3B - Enthalpy Changes

Question 1

What is a:

System

Surrondings

Answer 1

System = Chemicals in a reaction

Surroundings = Everything else (universe)

Question 2

What would the energy diagram of a exothermic reaction look like? Why?

Answer 2

Question 3

What would the energy diagram of a endothermic reaction look like? Why?

Answer 3

Question 4

Sign and effect on tempertature:

1. Exothermic
2. Endothermic

Answer 4

Exothermic = ΔH = -ve and increase in temperature

Endothermic = ΔH = +ve and decrease in temperature

Question 5

Equations for finding the enthalpy of a reaction

Answer 5

q = mcΔT

then

ΔH = q/number of moles

Question 6

SI units for q, m, c and ΔT when using q = mcΔT

Answer 6

q = J

m = 1g = 1cm³

c = Jg^-1K^-1

ΔT = K or ºC

Question 7

3 reasons why water is most frequently used as the heat sink?

Answer 7

1. Very high specific heat capacity (sensitive experiment). Reaction will be more accurate.
2. Don’t need to weight it. 1g = 1cm³
3. Assume that all heat energy will be given to water as the chemical reaction will take place in water, the chemicals are water soluble.

Question 8

Specific heat capacity of water?

Answer 8

4.18 Jg^-1K^-1

Question 9

Define Hess’s law

Answer 9

If you can go from reactants to products by 2 different routes. The enthalpy change of each route must be the same. the initial andfinal conditions are the same.

Question 10

What is the average bond enthalpy?

Answer 10

The average enthalpy change that takes place when breaking by homolytic fission 1 mole of a given type of bond in the molecules of a gaseous species

Question 11

Signs for breaking and making bonds. Which is exothermic/endothermic

Answer 11

Breaking bonds = Always positive (endothermic)

Making new bonds = Always negative (exothermic)

Question 12

Formular for percentage error

Answer 12

[(Apparatus error x Number of uses) / Value measured] x 100