study of energy changes that occur during chemical...

Unit 12: Thermochemistry

Study of energy changes that occur during chemical reactions and

changes in state

Energy Transformations

What are the ways in which energy changes can occur?

Energy

The capacity to do work.

It has no mass or volume.

Exchanged in both physical and chemical changes.

Can you think of an example of each?

Physical Change Chemical ChangeBoth usually involve the absorption or release of heat.

Types of energy

Potential – stored energy

Kinetic – energy of motion

Thermal – form of kinetic energy resulting from the motion of particles and is transferred as heat

Chemical – form of potential energy related to the structural arrangement of atoms or molecules

LAW OF CONSERVATION OF ENERGY

The Law of Conservation of Energy states that the total amount of energy in a closed system remains constant.

Energy cannot be created nor destroyed.

In a closed system, energy can only change form.

a chemical reaction = a closed system

http://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Closed_systemhttp://en.wikipedia.org/wiki/Closed_system

Conservation of Energy

Energy is measured in joules (J).

• According to the Law of Conservation of Energy:

If a system started with 1000 joules of electrical energy and then the energy was converted to light, how much light energy would there be?

1000 joules

Heat

Energy flows from something

warm to something cool

A hotter substance gives energy to a cooler one

When heat is transferred (lost or

gained), there is a change in the

energy within the substance

Heat can be

transferred in

three ways:

Conduction

Convection

Radiation

Conduction

The direct transfer of heat through contact (move from higher to lower temperature)

Ex: frying eggs on a pan

Convection

Transfer of heat by the motion of fluids (liquids and gases).

ex: convection oven heating up a piece of meat

Radiation

Radiation is heat that is transferred through electromagnetic waves.

Radiation is the only form of heat transfer that does not require matter, and can move through space.

Examples: radiation heat from sunlight, UV rays, microwave.

The Universe is divided into two halves - the system and the surroundings.

The system The surroundings

Exothermic reactions release energy into the surroundings.Endothermic reactions absorb energy from the surroundings.

Heat

Heat

Energy in chemical reactions.

4Fe + 3O2 → 2Fe2O3 + 1625 kJ

Hot packs are exothermic (energy is a product).

Are your hands the system or surroundings?

Activation Energy

The energy that must be overcome in order for a chemical reaction to occur.

Or…the energy required to start a chemical reaction

Energy in= Energy Out

http://en.wikipedia.org/wiki/Chemical_reaction

Activation Energy

Catalyst

– substance that increases reaction rate without being consumed in the reaction

– lowers the activation energy

Fast and slow reactions

The smaller the activation energy, the faster the reaction will occur regardless if exothermic or endothermic.

If there is a large activation energy needed, that means that more energy (and therefore, time) is being used up for the successful collisions to take place.

Exothermic Reaction

Reaction that releases energy

Products have lower PE than reactants

energyreleased

2H2(l) + O2(l) 2H2O(g) + energy

CH + 2O CO + 2H O + Heat4 2 2 2

2O + CH 24

CO + 2 H O 2 2Pote

nti

al e

ner

gy

Heat

Endothermic Reaction

Reaction that absorbs energy

Reactants have lower PE than products

energyabsorbed

2Al2O3 + energy 4Al + 3O2

22 O + N

Pote

nti

al e

ner

gy

Heat

2NO

N + O 2NO2 2 + heat

Analyzing the potential energy diagram

What is the Eafor the forwardreaction?

The activation energy (Ea) for the forward reaction is shown by (a) on the diagram

Ea (forward) = PE (activated complex) - PE (reactants) = 400 - 100 = 300 kJ mol-1


What is the Eafor the reversereaction?The activation energy (Ea) for the reverse reaction is shown by (b) on the diagram.

Ea (reverse) = PE (activated complex) - PE (products) = 400 - 300 = 100 kJ mol-1

Enthalpy (H)

Enthalpy is a fancy word to mean the energy in a system at constant pressure.

H is the variable used for enthalpy.

We can never fully measure the enthalpy of a system, but we can measure changes in enthalpy.

ΔH = change in enthalpy = Hproducts – Hreactants

The sign of ΔH tells us a lot….

This diagram represents the following: CH4 + 2O2 → CO2 + 2H2O + 890 kJ

It is losing enthalpy, so the 890 kJ is listed as a product.

When we think about ΔH = Hproducts –Hreactants, we see that it is a negative value.

Exothermic

o System loses energy

o Energy listed as a product

o ΔH is negative (–ΔH)

Endothermic

o System gains energy

o Energy listed as a reactant ΔH is positive (+ΔH)


What is the ΔHfor the forwardand reverse reactions?

