reactivity 1.1 Flashcards
temperature is the measure of _____
the average kinetic energy of the particles
heat is the measure of _____
the energy content of a substance
energy is a measure of ______
the ability to do work
what can/can’t open systems transfer? x2
can transfer energy AND matter
what can/can’t closed systems transfer? x2
can transfer energy BUT not matter
what can/can’t isolated systems transfer? x2
can not transfer energy NOR matter
inside the reaction vessel is known as ___
the system
outside the reaction vessel is known as ___
the surroundings
what does the law of conservation of energy state?
that energy is neither created or destroyed so the total energy of a process remains the same
define enthalpy
the chemical potential energy of a system/the measurement of energy in a thermodynamic system
when does enthalpy change?
when chemical reactions take place, changes in chemical energy take place
what symbol is an enthalpy change is represented by?
ΔH
an enthalpy change can be _______ or _________
positive or negative
what makes a reaction exothermic?
when the products have less enthalpy than the reactants
how does temperature in exothermic reactions change in surroundings/system?
heat energy is given off by the system to the surroundings (surroundings temp ↑, system temp ↓)
ΔH value in exothermic reactions
negative (because there is an enthalpy decrease)
why are exothermic reactions thermodynamically possible?
because Hreactants > Hproducts)
if the rate of the an exothermic reaction is too slow, what will happen?
the reaction may not occur, so the reaction becomes kinetically controlled
what makes a reaction endothermic?
when the products have more enthalpy than the reactants
how does temperature in endothermic reactions change in surroundings/system?
heat energy is absorbed by the system from the surroundings (surroundings temp ↓, system temp ↑)
ΔH value in endothermic reactions
positive (because there is an enthalpy increase)
“the substances that are reacting” is called ______
the system
what does an energy profile show? x3
energies of reactants, transition state(s) AND products of the reaction with time
define transition state
a stage during the reaction at which chemical bonds are partially broken and formed
is the transition state stable? why or why not?
no, it cannot be isolated and is higher in energy than reactants & products
define activation energy (Eₐ)
‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’ (the energy needed to reach the transition state)
between exo and endo thermic reactions, which has a lower activation energy? why?
exothermic because the reactants are closer in energy to the transition state
how to determine Eₐ from a drawn energy profile
the energy difference from the energy level of the reactants to the top of the ‘hump’
what does the standard enthalpy change for a chemical reaction (ΔH^Ꝋ) refer to?
the heat transferred at constant pressure under standard conditions and states
what are the standard conditions for ΔH^Ꝋ x3
pressure = 100 kPa, concentration = 1 mol dm⁻³ (for all solutions) AND each substance is in its standard state (gas/liquid/solid)
what is the commonly given temperature for ΔH^Ꝋ?
298 K (25℃)
what does Ꝋ signify?
that a reaction has been carried out under standard conditions
define standard enthalpy change of reaction (ΔH^Ꝋᵣ), is it for both exo and endothermic reactions?
the enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions (BOTH ENDO & EXO)
define standard enthalpy change of formation (ΔH^Ꝋf), is it for both exo and endothermic reactions?
the enthalpy change when one mole of a compound is formed from its elements under standard conditions (BOTH ENDO & EXO)
define standard enthalpy change of combustion (ΔH^Ꝋc), is it for both exo and endothermic reactions?
the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions (ONLY EXO)
define standard enthalpy change of neutralisation (ΔH^Ꝋneut), is it for both exo and endothermic reactions?
the enthalpy change when one mole of water is
formed by reacting an acid and alkali under standard conditions (ONLY EXO)
define calorimetry
a technique used to measure changes in enthalpy of chemical reactions
examples of what can a calorimeter be made of x3
polystyrene drinking cup, vacuum flask OR metal can
define specific heat capacity (c) (of the liquid)
the energy needed to raise the temperature of 1g of a substance by 1K
what is the specific heat capacity of water?
4.18 J g⁻¹ K⁻¹
how can energy transferred as heat be calculated? what do each of the letters mean & what are their units? x2
q = m∙c∙ΔT
q ⇒ heat transferred (J)
m ⇒ mass of water (g)
c ⇒ specific heat capacity (J g⁻¹ K⁻¹)
ΔT ⇒ temperature change (K)
ΔH = - Q / n
ΔH ⇒ change in enthalpy (kJ mol⁻¹)
q ⇒ heat transferred (J)
n ⇒ # of moles (mol)
two types of calorimetry experiments
enthalpy changes of reactions in a solution AND enthalpy changes of combustion
principle of ΔH for reactions in solution
the principle of these calorimetry experiments is to carry out the reaction with an excess of one reagent and measure the temperature change over the course of a few minutes
define reagent
a substance or compound added to a system to cause a chemical reaction
what assumptions are made for ΔH for reactions in solution? 5
c of the solution is = pure water (4.18 J g⁻¹ K⁻¹), density of solutions = pure water (1g cm⁻³), c is ignored, reaction is complete AND there are negligible heat losses
what are temperature correction graphs used for?
for reactions which are not instantaneous, there may be a delay before the maximum temperature is reached
for non-instantaneous reactions, why may the true maximum temperature not be reached?
during the delay substances may be losing heat to surroundings
steps to making a temperature correction graph x3
- take temperature reading before adding reactants to get a steady value
- add second reactant and continue recording temperature & time
- plot the graph and extend the cooling part of the graph until you intersect the time where the second reactant was added
principle of ΔH of combustion experiments
the principle here is to use the heat released by a combustion reaction to increase the heat content of water
components of a typical & simple calorimeter used to measure the temperature changes to the water (in an ΔH of combustion experiment) x6
insulating lid, water, copper cam, thermometer, draught/draft shield AND spirit burner
where can some of the heat produced by the ΔH of combustion reaction escape to (not being transferred to the water)? x2
surroundings AND absorbed by calorimeter
ways to minimize heat losses in a calorimeter measuring temperature changes to the water (ΔH of combustion) x3
copper calorimeter should not be too far above the flame, lid placed overtop AND draught/draft shields to block any air currents
main sources of error in ΔH of combustion calorimeter experiments x2
heat losses AND incomplete combustion
how is water contained in experiments to determine H changes in solution?
in an insulated calorimeter
what does a thermochemical equation give?
the equation for the reaction and the corresponding enthalpy change
