Unit 3 - Chemical Energy

Question 1

State the units of enthalpy

Answer 1

kJ mol-1

Question 2

Exothermic: ΔH is ______

Answer 2

Negative

Question 3

Endothermic: ΔH is ______

Answer 3

Positive

Question 4

Why is bond breaking endothermic (ΔH = positive)?

Answer 4

Needs energy to be put into the bonds for them to break

Question 5

Why is bond making exothermic (ΔH = negative)?

Answer 5

energy is released when bonds are formed

Question 6

If more energy is needed to break bonds than is released when bonds made, ΔH is _____

Answer 6

positive

Question 7

Define Hess’s Law

Answer 7

Total enthalpy change of a reaction does not matter with the route taken.

A —-> C = A —-> B ——> C

Question 8

What is Hess’s law used for?

Answer 8

Working out enthaply changes that you can’t find directly by doing an experiment

Question 9

What is bond enthalpy?

Answer 9

Energy required to break bonds

Question 10

Where can you find some common bond enthalpies?

Answer 10

In the data booklet

Question 11

Mean Bond Enthalpies

What formula do you use to calculate enthalpy changes?

Answer 11

Enthalpy change = bonds broken - bonds formed

Question 12

What equation is used to calculate enthalpy of combustion?

Answer 12

E = cmΔT

Question 13

Define c in E = cmΔT

Answer 13

c is the specific heat capacity of whatever was being heated up. It is the energy required to heat 1kg up by 1 degree Celsius