Textbook Unit 1.4 Flashcards
(27 cards)
what is Enthalpy change
the change in energy when a chemical reaction happens
whats the definition of enthalpy change
it is the heat energy transferred in a reaction at constant pressure. its units are delta H and kj mol-1
what does the circle symbol with a line thru it mean
substance were in the standard states and the measurements were made under standard conditions
- symbo usually comes after delta h
what are standard conditions
100 kPa pressure
exothermic reactions = delta H is negative
- bc give out energy
endorthermic reactions =
delta h is positive
- bc absorb energy
what is an example of an exothermic reaction
- combustion of a fuel like methane
- oxidaions of a carb like glucose in respiration
what is an example of an endothermic reaction
- thermal decomposition of calcium carbonate
- main reactions of photosynthesis like sunlight supplying the energy
essentially what is a reaction about
making and breaking bonds
when a reaction happens what happens to the reactant bonds and the product bonds
reactant bonds are broken and product bonds are formed
when and why would the delta h be positive
in an endothermic reaction (BREAKING bonds)
- bc you need energy to break bonds
- stronger bonds = more energy needed
when and why would the delta h be negative
- in an exothermic reaction (FORMING bonds)
- bc energy is released when bonds are formed
- stronger bonds release more energy when they form
what is the enthalpy change for a reaction
the overall effect of these 2 changes
- if more energy is needed when breaking than forming, then the delta h becomes positive
- if more energy is released when forming rather than breaking, then delta h becomes negative
what is bond enthalpy
the energy required to break bonds
what does the energy needed to break a bond usually depend on
the ENVIRONMENT
in calculations.. use what?
use mean bond enthalpy (average energy needed to break a certain bond over a range of compound)
why are mean bond enthalpies always positive
because breaking bonds are always endothermic
how to work out the enthalpy change of reaction?
total energy absorbed - total energy released
give two different types of enthalpy
(f) standard enthalpy of formation - the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
(c) standard enthalpy of combustion -enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions under standard conditions
what can you find with calorimetry
find out how much heat is given out by a reaction by measuring a temperature change
how to use calorimetry to find enthalpy change
- to find enthalpy of combustion of a flammable liquid, you burn it inside an apparatus (calorimeter)
- as fuel burns, water heats up
- can work out heat energy that has been absorbed if you kno the
mass of the water
temp change
shc - all the heat given out by the burning fuel would be absorbed by the water, so can work out the enthalpy change of combustion exactly
- in any calorimetry experiment, heat will always be lost to surroundings making it hard to get accurate results
- when you burn fuel, combustion may be incomplete (less energy is given out)
- flammable liquids are quite volatile too so may lose fuel to evaporation
how can a calorimetry be used to measure enthalpy changes in solutions like nuetralisation, dissolution (dissolving) and displacement
- for a NUETRALISATION reaction, add a known volume of acid to an insulated container ( e.g. polystyrene cup) and measure temp
- add a known volume of alkali and record the temp of the mixture at regular intervals (stir the solution to make sure its evenly heated)
- find temp change for experiment. use it to calculate enthalpy change of the reaction (w/ formula)
q=m c DELTA T
q = heat lost or gained (joules)
- same as enthalpy change if the pressure is constant
m = mass of water/solution (grams)
c = shc (amount of energy it takes to raise 1g of water by 1K)
delta t = change in temp of water/solution (kelvin)
what unit has the same value as kelvin
degrees
what is Hess’s law
the total enthalpy change of a reaction is independent of the route take
