Thermochemistry of Chemical Reactions

Learning Goals

Utilize calorimetry to measure the energy associated with bond-breaking and bond-formation processes in an acid-base neutralization reaction

•To determine quantitatively the heat of reaction between HCl and NaOH by measuring the temperature change

Thermochemistry

the study of energy changes that occur during chemical reactions and changes in state

First Law of Thermodynamics

Energy can be transferred and transformed, but it cannot be created or destroyed; total energy in the universe must remain constant

How do we use calorimetry to measure ∆H of a reaction??

- Assemble a coffee cup calorimeter

• Insulated 🡪 no heat loss to the environment (in a perfect world) 🡪 can assume all temperature changes are due to the reaction • Constant pressure 🡪 can assume the heat flow (q) for the process is equal to the change in enthalpy (∆H)

- Mix two reactants together in the coffee cup and measure the temperature change

endothermic

(of a chemical reaction or compound) occurring or formed with absorption of heat

exothermic

Chemical Reaction in which energy is primarily given off in the form of heat

q of solution

m (mass of solution) C (specific heat of solution) ΔT(temperature change)

q calorimeter

C (constant of calorimeter) * ΔT

total enthalpy change

-ΔH = qrn = -(q soln + q cal)

Learning objectives of the Thermochemistry experiment include:

-Using calorimetry to relate measured temperature changes in chemical reactions to the reactions' thermochemical properties.

- Demonstrating concepts, terms, and calculations of thermochemistry.

= Illustrating the concept of a state function for heats of reaction.

In an exothermic reaction, will the enthalpy change be positive or negative? Why?

Enthalpy change will be negative; the energy of the starting material is higher than the energy of the products.

Hess's Law states that if a reaction can be written as the sum of two or more other reactions, the enthalpy change (ΔH) for the overall process must be the sum values of the constituent reactions

True

Enthalpy change ( ΔH ) is referred to as a "path function" because it is dependent on the specific mechanism of reaction, or pathway

False

According to the Safety Data Sheet (SDS) for hydrochloric acid (HCl) the following statements are listed as hazard statements?

May be corrosive to metals.

May cause respiratory irritation.

Causes severe skin burns and eye damage.

Why do you weigh the solid NaOH by difference instead of directly weighing it on a weigh boat?

Sodium hydroxide is hygroscopic and will absorb water from the air, making it difficult to get an accurate mass.

What is the balanced chemical reaction and the net ionic reaction for reaction 3, NaOH solid in HCl?

Balanced:

NaOH (s) + HCl (aq) -> NaCl (aq) + H2O (l)

Net Ionic:

NaOH (s) + H+ (aq) -> Na+ (aq) + H2O (l)

heat change

The change when heat flows into an object, is q

Hess's Law

the overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process

enthalpy

The heat content of a system at constant pressure

state function

A property (like enthalpy change) that is independent of the specific mechanism or pathway a reaction follows. It only depends on the relative energy of the reactants and products.

What is an exothermic reaction?

A reaction that releases heat energy to the surroundings.

What happens to the temperature of the surroundings during an exothermic reaction?

The temperature of the surroundings increases.

What is the sign of ΔH in an exothermic reaction?

ΔH is negative.

In an exothermic reaction, what is true about the energy of bonds?

Energy released when new bonds form is greater than energy required to break old ones.

What is an endothermic reaction?

A reaction that absorbs heat energy from the surroundings.

What happens to the temperature of the surroundings during an endothermic reaction?

The temperature of the surroundings decreases.

What is the sign of ΔH in an endothermic reaction?

ΔH is positive.

What is required to break bonds in an endothermic reaction?

Energy is required to break bonds, which is added from the surroundings.

Calorimetry

A technique used to measure the heat of a reaction by monitoring the temperature change of the surroundings using a calorimeter

Specific Heat

The amount of heat energy required to raise the temperature of one gram of a substance by 1 C

Extrapolation

Used in cooling curves to determine the exact temperature at the "time of mixing". This accounts for the fact that heat transfer is not instantaneous and insulation is not perfect