Chemical Changes In Thermochemistry

Question 1

Changes in thermochemistry

Answer 1

Most chemical changes require or release energy to break bonds and from bonds.

•If extra energy remains it is released to the surroundings and is a EXOTHERMIC reaction.

•if the reaction requires more energy than us available it is ENDOTHERMIC reaction.

Question 2

Mixtures / what’s happening ?

Answer 2

Two substances mixed together at different temperatures, there is a transfer of energy from the ‘HOT’ to ‘COLD’ substance

One cools - gives energy (exo)
One warms - gains energy (Endo)

Always set up as:
-Q = Q /. -Q + Q

Question 3

Phase changes (adding energy / Endo)

Answer 3

Adding Energy
—> melting = Q n^H fusion
—> boil/vaporize = Q n^HVaporization

Where:
Q = Energy ( KJ )
n = moles ( Mol )
^H fus = molar enthalpy of fusion
( kj / Mol)

^H vap = molar enthalpy of vaporization ( kj/Mol)

Water:
• ^Hvap = 40.65 kj/Mol
• ^Hfus = 6.01 kj/Mol

Question 4

Measuring Energy changes

Answer 4

• energy changes will be in units ( J ) or ( KJ )

1. Temperature change
   Q= mc^t
   Q= energy ( J )
   m= mass (g)
   C= specific heat capacity ( J/g degrees Celsius
   ^t = change in temperature ( degrees Celsius)
   TF - TI

Temp determines energy LOST or GAINED
^T = negative (lost) exo
^T = positive (gained) Endo

Question 5

Exothermic A)
Endothermic B)

Answer 5

A) exo - releasing energy * X as in xscape / energy escaping

B) Endo - absorbing energy

Question 6

Specific heat capacity of water

Answer 6

Solid = 2.00 J/ g degrees Celsius
Liquid = 4.19 J/g degrees Celsius
Vapour = 2.02 J/g degrees Celsius
(Steam)

• use number for the final state of water in the problem if melting use 4.19 final state would be liquid
  —> in the questions if it starts as ice than melts to water use 4.19