Chapter 10

Energy

Chemistry 101

Energy

Matter Energy Empty space

Universe

Energy: ability to do work or produce heat.

Energy

Kinetic energy (KE): energy of motion

KE = ½ mV2 V: velocity

Potential energy (PE): stored energy

Law of conservation of energy

(Position & Composition)

Kinetic and Potential Energy

A

BB

A

Kinetic energy Potential energy

Change in potential energy for ball A (change in level)

Work: change the position of ball B

Heat: friction between surface & ball

Frictional heating (heat is a form of energy).T of hill increases

Heat and Temperature

Temperature: measure of random motions of the components of substance.

T ↑ move fasterKinetic energy ↑

Energy is transferred from high T to low T.

Heat: Flow of energy due to a T difference.

Heat and Temperature

ColdWater

HotWater

Tfinal =Thot initial + Tcold initial

2T final T final

Heat

units of heat: calorie (cal) English system

joule (J) SI system

1 cal = 4.184 J

Joule: Energy (heat) required to raise T of one gram of water by 1C.

Food energy is measured in Calories (note the capital C).

1 Cal = 1 kcal = 1000 cal

Amount of heat = specific heat × mass × change in temperature

Amount of heat = SH × m × (T2 – T1)

T2 = final temperature

T1 = initial temperature

SH = Specific heat capacity (cal/g °C)

m = mass (g)

Heat

Note: ALEKS uses “c” instead of “SH” to stand for specific heat capacity.

• Specific heat capacity is the energy required to change the temperature of a mass of one gram of a substance by one Celsius degree.

Heat

• Calculate the amount of heat energy (in joules)

needed to raise the temperature of 6.25 g

of water from 21.0°C to 39.0°C.

• We are told the mass of water and the temperature increase. We look up the specific heat capacity of water, 4.184 J/g°C.

Q = SH x m x T

Q = (4.184 J/g°C) x (6.25 g) x (39.0°C – 21.0°C)

Q = 471 J

Practice 1:

Heat

• A silver-gray metal weighing 15.0 g requires 133.5 J to raise the temperature by 10.°C. Find the heat capacity.

Q = SH x m x T

(133.5 J) = SH x (15.0 g) x (10.°C)

SH = 0.89 J/g°C

Can you determine the identity of the metal using

Table 10.1?

Heat

Practice 2:

Al

Heat of reaction

C3H8(s) + 5O2(g) 3CO2(g)+ 4H2O(l) + heat (energy)

2HgO(s) + heat (energy) 2Hg(l) + O2(g)

Endothermic reaction

Exothermic reaction

All combustion reactions are exothermic.

Heat of reaction

Surroundings Surroundings

System System

Energy Energy

Exothermic Endothermic

(PE)

Reactant

Product

Energy released to the surroundings as heat

Surrounding

Exothermic (burning)

Exothermic = exit!

• Classify each process as exothermic or endothermic. Explain why. (The system is underlined.)

a) Your hand gets cold when you touch ice.

b) The ice gets warmer when you touch it.

c) Water boils in a stove-top kettle.

d) Water becomes ice in the freezer.

e) Water vapor condenses on a cold pipe.

f) Ice cream melts.

Exo

Endo

Endo

Exo

Exo

Endo

Heat of reaction

Practice:

Thermodynamics

Thermodynamics: study of energy

Internal energy (E): sum of the kinetic and potential energies.

E = q + w “delta”: change

WorkHeat

Law of conservation of energy: energy of the universe is constant.

The first law of thermodynamics:

SurroundingsSurroundings

SystemSystem

EnergyEnergy

ExothermicEndothermic

E 0E 0

Thermodynamics

E = q + w

Energy flows into system via heat (endothermic): q = +x

Energy flows out of system via heat (exothermic): q = -x

System does work on surroundings: w = -xSurroundings do work on the system: w = +x

Enthalpy

For a reaction in constant pressure,the change of enthalpy is equal to energy that flows as heat.

Hp = heat

Constant pressure

“-” heat or Hp: exothermic: heat flows out of the system.

“+” heat or Hp: endothermic: heat flows into the system.

Enthalpy (Thermochemistry): heat of chemical reactions.

Calorimetry

Calorimeter:

A device to measure the heat energy released or absorbed by a reaction.

T H

S(s) + O2(g) SO2(g) ΔH = –296 kJ

•Calculate the quantity of heat released when 2.10 g of sulfur is burned in oxygen at constant pressure.

Use the H value like a conversion factor.

2.10 g S = 0.0655 mol Sx1 mol S

32.26 g S

0.0655 mol S = – 19.4 kJx– 296 kJ

1 mol S

Enthalpy

Practice:

Hess’s Law

State function: a property of system that changes independently of its pathways.

Enthalpy is a state function.

In a chemical reaction, change of enthalpy is the same whether the reaction takes place in one step or in a series of steps.

N2(g) + 2O2(g) 2NO2(g) H1 = 68 kJ1 Step

N2(g) + O2(g) 2NO(g) H2 = 180 kJ

2NO(g) + O2(g) 2NO2(g) H3 = -112 kJ

N2(g) + 2O2(g) 2NO2(g) H2 + H3 = 68 kJ

2 Steps

Two rules about enthalpy

1. If a reaction is reversed, the sign of H is also reversed.

N2(g) + 2O2(g) 2NO2(g) H1 = 68 kJ

2NO2(g) N2(g) + 2O2(g) H1 = -68 kJ

2. If the coefficients in a balanced reaction are multiplied by an integer, the value of H is also multiplied by the same integer.

N2(g) + 2O2(g) 2NO2(g) H1 = 68 kJ

2N2(g) + 4O2(g) 4NO2(g) H1 = 2 68 kJ = 136 kJ 2

Law of conservation of energy

Quality-Quantity of Energy

Why are we concerned about energy?

Gasoline + O2 CO2 + H2O + energy

Spread in universe

Concentrated energy Spread energyUse of energy to do work

Quantity Quality Heat death

Energy and Our World

Sun

Woody plants

Coal

Natural gas

Petroleum

Photosynthesis Source of

energy

6CO2 + 6H2O + energy of sun C6H12O6 + 6O2

Photosynthesis

glucose

1. Natural gas – 90 to 95 percent methane. – 5 to 10 percent ethane, and a mixture of other low-boiling alkanes.

2. Petroleum– A thick liquid mixture of thousands of compounds, most of them

hydrocarbons.

Energy and Our World

Fossil Fuels: formed from the decomposition of marine plants and animals.

(C5-C10)

(C10-C18)

(C1-C4)

(C15-C25)

(C25)

3. Coal

– Was formed from the remains of plants that were buried (under high P and T).

– 20% of our energy.

– Expensive, dangerous, and produces pollution (CO & SO2).

Energy and Our World

Greenhouse Effect

Driving forces

Energy spread: concentrated energy is dispersed widely.

(Exothermic process)

Matter spread: molecules of a substance are spread out and occupy a larger volume.

Dissolving is endothermic process,but because of matter spread, it occurs.

heat

Entropy (S)

A measure of disorder or randomness.

Energy spread Faster random motions of the molecules in surroundings.

Matter spread Components of matter are dispersed (occupy a larger volume).

A Spontaneous process is one that happens in nature on its own.(because of increasing entropy)

Dissolving

The second law of thermodynamics:

The entropy (S) of the universe is always increasing.

We run towards a disorder (heat death of universe).

Midterm 2

•Bring a 2B pencil.

•One 30423 Scantron form (available in Runner Bookstore).

•A calculator (scientific, not graphing).