Energetics

Question 1

What is Enthalpy change?

Answer 1

Enthalpy change is the heat energy change measured under conditions of constant pressure

Question 2

What is the symbol for enthalpy change?

Answer 2

∆H^Ɵ
(circle with a line means in standard conditions)

Question 3

What are standard conditions classed as?

Answer 3

100kPa pressure and 298K (25C) temperature

Question 4

What are the units for enthalpy change?

Answer 4

kJmol-1

Question 5

When breaking bonds, explain if the bond enthalpy is positive or negative.

Answer 5

• Energy needs to be absorbed to break bonds
• Bonds are broken in the reactants which is an endothermic process
• So enthalpy change is positive

Question 6

When making bonds, explain if the bond enthalpy is positive or negative.

Answer 6

• Energy is released when bonds are formed
• Bonds are made when products are being produced which is an exothermic reaction
• so enthalpy change is negative

Question 7

What is classed as an endothermic reaction?

Answer 7

If the energy needed to break bonds is > than the energy released from forming bonds

Question 8

What is classed as an exothermic reaction?

Answer 8

If the energy released from forming bonds > than the energy needed to break bonds

Question 9

How do you calculate the enthalpy change?

Answer 9

total energy needed to break bonds - total energy released from forming bonds

Question 10

What are Calometry reactions used for?

Answer 10

They are used to work out the bond enthalpy change for a specific reaction

Question 11

Tell me about the calorimetry reaction for combustion.

Answer 11

• An alcohol/fuel is burnt to increase the temperature of the water
• Make sure to weigh the fuel before and after (to see the mass burnt)
• Most energy from the fuel is transferred into the water, however, some is lost to its surroundings too (since it is a flame)
• To combat this, a lid is placed on top to prevent heat loss and windshields are placed on either side of the flame to prevent wind from moving the flame

Question 12

What is the difference between enthalpy change and heat energy change?

Answer 12

enthalpy change is a specific type of heat energy change that occurs at constant pressure and is used to describe chemical reactions, while heat energy change is a more general term for energy transfer due to temperature differences

Question 13

Explain fully how to calculate the enthalpy change.

Answer 13

You need 2 things, the energy transferred and the moles of the fuel burnt / used.
To get the energy transferred use the equation: q=mc∆T
To get the moles of the fuel burnt / acid used, use the equation: mass/mr or conc x vol

Question 14

Explain the equation for energy transferred.

Answer 14

q=mc∆T

q = energy transferred
m = mass of water / solution (total mass of BOTH liquids mixed)
c = shc of water ( 4.18Jg^-1K^-1 )
∆T = temp change

Question 15

what are the units for shc?

Answer 15

Jg^-1K^-1

Question 16

what is the shc of water?

Answer 16

4.18Jg^-1K^-1

Question 17

Tell me about the calorimetry reaction for solutions.

Answer 17

• Fill a polystyrene cup with an amount of acid
• measure the temperature of it
• add an amount of alkali to the acid and stir
• calculate the temperature change

Question 18

Why is Hess’s law used?

Answer 18

To work out the enthalpy changes that you can’t determine by doing an experiment.

Question 19

What is Hess’s law?

Answer 19

The total enthalpy change of a reaction is independent of the route taken.

Question 20

What is the enthalpy change of formation?

Answer 20

The enthalpy change when 1 mole of a substance is formed from is elements in their standard states under standard conditions.