Enthalpy And Equilibrium

Question 1

At what temperature do atoms stop moving?

Answer 1

Absolute zero.

Question 2

True or false? You can get colder than zero on the kalvin scale?

Answer 2

False.

Question 3

Why is the energy involved in reactions important?

Answer 3

• We can measure fuels energy values
• Calculate energy requirements for industrial processes.
• Work out theoretical amount of energy to break and form a bond.
• Helps us predict whether a reaction or not will take place.

Question 4

What is thermochemistry?

Answer 4

Study of heat changes during chemical reactions.

Question 5

Define exothermic and endothermic.

Answer 5

Energy given out = exothermic

Energy taken in = endothermic

Question 6

What is enthalpy change? How is it measured?

Answer 6

Measuring heat change at a constant pressure.

Only measured at 100kPa and temperature 298K.

Question 7

Define enthalpy.

Answer 7

Is the thermal energy stored in a chemical system.

Question 8

What’s the equation for enthalpy?

Answer 8

H = U + pV

H = enthalpy
U = internal energy of the system
P = pressure
V = volume

Question 9

As we can’t measure enthalpy, what do we measure instead? And what is it?

Answer 9

Enthalpy change.

Heat exchange with surroundings during a reaction. Measured with Kelvin scale.

Differences between enthalpy of products and reactants.

Question 10

Differences between exothermic and endothermic conditions in reactions?

Answer 10

Exothermic:
Releases heat
Negative enthalpy change
Enthalpy change of products is less than reactants
Heat is lost from system to surroundings

Exothermic - excess energy lost so surroundings temperature will rise.
Endothermic - energy from surroundings taken in so surrounding temperature falls.

Endothermic:
Absorbs heat
Positive enthalpy change
Enthalpy change of products is greater than reactants.
Heat is absorbed by the system from the surroundings.

Question 11

What is enthalpy change of - formation?

Answer 11

Energy change takes place when 1 mole of a compound is formed from its constituent elements.

Question 12

What is enthalpy change of - combustion?

Answer 12

Energy change when 1 mole of a substance is completely burned in oxygen.

Question 13

What is hydration enthalpy?

Answer 13

Measure of energy when attractions form between positive and negitive ions in water molecules.

Question 14

Equation for measuring enthalpy change?

Answer 14

Q = mc∆T

Q = Heat exchanged with surroundings in joules
m = mass of substance in grams
c = specific heat capacity of substance
∆T = Change in temperature in Kelvin.

Them later divide by RMM and your answer for moles afterwards.

Question 15

What are the 4 factors of equalibrium?

Answer 15

Equalibrium can only be reached in a closed system (products and reactants can’t escape)

Equilibrium can be approached by either direction - final result will be the same.

Equilibrium reached when rates of 2 opposing processes are the same - dynamic process.

Equilibrium has been reached when macroscopic properties of the system don’t change with time.

Question 16

What is the technical definition for equalibrium?

Answer 16

The point at which there the same number of particles changing from reactants to product that are changing from product to reactants. That is equilibrium.

Has to be a reversible reaction though.

Question 17

What conditions can effect the properties of equilibrium mixture?

Answer 17

Temperature
Pressure
Concentration

Question 18

What is changing the position of equilibrium?

Answer 18

Changing proportions of reactants to products to get a greater yeild of products.

Question 19

What shifts equilibrium right and left?

Answer 19

Increasing products in equilibrium mixture means equalibrium had been moved to right - foreward direction.

Increasing reactants in equilibrium mixture means equilibrium has been shifted to left - backwards direction.

Question 20

What is Le Cháteliers principle? And what does it tell us? What’s it’s flaw?

Answer 20

Principle: If a system at equilibrium is disturbed, it moves in the direction that tends to reduce the disturbance.

Tells us direction of equilibrium movement when conditions are changed.

Flaw: Doesn’t say how far so we can’t predict quantities involved.

Question 21

What happens if you raise the concentration of one of the reactants?

Answer 21

Equilibrium will shift in the direction to reduce concentration of that reactant.

Question 22

How does changing pressure affect equilibrium?

Answer 22

Equilibrium will only change if there are different amounts of molecules in either side of the equation.

Higher pressure = more molecules in given volume

Higher pressure = equilibrium will shift to decrease pressure

Equilibrium will shift to the left - fewer molecules exert less pressure.

Lower pressure = equilibrium will move right.
So to help visualise in case of N2O4 2NO2
Molecules of N2O4 will decompose to form molecules of NO2. So it goes right, get it?

Question 23

What are the effects of temperature on shifting equalibrium?

Answer 23

Reversible reactions are exothermic in one direction and endothermic in the other. The size of the enthalpy determines how much equilibrium shifts in one direction.

E.g To Increase the temperature of an equilibrium mixture that’s exothermic in the foreward direction.

2SO2 + O2 2SO2 ∆H∅ = - 197KJmol-1.

• Negative sign means heats given out in foreward direction.
• Means heat is absorbed if reaction goes in reverse.
• Le Cháteliers principle tells us that if we increase the temperature the equalibrium moves in the direction that cools the system down.