Enthalpy

Question 1

What is the sign for enthalpy change for an exothermic reaction and endothermic reaction

Answer 1

exo= negative
endo= positive

Question 2

explain term standard conditions

Answer 2

a pressure of 100kPa, temperature of 298K and conc 1moldm3

Question 3

enthalpy change of reaction

Answer 3

enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions

Question 4

enthalpy change of formation

Answer 4

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

Question 5

enthalpy change of combustion

Answer 5

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

Question 6

enthalpy change of neutralisation

Answer 6

enthalpy change that takes place when one mole of water is formed from an acid and an alkali under standard conditions

Question 7

average bond enthalpy

Answer 7

energy required to break one mole of a specific type of bond in a gaseous molecule

Question 8

explain exothermic reactions in terms of enthalpy changes associated with the breaking and making of bonds

Answer 8

For an exothermic reaction, the energy released when making bonds in the products is greater than the energy required to break the bonds in the reactants.

Question 9

explain endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds

Answer 9

For an endothermic reaction, the energy required to break the bonds in the reactants is greater than the energy released when new bonds are made in the products

Question 10

Explain how you would conduct an experiment to find the enthalpy change of solution

Answer 10

-Pipette 25cm3 of water into a polystyrene cup. Measure the initial temperature every 30 seconds for 2 and a half minutes.
-Add a measured amount of solid after 3 minutes and stir. -Continue to measure the temperature every 30 seconds from 3 and a half minutes until a total of ten minutes has elapsed.

-Plot a cooling curve to calculate deltaT
- use q=mcDELTAt to calculate the energy exchanged, m = mass of water
-DELTA H can be found by dividing Q in kj by number of moles of solid.
-if temp increases, the value for DELTA H is negative
-if temp dec, the value is positive

Question 11

Explain how you would conduct an experiment to find the enthalpy change of neutralisation

Answer 11

1. Pipette 25cm3 of 1 moldm-3 HCl into a polystyrene cup and measure the temperature every 30 seconds for 2 and a half minutes using a thermometer.
2. Measure 25cm3 of 1 moldm-3 NaOH using a second pipette and add it to the polystyrene cup after 3 minutes stirring the mixture.
3. Continue to record the temperature of the mixture every 30 seconds until 10 minutes has elapsed.
4. Plot a cooling curve to calculate DELTA T
   value for DELTA H is negative

Question 12

Explain how you would conduct an experiment to find the enthalpy change of combustion

Answer 12

1. Using a measuring cylinder, measure out 150cm3 water into a beaker. Record the initial temperature.
2. Weigh a spirit burner containing the fuel.
3. Place the spirit burner under the beaker. Light the burner and stir the water with the thermometer until a temperature rise of 20˚C is obtained. Extinguish the flame.
4. Reweigh the spirit burner.
5. use Q=mcx change in temp to calculate the heat energy transferred in J, divide Q in kj by number of moles burned
   delta h is negative because combustion is exothermic