Energetics & Thermochemistry

Ms. KielyIB Chemistry SLCoral Gables Senior High School

What energy changes occur when chemical bonds are formed and broken?

A. Energy is absorbed when bonds are formed and when they are broken.

B. Energy is released when bonds are formed and when they are broken.

C. Energy is absorbed when bonds are formed and released when they are broken.

D. Energy is released when bonds are formed and absorbed when they are broken.

Bell-Ringer - IA REVIEW

TURN IN TOPIC 4 REBUILDS

What energy changes occur when chemical bonds are formed and broken?

D. Energy is released when bonds are formed and absorbed when they are broken.

ANSWER

Bell RingerWhich statement is correct for an endothermic reaction?

A. The products are more stable than the reactants and ΔH is positive.

B. The products are less stable than the reactants and ΔH is negative.

C. The reactants are more stable than the products and ΔH is positive.

D. The reactants are less stable than the products and ΔH is negative.

READ TOPIC 5.1 and complete exercises 12 - 15!!!

New quarter means new pop quizzes :)

ANSWERWhich statement is correct for an endothermic reaction?

C. The reactants are more stable than the products and ΔH is positive.

Energy comes in many forms, and we will be discussing mainly chemical energy in this topic. It will be referred to in this topic as heat energy or changes in heat energy since

the change in heat energy during a reaction can tell us a lot about what is occurring between the reactants.

Chemical Energy: Heat energy that is stored in the chemical bonds, (including intermolecular forces), that hold molecules and compounds together.

This heat energy is stored as potential energy in chemical bonds.

Thermochemistry

In order for a chemical reaction to occur, energy of some sort must be present.

-Energy is released when new bonds are formed between atoms. (Exothermic)

-And energy is absorbed when you need to break the bonds of a molecule. (Endothermic)

VIDEO: https://www.youtube.com/watch?v=5-TPVHIi39w

Exothermic Endothermic

Thermochemistry

Chemical reactions take place in many different environments, such as test tubes, beakers, flasks, living cells, the ocean, etc. It is therefore useful to distinguish between the system- the area of interest, such as the sample or reaction vessel you are using to conduct a chemical reaction; and the surroundings- in theory everything else in the universe!

When the system releases heat to the surroundings = exothermicWhen the system absorbs heat from the surroundings = endothermic

System and Surroundings

Exothermic Endothermic

SystemSystemSurroundings Surroundings

Surroundings

Most chemical reactions take place in an open system, which can exchange BOTH energy and matter (for instance gas, which is matter, can escape in an open system).

A closed system can exchange ONLY energy (such as heat escaping through a closed container- conduction) but not matter with the surroundings.

System and Surroundings

IMPORTANT: Although energy can be exchanged between a system and the surroundings, the total energy cannot change during the process:

Any energy lost by the system is gained by the surroundings and vice versa. (Law of

Conservation of Energy)

ALL CHEMICAL REACTIONS ARE ACCOMPANIED BY ENERGY CHANGES

The heat content of a system is called its enthalpy (H), which is a Greek word meaning ‘heat inside’.

A change in enthalpy (ΔH) allows us to observe the heat transfer that occurs between the system and the surroundings, which can help us understand whether the reactants

undergo a chemical reaction that is exothermic or endothermic.

-When heat is added to a system from the surroundings (endothermic) such as if we melt ice, the enthalpy of the system (the ice) increases, and we symbolize this change in

enthalpy as +ΔH.

-When heat is released from the system to the surroundings (exothermic), such as if we freeze water into ice, the enthalpy of the system (the ice) decreases, and we symbolize

this change in enthalpy as -ΔH.

System and Surroundings

+∆H -∆H

System SystemSurroundings

Surroundings

Surroundings

VIDEO: https://www.youtube.com/watch?v=i3mYWB2fNp4

Endothermic Exothermic

a) An exothermic reaction. The enthalpy of the products is less than the enthalpy of the reactants.

b) An endothermic reaction. The enthalpy of the products is greater than the enthalpy of the reactants.

Let’s consider the following and apply it to chemical reactions:

-Energy is released when new bonds are formed between atoms. (Exothermic)

-And energy is absorbed when you need to break the bonds of a molecule. (Endothermic)Pg. 167, Figure 5.4

Can you explain the following graphs in terms of changes in enthalpy?

SUMMARY

-When substances react, the difference in the enthalpy between the reactants and products results in a heat change which can be observed and can help deduce whether a reaction is exothermic or endothermic.

-If the amount of energy gained is more than the energy released in a rx, then the rx is endothermic (and the products contain more enthalpy, since reactants need to absorb energy in order to break their very natural forces of attraction (bonds).

