4.3- MEASURING ENTHALPY CHANGES Flashcards
What is the general name for the enthalpy change for any reaction?
standard molar enthalpy change of reaction ΔH°
What is standard molar enthalpy change of reaction ΔH° measured in?
kilojoules per mole kJmol-1
What is the standard molar enthalpy of formation ΔfH°?
enthalpy change when one mole of substance formed from its constituent elements under standard conditions, all reactants + products being in their standard states
What is the standard molar enthalpy of combustion ΔcH°?
enthalpy change when one mole of substance completely burnt in oxygen under standard conditions, all reactants + products being in their standard states
What is temperature related to?
average kinetic energy of particles in system
What happens to average KE and temperature as particles move faster?
average KE increases + temp goes up
Does the number of particles present affect temperature?
no
What is temperature measured with?
thermometer
What is heat a measure of?
total energy of all particles present in given amount of substance
What does heat depend on?
how much of substance present
What is included in the measure of heat?
energy of every particle
Where does heat flow from?
always flows from high to low temp
What is the enthalpy change of a reaction?
heat given out / taken in as reaction proceeds
Is there an instrument to measure heat directly?
no
How can you measure the enthalpy change?
arrange for heat to be transferred into particular mass of substance, often water
What three things do you need to know when trying to work out enthalpy change?
mass of substance that’s being heated up/cooled down
temp change
specific heat capacity of substance
What is specific heat capacity?
amount of heat needed to raise temperature of 1g of substance by 1K
What is the units for specific heat capacity?
Jg-1K-1
Enthalpy change equation
q = mcΔT
How is a simple calorimeter used?
burn fuel to heat known mass of water + then measure temp rise of water
assume all heat from fuel goes into water
What can the simple calorimeter be used to compare?
ΔcH° values of series of similar compounds as errors will be similar for every experiment
Is the flame calorimeter better than the simple calorimeter?
yes
Why is the flame calorimeter better than the simple calorimeter?
designed to reduce heat loss even further
How is the flame calorimeter designed to reduce heat loss even further than the simple calorimeter? (3)
spiral chimney made of copper
flame enclosed
fuel burns in pure oxygen, rather than air
How easy is it to measure heat changes for reactions that take place in reactions?
relatively easy
How is the heat generated in solutions?
by themselves + only has to be kept in calorimeter
What is often used for the calorimeters? (measuring enthalpy change of reactions in solutions)
expanded polystyrene
What are expanded polystyrene good at?
good insulators
As expanded polystyrene is a good insulator what do they do?
reduce heat loss through their sides
What does expanded polystyrene have a low of?
low heat capacity
What does it mean as expanded polystyrene has a low heat capacity?
absorb very little heat
What is the specific heat capacity of dilute solutions usually taken to be?
same as water- 4.2Jg-1K-1
What type of reactions are neutralisation reactions in solution?
exothermic- give out heat
General equation for acid neutralised by alkali?
acid + alkali -> salt + water
How do you find the enthalpy change for a reaction?
use quantities in moles given by balanced equation
When will a metal displace another one from a compound?
when a metal is more reactive than another, it will displace the less reactive one from a compound
If a metal that’s more reactive than another and displaces the less reactive one in a compound + compound can dissolve in water, how can the reaction be investigated?
using polystyrene beaker
What will happen even if expanded polystyrene cups are good insulators?
some heat will still be lost from side + top leading to known values for enthalpy changes measured by this method
