Transcript KineticAndPotentialEnergy

Unit 1: Energy Changes in
Chemical Reactions
Not All Reactions Go Off with
a Bang!
Energy
The capacity to do work or to produce heat
Forms of Energy:
1. Kinetic Energy– The energy of motion
2. Potential Energy
– The energy of position (gravitational
potential energy)
eg. Diefenbaker Dam
– The energy stored in chemicals
because of their composition
eg. Chocolate bars (Yum!)
Measuring Energy
☼ Common Units
→ calorie (cal)
- food
≈ 2000 – 3000 kcal/day
1000 cal = 1 kcal (kilocal)
1000 cal = 1 Calorie
→ British Thermal Unit (BTU)
- fuels
fuel = 20 900 BTU/pound
coal = 9 600 BTU/pound
plastic bags = 18 700 BTU/pound
☼ Scientific Unit
→ joule (J)
1 J = 1 newton ● metre (N●m)
i.e. The work required to move an object
one metre using a force of one newton
(N = kg●m/s2).
4.184 J = 1 cal
☼ Most chemical reactions involve energy or
changes in energy.
☼ In a Chemical Reaction (rxn)…
- existing bonds are broken (requires E)
- atoms are rearranged
- new bonds are formed (releases E)
Thus, almost all chem. Rxns either absorb or
release energy. This results in an exchange of
energy (aka HEAT)
Heat-- The energy that is transferred
from one object to another due to a
difference in temperature (flow of energy is
usually from hot to cold until equilibrium is
reached)
Thermochemistry-- The study of
the changes in heat in chem. rxns (part of
thermodynamics-- energy/work)
→ thermes = greek for heat
→ we look at systems during changes of
heat (i.e. A beaker or flask)
→ we also look at surroundings into which
heat may be lost or gained
→ A system is part of the universe on
which we focus our attention, the
surroundings include everything else in the
universe
→ energy is neither created or destroyed
when heat is transferred (1st Law of
Thermodynamics)
→ closed vs open system
→ we can’t tell how much energy is in
something until it is released
Temperature = degree of hotness or
coldness of an object, which is a measure of
average kinetic energy of the molecules
Heat = the energy transferred from one body to
another because of the temp. difference
Heat is a form of energy; temp. in NOT!
There is more heat in a large iceberg than in a cup
of boiling water! This is b/c heat is trapped
inside as opposed to being released so the ice
berg doesn’t feel warm.
Exothermic Reactions
→ rxns that release heat into the
surroundings
→ PE is converted to
heat energy; temp. ↑
Endothermic Reactions
→ rxns that absorbs heat from the
surroundings
→ KE decreases; temp. ↓
Examples:
1. Combustion of propane
C3H8(g) + 5O2 → 3CO2(g) + 4H2O(g) + 2043 kJ
1 mole of propane produces 2043 kJ of heat
Energy released as new bonds are formed
in the products is greater than the energy
required to break the old bonds in the
reactants.
All combustion rxns are exothermic!
2. Water Gas Reaction
C(s) + H2O(g) + 113 kJ → CO(g) + H2(g)
1 mole of solid carbon requires 113 kJ of
heat to produce 1 mole of carbon
monoxide.
This is an endothermic rxn.
Chemical equations with heat values
incorporated into them are known as
thermochemical equations.
Enthalpy (H) is…
… the heat content or the amount of heat a
substance has at a given temp. and pressure
(the total energy stored by a substance is the
sum of KE & PE. The enthalpy is this energy
plus a small added term that takes into account
the pressure and volume.)
∆H = Change in enthalpy
∆H = Hproducts – Hreactants
∆H = + (pos.) = heat absorbed = endo
∆H =  (neg.) = heat released = exo
Energy Diagrams
SATP
For reporting enthalpy changes, chemists use
1 atm = 101.3 kPa and 25°C (298K) as standard
enthalpy changes ∆H°.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ∆H° =  890.3 kJ
½H2(g) + ½I2(g) → HI(s) ∆H° = + 26.5kJ
Equations are stoichiometrically correct!
Your Turn!
1. Draw the Energy diagram for…
a) the combustion of propane
b) the formation of CO(g)
2. Write a balanced equation and state the
∆H° value (i.e. don’t include the value in
the eqn but state it separately as pos. or
neg.)
Summary
Exothermic (∆H°)
→ energy is released
→ energy appears as a product
→ surroundings increase in temp. (warmer)
Endothermic (+∆H°)
→ energy is absorbed
→ energy appears as a reactant
→ surroundings decrease in temp. (cooler)
☼ Equations are stoichiometrically correct!
Calculation Questions:
1. How much heat is transferred when 9.22 g
of glucose (C6H12O6) in your body reacts
with O2 according to the following eqn.?
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)
∆H° =  2803 kJ
9.22g C6H12O6 x 1 mol x
2803 kJ
180.0 g 1 mol C6H12O6
= 143.57589 kJ = 144 kJ released
2. How much heat is transferred when 147g
of NO2(g) is dissolved in an excess of H2O?
3NO2(g) + H2O (l) → 2HNO3(aq) + NO(g)
∆H° =  138 kJ
147g NO2 x 1 mol NO2 x
138 kJ
46.0 g
3 mol NO2
= 147 kJ released
Assignment:
Pg. 187, #10 & 11
Pg. 191 – 195, #30 – 33
(Addison-Wesley Text)