Energetics

Question 1

Define enthalpy change

Answer 1

Enthalpy change is the heat energy change/ heat energy that is transferred between the system and surroundings. It is measured at constant pressure.

Question 2

Difference between endo and exothermic reactions?

And is the enthalpy change positive or negative for each?

Answer 2

Exothermic- heat energy is transferred from system to the surroundings. Enthalpy change is negative.

Endothermic- heat energy is transferred from the surroundings to the system. Enthalpy change is positive.

Question 3

What are:
system
surroundings

in a chemical reaction?

Answer 3

System refers to the reaction mixture, and everything outside of the system is called the surroundings.

Question 4

what are standard conditions?

Answer 4

standard conditions are 100 kPa and a specified temperature (usually 298K)

under standard conditions, enthalpy change is called standard enthalpy change and has a diff symbol (one with the circle and horizontal line through it)

Question 5

Draw the enthalpy level diagrams for endo and exothermic reactions with labelled axes

Answer 5

google it

Question 6

define standard enthalpy change of:
formation
combustion
reaction
neutralisation
atomisation

Answer 6

formation- the enthalpy change measured at standard conditions when one mole of a substance is formed from its elements in their standard states.

combustion- the enthalpy change measured at standard conditions when one mole of a substance completely burns in oxygen.

reaction- the enthalpy change measured at standard conditions when matter is transformed by a given chemical reaction, when all reactants and products are in their standard states.

neutralisation- the enthalpy change measured at standard conditions when one mole of water is produced by the neutralisation of an acid with an alkali.

atomisation- the enthalpy change measured when one mole of gaseous atoms is formed from an element in its standard state.

Question 7

Outline the experimental procedure for determining the standard enthalpy change of combustion of a liquid.

How is the enthalpy change calculated at the end?

Answer 7

1- A spirit burner containing the liquid being tested is weighed.
2- A known volume of water is added to a copper can.
3- The temperature of water is measured.
4- The burner is lit.
5- The mixture is constantly stirred with the thermometer.
6- When the temperature of the water reaches approximately 20 degrees celcius higher than its initial temperature, the flame is extinguished and the burner is immediately reweighed.
7- The final temperature is measured.

Q= mc/\T
/\H= -Q/n

Q= heat energy transferred to the water
m= mass
c= specific heat capacity of water
/\T= change in temperature
n= moles

Question 8

State the possible sources of error for measuring standard enthalpy change of combustion.

Answer 8

• Some of the heat energy produced is transferred to the air or the copper can and not the water.
• Some of the ethanol may not burn completely to produce carbon dioxide and water.
• The conditions are not standard, for example water vapour forms instead of liquid water.
• The experiment takes a long time, which means not all of the heat energy transferred from the water to the surroundings is compensated for. So, the longer the experiment takes, the more heat is lost to the surroundings (not transferred to the copper can).

Question 9

Outline the experimental procedure for determining the standard enthalpy change of neutralisation.

Answer 9

• Wear safety glasses and a lab coat.
• Using a pipette fitted with a safety filler, place 25 cm^3 of 1 mol/dm^3 acid into an expanded polystyrene cup.
• Measure the temperature of the acid.
• Using another pipette, place 25cm^3 of an alkali (usually dilute sodium hydroxide of a slightly higher conc than 1 mol/dm^3 to ensure all the acid is neutralised) into a beaker.
• Measure the temperature of the alkali.
• Add the alkali to the acid, stir with a thermometer and measure the maximum temperature reached.

Question 10

Identify sources of error in measuring /\Ho of neutralisation.

Answer 10

Uncertainties of measurements when using the thermometer and pipette.

(which means you arent 100% sure if youre using exactlyyy 25cm^3, and when you measure temperature you’re only estimating where you think the mercury of the thermometer is closest to, so you don’t really know the exact temperature)

Question 11

Why is it difficult to measure most standard enthalpy changes of formation experimentally?

Answer 11

Because measuring the enthalpy of individual elements, being toxic/gaseous is in itself difficult.

So measuring the temperature change when something is heating is difficult, when something is gaseous esp. and there could also be heat losses due to the high temperatures

Question 12

What is Hess’s Law?

Answer 12

Hess’s Law states that the enthalpy change of a reaction is independent of the path taken in converting reactants into products, provided the initial and final conditions are the same in each case.

Question 13

Study the enthalpy cycles from tb or CGP

Answer 13

do pp question practice for them

Question 14

What are mean bond enthalpy, and bond enthalpy change?

Answer 14

Mean bond enthalpy is the energy required to break one mole of bonds.

/\HB is the enthalpy change when one mole of a bond in the gaseous state is broken.

Question 15

What is mean bond enthalpy? How do you denote it?

Answer 15

For some bonds the bond enthalpy varies with the environment. Therefore it is useful to quote mean bond enthalpy.

You denote it like this E(C–C)= whatever

this example is for a carbon-carbon bond.

Question 16

How to calculate standard enthalpy change of reaction using mean bond enthalpy?

Answer 16

1- Calculate the sum of the mean bond enthalpies of the bonds broken.
2- Calculate the sum of the mean bond enthalpies of the bonds made.
3- Calculate the enthalpy change of reaction using:

/\Hr = sum of(bonds broken) - sum of(bonds made)

Question 17

How does bond enthalpy data give indication about which bond will break first in a reaction, how easy or difficult it is and how rapidly a reaction will take place at room temperature?

Answer 17

Bonds with higher bond enthalpy will require more energy to break them.

This means bonds with lower bond enthalpy will break first and more easily.

A reaction that involves breaking bonds with low bond enthalpies will be likely to take place at room temperature and more rapidly.

Reactions with breaking bonds that have high bond enthalpies would require heating or a catalyst.

Question 18

What are the limitations of using bond enthalpies to calculate the enthalpy change of reaction?

Answer 18

• Bond enthalpies are always measured in the gaseous state, which could be different than the states of the elements in the reaction.
• Using mean bond enthalpies is inaccurate as they might not correspond to the bond enthalpies in the molecules of the reaction itself.