energy physical chemistry Flashcards
go over the different types of reactions like combustion, metals + acids etc, go over all the practicals
what type of reactions are there (2)
exothermic and endothermic
what is an exothermic reaction?
one which gives out energy to the surroundings, usually in the form of heat and is usually shown by a rise in temperature of the surroundings.
(basically the test tube gets warmer - in the reaction chemical energy (stored in the bonds of chemicals) is converted to heat energy, which is released to the surroundings and rthe temp of the mixture and its surroundings go up)
what is an endothermic reaction?
one which takes in (absorbs) energy from the surroundings, usually in the form of heat and is usually shown by a fall in temperature of the surroundings
(if you hold the test tube, it gets cooler beause the products have more energy than the reactants, meaning you need to get the extra energy by using heat from the surroundings. THis is converted into chemical energy and the temperature of the reaction mixture and its surroundings goes down because heat energy has been converted into a different form of energy)
what is enthalpy change?
the overall change in energy in a reaction
symbol for enthalpy change?
ΔH
if the reaction is endothermic the enthalpy change is….
positive because the reaction is taking in energy
if the reaction is exothermic the enthalpy change is….
negative because the reaction is giving out energy.
what is a reaction profile?
it shows energy cchanges
what is the symbol for activation energy
Ea (tiny a)
what is activation energy?
the minimum amount of energy the reactants need to collide with eachother and react
is thermal decomposition exo/endothermic:?
endothermic
name one type of reaction which is definitely exothermic.
any reaction that produces a flame i.e., combustion reactions
name all reactions that are exothermic.
reactions of metals with acids, neutralisation reactions, displacement reactions
what happens to enthalpy change in an exothermic reaction?
the products are more stable than the reactants. As the reaction happens, energy is given out in the form of heat. The energy warms up both the reaction itself and its surroundings. TGhe enthalpy change is negative
what does the term stability?
it is usually used to describe the relative energies of the reactants and the products in a chemical reaction. The more energy a chemical has, the less stable it is.
what are the units for delta H?
it can be written as kJ/mol or kJ mol-1
what happens to enthalpy change in an endothermic reaction?
enthalpy change is positive. this is because the products have more energy than the reactants (remember energy is required and must be put in for the reaction to take place). The products are less stable than the reactants
what is specific heat capacity?
IT IS THE AMOUNT OF HEAT NEEDED TO RAISE THE TEMPERATURE OF 1 GRAM OF A SUBSTANCE BY 1 DEGREES.
it tells us how much energy has to be put in to increase the temperature of something.
what are the units for specific heat capacity?
J/g/ºC
what is the specific heat capacity of water?
4.18 J/g/ºC
When we do calculations this is the number you use for specific heat capacity (you sort off assume the liquid has a similar specific heat capacity to water)
What is specific heat capaqcity directly proportional to?
mass and temp change
what is the equation linking to specific heat capacity?
Q = mc ΔT
heat energy change = mass x specific heat capacity x temp change
what is the technique used to measure the amount of heat absorbed or given out in chemical reactions and physical changes?
calorimetry. it is based ont he idea that is we use the heat form a reaction to heat another substance, like water, then we can use Q=mcΔT to calculate the amount of heat released.
Note here the mass and temp changes are rferring to the substance heated
we can then work out the molar enthalpy change, ΔH, of the reaction in the unit kJ/mol.
when you do calimetry, how do you get the eneergy in kJ?
divide Q, energy given out, by 1000
what is the equation linking specific heat capacity (basically Q) and ΔH
you need to fine moles (n=m/mr)
then ΔH = Q / n
ΔH = heat energy change/ number of moles of x burned or otherwise
what should you do to make sure to get the correct ΔH when doing calculations?
check whether the reaction is endothermic or exothermic.
if it is endothermic, then ΔH is positive
if it is exothermic, then ΔH is negative
what is the number of carbon atoms and if you plot this, how must it be plotted?
they are a non- continuous variable, e.g., you can’t have 0.9 or 1.65 carbon atoms, you can only have 1,2,3, etc.
When you plot them, they must be plotted as a table
what is another way of finding out moles form conc and gases
n=cv
in gases - moles = volume (in dm³) / 24,
why do reactions either give out or absorb heat?
this is becasue energy has to be supplied to break the bonds in the hydrogen molecules and in the oxygen molecule (example is H2O). Energy is released when new bonds are formed between hydrogen aand oxygen atoms in the water molecules.
Whenthe covalent bonds are formed in the productt, the reaction is exothermis as the energy released making bonds in product is more than the energy required to break the covalernt bonds in the reactants.
(the expalnation doesnt relaly make sense - check p 219)
how can you calculate enthalpy change using bond energies?
calculate the energy required to break bonds and then required to make them. seubtract them from each otehr and you have your enthalpy change
how can you tell ammonia is produced in the haber process?
the ammonia, once cooled, condenses
what are the conditions for something to be in equilibrium?
the rate of reaction on each side is equal
the conc of reactandts and products are equal
what does it mean if a system is closed?
no reactants or products can escape
draw some position of equilibrium graphs
what happens if conc of something increases?
th eopposing reaction is to decrease it so you move away from the thing that has increased
if the conc ofsomething has decreased what happens?
you want that thing to increase again, so you woudl move towards that thing
what happens if pressure is increased?
you want to decrease the pressure, so equilibrium moves to the side with less moles and vice versa
what happens if temperature is increased
you want to decrease temp. this means you favour the reaction which is endothermic and vice versa
what is the eqaution for the haber process
N2 (g) + 2H2(g) = 2NH3 (g)
atmospheric nitrogen + hydrogen = ammonia
EXOTHERMIC
what happens as temp increases with the haber process
yield decreases
what does high pressure do?
shift the equilibrium to the right
what happens if temp is too low?
the rate of reaction will not be high enough and money would be lost becasue RoR is too slow
what conditions are used in the Haber process? why?
450 degrees, 200 atm
temp is high enough for the yeld and the rate of reaction, and the pressure is high enough, but not too higgh for it to be dangerous
what effect does a catalyst have on yields?
no effecgt
What is bond energy
The amount of energy needed to break 1 mole of covalent bonds in gaseous molecules