The enthalpy change for the reaction is shown by (c):

ΔH = PE (products) - PE(reactants) = 300 - 100 = +200 kJ mol-1

*for the reverse reaction (Change Sign).*

1. Determine the change in enthalpy for this reaction.

NaOH(s) + HCl(g) ----> NaCl(s) + H2O(g)

Compound Hf (kJ/mol)

NaOH(s) -426.7

HCl(g) -92.3

NaCl(s) -411.0

H2O(g) -241.8


2 CO(g) + O2(g) ---> 2 CO2(g)


CO(g) -110

O2(g) 0

CO2 (g) -393.5



B5H9(g) 73.2

B2H3(g) -1272.77

O2(g) 0

H2O(g) -241.82

Thermochemical Equations

A thermochemical equation includes a chemical equation and the corresponding ΔH.

2HBr → H2 + Br2 ΔH = +72 kJ

What do we know about the decomposition of hydrobromic acid?

It is endothermic and requires energy.

A thermochemical equation includes a chemical equation and the corresponding ΔH.

2HBr → H2 + Br2 ΔH = +72 kJ

Which has more energy in it, the reactants or the products?

The products have more energy. The reactants absorbedenergy as they reacted and turned into the products.

2HBr → H2 + Br2 ΔH = +72 kJ

The 72 kJ corresponds to the reactants and products as written. o 72 kJ are required for every two moles of HBr.

o 72 kJ are required for the formation of one mole of H2.

o 72 kJ are required for the formation of one mole of Br2.

2HBr → H2 + Br2 ΔH = +72 kJ

1. How much energy is required to decompose 57 grams of HBr?

2Ba + O2 → 2BaO ΔH = -1107 kJ

2. A reaction between barium and oxygen was conducted and we measured 557 kJ of energy released. How much barium reacted?

Enthalpies (Heats) of Combustion

Combustion reactions are very common and release a lot of energy.

The energy change of combustion reactions is usually given as the “Standard Enthalpy of Combustion,” or Δ Hcomb

The value is in the unit “kJ/mol” – It means that many kJ of energy are released for every mole of the substance burned.

Enthalpies of combustion

For sugar, C12H22O11 (s) ΔHocomb = -5644 kJ/mol

C12H22O11 + 12O2 → 12CO2 + 11H2O ΔH = -5644 kJ

Octane, C8H18 Δ Ho comb = -5471 kJ/mol

2C8H18 + 25O2 → 16CO2 + 18H2O ΔH = ?

3. Calculate the energy released when 0.500 grams of octane burn.

HESS’s LAW

• How can you measure the enthalpy change when it can’t be directly measured?

– Determine heat of reaction by using the known heats of reaction of two or more thermochemical equations

SPECIFIC HEAT AND CALORIMETRY

Specific Heat

Does everything heat up and cool down at the same rate?

Different substances have different capacities for storing energy.

It can take 20 minutes to heat water to 75°C. The same mass of any metal might require only 5 minutes.

Specific Heat

Specific heat is the amount of heat needed to raise the temperature of 1 g of a substance by 1° C.

Conductors and Insulators

Conductors are materials that transfer heat easily and quickly. Metals are the best conductors of heat because they have a low specific heat.

Insulators do not conduct heat easily. They have a high specific heat. Examples are water, glass, plastic, rubber and air.

• Materials that trap air, such as foam, fur and feathers, and double-paned windows are the best insulators.

Specific heat

Specific Heat can be calculated.

Q = mcp ΔT

Q is the heat energy (Joules)

m is the mass of the substance (grams)

cp is the specific heat capacity (J/g°C)

ΔT is the change in temperature (°C)

Heat loss=Heat Gain

Measuring Energy Changes

Device called calorimeter

Reaction in cup either absorbs or releases energy causing water temp to rise or fall

Temperature change of water tells you whether energy is absorbed or released

1. How much temperature change can be expected when 250.0 g of water absorbs 5050 J of heat?

2. A 15.75-g piece of iron absorbs 1086.75 joules of heat energy, and its temperature changes from 25°C to 175°C. Calculate the specific heat capacity of iron.

3. To what temperature will a 50.0 g piece of glass raise if it absorbs 5275 joules of heat and its specific heat capacity is 0.50 J/g°C? The initial temperature of the glass is 20.0°C.

4. 100.0 mL of 4.0°C water is heated until its temperature is 37°C. If the specific heat of water is 4.18 J/g°C, calculate the amount of heat energy needed to cause this rise in temperature.

5. If a sample of chloroform is initially at 25°C, and its final temperature 236oC. If it absorbs 1.0 kilojoules of heat, and the specific heat of chloroform is 0.96 J/g°C, what is the mass of the sample in grams?

study of energy changes that occur during chemical...

Documents