enthalpy (H)

All reactions have a standard enthalpy
change
◦ Found at 1 atmosphere of pressure and 25˚C

ΔH = Hproducts – Hreactants
Enthalpy changes (ΔH)
If ΔH is positive, it’s an endothermic
reaction and heat is absorbed.
 If ΔH is negative, the reaction is
exothermic and heat is released.

What does it all mean?

How much heat is transferred when 9.22
g of glucose (C6H12O6) in your body react
with O2 according to the following
equation?
C6H12O6 + O2  6CO2 + 6H2O
ΔH˚ = -2803 kJ
Using Enthalpy

In going from a particular set of reactants
to a particular set of products, the change
in enthalpy is the same whether the
reaction takes place in one step or in a
series of steps.
Hess’s Law
N2 + O2  2NO
2NO + O2  2NO2
ΔH = 180 kJ
ΔH = -112 kJ
Net: N2 + 2O2  2NO2 ΔH = 68 kJ
Hess’s Law
S(s) + 3/2 O2(g)  SO3(g) ΔH = -395.2 kJ
2SO2(g) + O2(g)  2SO3(g) ΔH = -198.2 kJ
What is the ΔH for the following net
reaction?
S(s) + O2(g)  SO2(g)
ΔH = ?
How many grams of sulfur dioxide gas can
be created if 350 kJ of heat is transferred?