energetics

Question 1

What is enthalpy change ?

Answer 1

• the amount of heat energy taken in or given during any reaction in a system when the pressure is constant

Question 2

What happens during an exothermic enthalpy change ?

Answer 2

• energy is transferred from the system (chemicals) to the surroundings
• so the products have less energy than the reactants

Question 3

What happens during an endothermic enthalpy change ?

Answer 3

• energy is transferred from the surroundings to the system ( chemicals )
• so there must be an input of heat energy
• and the products have more energy than the reactants

Question 4

What is the definition for the standard enthalpy change of formation ?

Answer 4

• the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions and all reactants and products being in their standard states

Question 5

What is the enthalpy change of formation for an element ?

Answer 5

• 0 kJ/mol

Question 6

What is the definition for the standard enthalp change of combustion ?

Answer 6

• the enthalpy change when 1 mole of a substance is completely combusted in oxygen under standard conditions and all reactants and products are in there standard states

Question 7

What are the standard conditions for enthalpy change ?

Answer 7

• 100 kPa
• 298 K
• solutions at 1 mol/dm3
• all substances in their normal state at 298 K

Question 8

What is calorimetry used for ?

Answer 8

• to find the enthalpy change of a reaction by measuring the temperature change
• it can be used to heat or cool a known mass of water

Question 9

How do we get the temperature change from the calorimetry graph and why do we do it this way ?

Answer 9

• we have to extrapolate the temperature curve back to the time that the reactants were added together
• by taking temperature readings at regular time intervals
• because is the reaction is too slow the exact temperature rise is difficult to get as cooling is occurring as the reaction is still going

Question 10

Describe the general method for calorimetry

Answer 10

• wash and dry equipment before using
• use a polystyrene cup for insulation
• measure out desired volumes / masses of substances
• measure the temperature of the 1st reactant every minute for 3 minutes
• at the 3rd minute add the 2nd reactant an then measure the temperature every minute for some more minutes

Question 11

What are some errors with the calorimetry method ?

Answer 11

• energy is usually lost to surroundings
• in calculation it assumes all substance have the specific heat capacity of water
• doesn’t include the specific heat capacity of the calorimeter
• reaction may be incomplete or slow
• density of solution is assumed to be the same as water

Question 12

What is the formula for energy transferred ?

Answer 12

• energy transferred = mass x specific heat capacity x temperature change

Question 13

What is the formal a for enthalpy change per mole ( also called enthalpy change of the reaction ) ?

Answer 13

• energy transferred / moles = enthalpy change

Question 14

What is Hess’ law ?

Answer 14

• the total enthalpy change for a reaction is independent of the pathway taken

Question 15

When is Hess’s law used ?

Answer 15

• when it is impossible to measure the enthalpy change directly by experiments so alternative reactions are carried out

Question 16

How do you use Hess’s law to work out the enthalpy change of formation of a reaction ?

Answer 16

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

Question 17

How do you use Hess’s law to work out the enthalpy change of combustion of a reaction

Answer 17

Enthalpy change of combustion = ( sum of enthalpy changes of combustion of reactants ) - ( su of enthalpy changes of combustion of products)

Question 18

What is the definition of mean bond energy ?

Answer 18

• the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules

Question 19

Why is enthalpy change calculated using mean bond energies less accurate than calculating it using data for enthalpy changes of formation or combustion

Answer 19

• because the mean bond energies are not exact
• they are averages