Energetics Flashcards
What would you describe a chemical reaction that of which heat energy is given out?
exothermic reaction
What would you describe a chemical reaction that of which heat energy is taken in?
endothermic reaction
How would you define an exothermic reaction?
a reaction that releases energy and gets hot so it loses energy to its surroundings
How would you define an endothermic reaction?
a reaction that absorbs energy and gets cold so it takes energy in from surroundings
What is the meaning of enthalpy change for a process?
the TOTAL heat energy exchanged with the surroundings at constant pressure
What is the symbol for enthalpy change?
Q
What is the unit for enthalpy change?
kJ/mol^ -1
What enthalpy change symbol would an exothermic reaction have and why?
-ΔH
because heat is given out to its surroundings
What enthalpy change symbol would an endothermic reaction have and why?
+ΔH
because heat is absorbed from its surroundings
What kind of conditions are enthalpy changes usually measured in?
usually measured under standard conditions
What is the Law of Conservation of Energy?
the overall amount of energy in a reaction does not change as energy is conserved in reactions
Can energy be destroyed or created?
NO it can only be transferred
What happens to temperature in an exothermic reaction?
there is an increase in temperature, the temperature of the surroundings increases and the heat content of the system falls
What kind of reactions are classes under exothermic?
neutralisation and combustion
What happens to temperature in an endothermic reaction?
there is a decrease in temperature, the temperature of the surroundings falls and the heat content of the system increases
Why can displacement reactions be exothermic OR endothermic?
displacement reactions can either take in or give out (heat)energy
What is common between combustion and neutralisation reactions?
both type of reactions always give out energy (exothermic)
What do we use simple calorimetry experiments for?
to experimentally determine the relative amounts of energy released by a fuel
What are the two types of simple calorimetry experiments we need to know?
-enthalpy changes of reactions in solution
-enthalpy changes of combustion
How would you calculate the amount of energy produced by a chemical reaction in solution?
measure the temperature change when the solutions are mixed together
Why would you use an insulated container when measuring the temperature change using a simple calorimeter?
to prevent heat loss
What is always assumed about the specific heat capacity of the solution?
that it is ALWAYS 4.18 J/g/*C
(unless told otherwise)
What is the method for calculating the enthalpy change in a chemical reaction using a simple calorimeter?
-a fixed volume of one reagent is added to the calorimeter and the initial temperature taken with a thermometer
-an excess amount of the second reagent is added and the solution is stirred continuously
-the maximum temperature is recorded and the temperature rise calculated
-the energy given out would be calculated using the equation
Q = m x c x ΔT
What can be calculated using Q = m x c x ΔT ?
energy given out by burning fuels
What does each letter stand for in the equation
Q = m x c x ΔT ?
Q = the heat energy change, J
m = the mass of the substance being heated, g
c = the specific heat capacity, J/g/°C (always 4.18 unless told otherwise)
ΔT = the temperature change, °C
What is the method for the experiment that calculates the enthalpy change for the combustion of fuels?
-measure a fixed volume of water into an aluminium can
-clamp the can so its base is approximately 10cm above the bench
-weigh the spirit burner INCLUDING THE LID containing a fuel using a balance
-place the burner underneath the can
-measure the initial temperature of the water
-remove the burner lid and immediately ignite the burner to prevent it drying out
-burn the fuel and stir the water using a thermometer for 30 seconds without touching the sides
-measure the maximum temperature of the water in the can
-accurately reweigh the burner including its lid
-calculate the enthalpy change of combustion using the equations
Why is not all the heat produced by the combustion reaction is transferred to the water when using a simple calorimeter?
some heat is lost to the surroundings
some heat is absorbed by the calorimeter
What happens in a reversible reaction?
the product molecules can react with each other or decompose and form the reactant molecules again
What is the symbol that shows a reaction is reversible?
⇌
What can reversible reactions be seen in?
-dehydration of hydrated copper (II) sulfate
-thermal decomposition of ammonium chloride
What can reversible reactions be classed as?
exothermic AND endothermic as they can do both
What is the symbol for molar enthalpy?
ΔH
What is the equation for molar enthalpy?
ΔH = -Q / n divided by 1000
What do the letter in the equation for molar enthalpy represent
ΔH = -Q / n divided by 1000 ?
ΔH = molar enthalpy change
Q = the heat energy change
n = moles of the fuel burned
When calculating the molar enthalpy change, why does the heat energy change (Q) have to be made negative?
because the reaction is exothermic meaning it releases energy
How would you calculate the moles of the fuel burned (n) when calculating the molar enthalpy change?
change in mass of the fuel / Mr of the fuel = moles of the fuel burned (n)
What are energy level diagrams?
energy level diagrams are graphical representations of the relative energies of the reactants and products in chemical reactions
What is on the y-axis of an energy level diagram and what does it look like?
increasing enthalpy (represented as an arrow heading upwards)
What is on the x-axis of an energy level diagram and what does it look like?
progress of reaction (represented as an arrow heading to the right)
What do the arrow in the middle of a energy level diagram indicate?
it indicates whether the reaction is exothermic or endothermic
What way the does arrow in the middle of the energy level diagram point for an exothermic reaction and why?
the arrow points DOWNWARDS because energy is given out
What way the does arrow in the middle of the energy level diagram point for an endothermic reaction and why?
the arrow points UPWARDS because energy is absorbed
On an energy level diagram for an exothermic reaction, are the reactants higher than the products?
YES because the reaction starts with more energy than it ends with
On an energy level diagram for an endothermic reaction, are the reactants higher than the products?
NO because the reaction ends with more energy than it starts with
What is drawn next to the downwards arrow on an exothermic energy level diagram?
-ΔH
What is drawn next to the upwards arrow on an endothermic energy level diagram?
+ΔH
Are the products before the reactants on an energy level diagram?
NO the reactants are before the products
Should you write the actual chemical formula on the reactants and products line on an energy level diagram?
YES
What type of process is bond-breaking and why?
an ENDOTHERMIC process because energy is absorbed/taken in
What type of process is bond-making and why?
an EXOTHERMIC process because energy is released
What does whether a reaction is endothermic or exothermic overall depend on?
the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed
Why would a reaction be exothermic in terms of energy?
energy needed to break the bonds is less than the energy released in making new bonds
Why would a reaction be endothermic in terms of energy?
energy needed to break the bonds is more than the energy released in making new bonds
What is usually given in a bond energies equation?
a chart with the average bond energy (kJ) for each bond to work out the difference between reactants and products
How would you perform a calorimetry study of the reaction between HCl and NaOH to investigate temperature changes?
-using a measuring cylinder, place 25cm^3 of the NaOH solution into the calorimeter
-measure and record the temperature of the solution
-add 5cm^3 of the dilute HCl and stir
-measure and record the highest temperature reached by the mixture
-repeat these steps increasing the amount of acid added by 5cm^3 each time
In a calorimeter study that investigates temperature changes what does a larger difference in temperature show?
the larger the difference in the temperature the more energy is absorbed or released