If the amount of energy released is more than the energy absorbed in a rx, then the rx is exothermic (and the reactants contain more enthalpy since they were likely not as stable as the products are, and so they spend a lot of energy trying to maintain their unstable states).

Never forget that bonds are attractions!

5.1: Calculating the changes in enthalpies of a reaction in solution from temperature changes: (∆H)[rx]A Calorimetry experiment is used in general to determine the changes in enthalpies of an exothermic rx, the reason being that a solution or object is being burned, which quite usually means a rx of combustion is being performed. Combustion rxs are always exothermic!

Let’s say you are heating this fuel, and want to know how much energy is released by the combustion of the fuel. You can figure out how much heat is actually released by the rx by looking at the changes in the temperature of the water.

The heat that is released by an exothermic rx would be absorbed by the water, causing the temperature of the water to increase.

Now, because you are trying to measure the energy released (exo) in this chemical rx of fuel burning, you must take into account the fuel itself; any changes in heat will depend on the amount of the fuel in the reaction, meaning, the amount of moles of fuel reacting in the reaction.

For this reason, enthalpy changes are generally expressed for both exothermic and endothermic rx’s in kJ/mol or kJ mol⁻¹: which basically means “the amount of energy released or absorbed per mole of the substance in the reaction”

Burning fuel is an exothermic rx. We cannot directly measure the heat being exerted by the fuel (the change in enthalpy of the reaction); we instead indirectly measure

the heat being released by measuring the changes in the temperature of the water.

Exothermic: (∆H)[rx] = -(∆H)[water] = -m(H₂O) x c(H₂O) x ∆T(H₂O)Endothermic: (∆H)[rx] = (∆H)[water] = m(H₂O) x c(H₂O) x ∆T(H₂O)

This will help with question #10:It is important to remember that it would only be extremely ideal in a high school calorimetry experiment that all the released heat from an exothermic chemical rx would be absorbed by the water in the can. However, since that is NOT LIKELY to happen because nothing is perfect in this ENIGMA OF A CONSCIOUS REALITY, we must account for other places where that released heat might go.

For example:-Some of the heat actually heats up the metal can containing the water, and some of the heat escapes to the surroundings. -If you are heating a liquid substance, then some of it may have evaporated to the surroundings as well, and thus its heat was never transferred to the water.

REFLECTIONGive respective examples on how you can physically determine whether a reaction is endothermic or exothermic.

Bell-Ringer

Which one of the following statements is true of all exothermic reactions?

a. They produce gasesb. They give out heatc. They occur quicklyd. The involve combustion

DO NOT TURN IN HW YET!

Bell-Ringer

Which one of the following statements is true of all exothermic reactions?

ANSWER

B. They give out heat

Exothermic rx’s always release heat.

Yes, most combustion rx’s are exothermic,but there are exceptions.

5.1: Measuring Energy ChangesAll changes in enthalpy, ΔH, between reactants and products depends on both the intra- and intermolecular bonds that hold those substances together.

You can find information on standard enthalpy changes for a given reaction in your IB data booklet. These values are measured at standard conditions:

● A pressure of 100 kPa● Concentrations of 1 mol dm³⁻

● All the substances are in their standard states of matter

*Standard states of matter are typically collected at 298 K

Here are examples of equations with their thermochemistry values and their calculated ΔH’s. These ΔH’s are actually calculated from temperature changes, as we

will see on the next slide.

*It is important to give state symbols in thermochemical equations as the enthalpy changes depend on the state of the reactants and the products. In you IB Data booklet, ΔH values

are different for the same substances in solid versus liquid versus gaseous state.

Heat changes can be calculated from temperature changes:

In general, the increase in temperature when an object is heated depends on 1) the mass of the object, 2) the heat added, 3) the nature of the substance

We can summarize this and calculate heat changes in an object, (q), via temperature changes in that object using the following equation:

q(J) = m(g) x c(J g⁻¹ K⁻¹) x ΔT(K)

C, stands for the specific heat capacity, which is the property of a substance which gives the heat needed to increase the temperature of unit mass by 1 K.

You will mainly be using the specific heat capacity of water, which is 4.18J g⁻¹ K⁻¹

In the last example we were calculating the heat change in ONE object based on temperature changes in that object.

IF INSTEAD, you want to consider the relationship between different objects, then you a more convenient formula is Heat Capacity (C):

C = heat change (q) temperature change (ΔT)

BELL RINGER:The temperature of a 2.0 g sample of aluminium increases from 25°C to 30°C. How many joules of heat energy were added? (Specific heat of Al = 0.90 J g⁻¹K⁻¹)

A. 0.36

B. 2.3

C. 9.0

D. 11

ANSWER:

C. 9.0