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Flashcards in 3.2 Deck (31)
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1

define enthalpy

the thermal energy stored in a chemical system. we can measure this by seeing if the temp of a reaction changes

2

how you know a Reaction has taken place

color change, effervescence, precipitate formed, temp/energy change

3

exothermic reactions

give out energy to their surroundings, make things feel hot, the energy of the products is less than that of the reactants

4

endothermic reactions

take in energy form their surroundings, make things feel cold, the energy of the products is greater than that of the reactants

5

activation energy

the energy needed to start breaking the bonds in the reactants. the minimum energy needed for a reaction to take place

6

how to calculate enthalpy change

enthalpy to break bonds - enthalpy to make bonds

7

negative enthalpy change means the reaction is

exothermic

8

positive enthalpy change mean the reaction is

endothermic

9

enthalpy change of neutralisation

(delta H neut) enthalpy change when one mole of water is formed from a neutralisation reaction can be exothermic or endothermic

10

enthalpy change of formation

(delta H f) enthalpy change when one mole of a compound is formed from its elements in their standard state usually exothermic

11

enthalpy change of combustion

(delta H c) enthalpy when one mole of a substance reacts completely with O2 always exothermic

12

standard conditions for enthalpy changes of reactions

pressure- 100 kPa (1 atm)
room temp- 289k (25`c)

13

measuring enthalpy change experimentally

Q=mcT
T=temp change
c=specific heat capacity of substance being heated
m= mass of object being heated
Q=heat lost or gained from reaction in J

14

Hess's law

The total enthalpy change of a reaction is independent of the route by which the reaction takes place provided the initial and final condition are the same

15

to increase the rate of reaction

increase pressure, increase surface area, increase temp, add a catalyst, increase concentration, increase surface area

16

why increase surface area

after rate of reaction as more particles are exposed and able to react

17

why use a catalyst to increase rate of reaction

lowers the activation energy

18

why increase the concentration

more particles per unit volume increase the chances go a successful collision

19

rate of reaction

change in concentration of x over time

20

two types of catalysts

heterogeneous catalyst, homogenous catalyst

21

heterogenous catalyst

catalyst is in a different phase to the reactants i.e. solid catalyst vs gaseous reactants

22

homogeneous catalyst

catalyst is in the same phase as the reactants i.e. all liquid or gaseous

23

benefits of catalysts in general

increase sustainability, reduce temp needed -> cheaper

24

Botzmann distribution

shows the distribution of molecules of certain energies, area under the curve= no. pf molecules in the system, if the temp is increased the curve becomes broader and flatter

25

Le Chateliers principle

if conditions change the position of the equilibrium will respond by also changing to minimise the change imposed

26

factors affecting the position of the equilibrium

concentrations reactants of products, pressure, temperature

27

how concentration affects the equilibrium position

if the either the products or reactants concentrations are changed the rate of the forwards/backwards reaction will increase to minimise the change

28

how pressure affects the equilibrium position

increasing the pressure favours the side with the lease moles

29

how temperature affects the equilibrium position

one of the reactions (forward or backwards) will be exothermic and the other endothermic increasing temp favours the endothermic reaction, decreasing it the exothermic reaction

30

equilibrium constant kc

kc>1 equilibrium to the right kc<1 to the left