P1: Energetics

Question 1

What is enthalpy?

Answer 1

Refers to all the heat energy stored in a chemical system,under constant pressure

Question 2

What is enthalpy change? (🔺H)

Answer 2

When you measure a heat energy change under a constant pressure

Question 3

What is Hess’s Law?

Answer 3

The enthalpy change in a reaction at constant pressure is independent of the route taken.

Question 4

What are the standard conditions?

Answer 4

100KPa
298K (25•c)

Question 5

Describe a formation cycle

Answer 5

🔺cH/🔺rH
Reactants ——> Products
^. ^. 🔺fH
^. ^
Elements in their
Standard states

1. Balance equation for direct route
2. Draw thermochemical cycle and note which arrow you flip
3. Calculate total enthalpy change of formations for reactants and products (ignoring H) (times moles)
4. Add formation values to cycle and flip the sign of the arrow you flipped.
5. Sum them together

(this is for each element)

Question 6

Describe a Combustion Cycle

Answer 6

🔺fH/🔺rH
Reactants ————> Products
v. v 🔺cH
v. v
Combustion products
CO2+ H2O

1. Draw combustion cycle and balance equations
2. Using the date from the table, replace 🔺cH with the combustion values (times any moles)
   (O2 is not included)
3. Flip the symbol (+ or -) if the route
   goes against the arrow
4. Add 🔺cH together to find 🔺fH

Question 7

What equation do you use to find energy?

Answer 7

q= mc🔺t

q= energy ( j )
m= mass (g)
c= SHC (4.18Jg^-1K^-1)
t= temp change (K)

Question 8

How do you find an enthalpy value?

Answer 8

Enthalpy = energy/moles

1. Find energy
   q=mc🔺t
2. Find moles
   moles= mass/Mr

Question 9

What is Calorimetry used for?

Answer 9

To work out the enthalpy change of combustion

Question 10

Describe the procedure to find the enthalpy change in a solution

Answer 10

-Use a polystyrene cup to prevent heat loss
-Add the acid and measure the temperature
-Add the alkali, stir, measure the temperature change
-Use q=mc🔺t (add mass of acid and alkali)
-Use mol= conc x vol
- Enthalpy= q/mol

Question 11

Define standard enthalpy change of reaction (🔺rH)

Answer 11

The enthalpy change of reaction according to the molar quantities in the equation under standard conditions

Question 12

Define standard enthalpy change of formation (🔺fH)

Answer 12

Enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 13

Define standard enthalpy change of combustion (🔺cH)

Answer 13

Enthalpy change when 1 mole of a substance is completely burnt in oxygen to make CO2 and H2O, under standard conditions

Question 14

Define standard enthalpy change of neutralisation (🔺neaut H)

Answer 14

Enthalpy change when an acid and an alkali react to form 1 mole of water under standard conditions

Question 15

What is an exothermic reaction?

Answer 15

-Give out heat energy to surroundings
-Gets hotter
-Make bonds
-Negative value

Question 16

What is an endothermic reaction?

Answer 16

-Absorbs heat energy from surroundings
-Gets colder
-Breaks bonds
-Positive bonds

Question 17

Describe an exothermic energy level diagram

Answer 17

-Reactant line at top
-Arrow down
-Products line at bottom

Question 18

Describe an endothermic energy level diagram

Answer 18

-Reactant line at bottom
-Arrow up
-Products line at top

Question 19

🔺H is…
and is measured in…

Answer 19

Enthalpy change
Measured in joules (J)

Question 20

What is on the x and y axis of an energy level diagram?

Answer 20

x= progress of reaction
y= energy

Question 21

What is different about an enthalpy level diagram compared to and energy level diagram?

Answer 21

Same but:
-Arrow is labelled with 🔺H=
-y axis is enthalpy