Chem Module 3 - enthalpy

Define enthalpy

A measure of the heat energy in a chemical system.

Define enthalpy change

The difference in enthalpy between products and reactants in reaction

Explain, in terms of bond breaking and bond forming, why this reaction is exothermic

The reaction is exothermic because more energy is released when new bonds are formed in the products than is required to break the bonds in the reactants.

How can enthalpy change be calculated?

Enthalpy change = enthalpy of products - enthalpy If reactants

Why does incomplete combustion occur

occurs when there is insufficient oxygen, so carbon is only partially oxidised, producing carbon monoxide or carbon (soot) instead of carbon dioxide.

Suggest two reasons why

standard enthalpy changes of combustion determined

experimentally are less exothermic than the calculated theoretical values.

1. Heat loss to the surroundings – not all the energy released is absorbed by the calorimeter.

2. Incomplete combustion – some fuel may not burn fully, so less energy is released than expected.

What does the law of conservation of energy state?

Energy cannot be created or destroyed.

Exothermic

Where products have a lower enthalpy than reactants and heat is lost from system to surroundings

endothermic

• heat is gained from the surroundings to the system (endothermic change) giving a positive enthalpy change. products have a higher enthalpy than reactants

What is activation energy?

The energy input required to break the chemical bonds in the reactants

Reactions don’t spontaneously occurred

Because it has a activation energy activation energy is the minimum energy that is required to start reaction by breaking the bond

What is |> Hf

Enthalpy Change of Formation

The and therapy changed when one mole of the compound is formed from. It’s elements

What is |> Hc

Enthalpy Change of Combustion

enthalpy change when one more of substance is burnt

|> H neu

Enthalpy Change of Neutralisation

The enthalpy change when solutions of an acid and an akali react together under standard conditions to produce 1 mole of water.

What is meant by the term “enthalpy change of reaction”?

The enthalpy change of reaction is the heat energy change that occurs during a reaction under constant pressure, with quantities of substances as shown in the chemical equation.

draw the endo and exo graphs

endo has the product higher

Explain bonding in a C=C double bond using the orbital overlap model.

A C=C double bond consists of a σ-bond formed by head-on overlap of orbitals, and a π-bond formed by sideways overlap of p-orbitals above and below the carbon–carbon axis.

Why are experimental standard enthalpy changes of combustion less exothermic than theoretical values?

Heat is lost to the surroundings and combustion may be incomplete, making the measured value less exothermic than the theoretical value.

Why is the average bond enthalpy of a C=C bond not twice that of a C–C bond?

Because a C=C bond contains a σ-bond and a weaker π-bond, the total energy is less than twice the σ-bond energy of two C–C single bonds.

What are standard conditions?

Standard conditions are 298 K, 100 kPa, and concentrations of 1 mol dm⁻³, with all substances in their standard states.

Why is it very difficult to determine the standard enthalpy change of formation of hexane directly?

Because forming hexane directly from carbon and hydrogen is difficult to control and would produce multiple products, so the enthalpy change cannot be measured directly.

Why is it not possible to measure the enthalpy change of formation of N₂O directly?

Because nitrogen and oxygen do not react directly to form N₂O under standard conditions.

|> Hf under standard conditions

temp is 298K, pressure is 100KPA

Measuring Enthalpy Changes

Q=Mc<T

Breaking Bonds and making bonds

Requires energy / endothermic

making bonds releases energy / expthermic

Exothermic

the energy required to break bonds is less than the energy given out when new bonds form

Average Bond Enthalpy

The average enthalpy change when one mole of gaseous covalent bonds is broken

More energy takes to break bonds = chain length

means shorter chain length

Bond enthalpues are determined under

by molecules in a gaseous state not under standard conditions

How to CALC volume with RTP

Volume = amount in mol x molar gas volume (Vm)

if finding volume as dm3 the Vm is 24

if cm3 then Vm is 24,000

Number of miles = volume in dm3/24

temperature

Define standard enthalpy change of formation

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions

Define standard enthalpy change of combustion

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions

Define standard enthalpy change of neutralisation

The energy change that accompanies the reaction of an acid by a base to form one mole of water

What is absolute zero?

0 K or -273 C

How is energy change calculated?

q=mc(change in)T

q = heat energy in joules
m = mass of off surroundings (thing changing temp)
c = specific heat capacity (normally of water 4.18)
(Change in)T = change in temp

Describe how you could use this apparatus to obtain data from which you could calculate a value for the enthalpy change of combustion of octane. You should also explain how you would use your experimental results to calculate this value.

Measure a known volume (or mass) of water in the conical flask and record its initial temperature with the thermometer.

2. Record the initial mass of the spirit burner containing octane before burning.

3. Place the burner under the flask and light the spirit burner, heating the water until the temperature rises by a suitable amount (e.g., 20 °C). Record the final temperature of the water.

4. Extinguish the flame and reweigh the spirit burner to find the mass of octane burned.

5. Use the temperature rise of the water to calculate the heat absorbed by the water using:

Q = mcΔT

(m = mass of water, c = specific heat capacity of water)

6. Convert Q to kJ, and calculate the enthalpy change per mole of octane burned:

ΔH = – Q / moles of octane burned

(negative because combustion is exothermic)

Why is the experimental value of standard enthalpy change of combustion of methanol less negative than the data book value of standard enthalpy change of combustion of methanol

-heat loss to the surroundings other than the water. This includes the bracket but mainly the air surrounding the flame
-Incomplete combustion of methanol. Some carbon monoxide may have been produced instead of carbon dioxide. Appearing as a black layer of soot on the beaker
-evaporation of methanol from the wick. Burner must be weighed as soon as possible archer extinguishing the flame

How can enthalpy change of reaction be determined experimentally?

1) Reaction solution is added to a polystyrene cup and initial temperature is recorded
2) when reaction has taken place a final temperature is recorded
3) use q=mc(change in)T. The solution itself is the immediate surroundings. The energy transfer is between gene chemical particles and the water molecules in the solution

What is an average bond enthalpy?

The average enthalpy change that takes place when breaking by homolytic fission one mole of a given type of bond in the molecules of a gaseous species.bond enthalpies are always endothermic

What is an average bond enthalpy calculated from?

The actual bond enthalpy can vary depending on the chemical environment if the bond. The average bond enthalpy is calculated from the actual bond enthalpies in different chemical environments.

Will enthalpy change be endothermic or exothermic when bond is broke/made

Energy is required to break bonds so bond breaking is endothermic
Energy is released when ones form so bond making is exothermic

How can enthalpy change of reaction be calculated?

Sum of enthalpy of formation for all products - sum of enthalpy of formation for all reactants

What does Hess' law allow to be determined?

Hess' law allows enthalpy changes to be determined indirectly

What does Hess' law state?

Hess' law states that

How can enthalpy changes of formation be calculated?

Enthalpy change of formation = sum of enthalpy changes of formation of products - sum of enthalpy changes of formation of reactants

How can enthalpy changes of combustion be calculated?

Enthalpy change of combustion = sum of enthalpy changes of combustion of products - sum of enthalpy changes of combustion of reactants