3B - Enthalpy Changes Flashcards Preview

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Flashcards in 3B - Enthalpy Changes Deck (12):
1

What is a:

System

Surrondings

System = Chemicals in a reaction

Surroundings = Everything else (universe)

2

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

A image thumb
3

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

A image thumb
4

Sign and effect on tempertature:

1. Exothermic

2. Endothermic

Exothermic =  ΔH = -ve and increase in temperature

Endothermic =  ΔH = +ve and decrease in temperature

5

Equations for finding the enthalpy of a reaction

q = mcΔT

then

 ΔH = q/number of moles

6

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

q = J

m = 1g = 1cm3

c = Jg-1K-1

 ΔT = K or ºC

7

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

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.

8

Specific heat capacity of water?

4.18 Jg-1K-1

9

Define Hess's law

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.

10

What is the average bond enthalpy?

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

11

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

Breaking bonds = Always positive (endothermic)

Making new bonds = Always negative (exothermic)

12

Formular for percentage error

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