Energetics Flashcards
Define enthalpy change
It is the heat energy transferred in a reaction at constant pressure in their standard states under standard conditions KJ mol-1
What happens to the reactants and products during a reaction?
Reactants- bonds are broken
Products- new bonds are made
Why do we measure enthalpy change?
Because enthalpy itself cannot be measured
What is the shape for enthalpy
Triangle
What is activation energy?
Minimum energy that a particle needs for a successful reaction to occur
What are the types of reactions involved with enthalpy
Exothermic and endothermic reactions
Draw the two enthalpy diagrams
Check physical flashcards to see if you’re right
Give the standard conditions values for pressure, temperature and concentration for all solutions
Pressure= 100kPa
Temperature= 298k
All solutions= 1.0mol dm-3
Define standard enthalpy of formation
Enthalpy involved when one mole of a compound is formed from its elements in their standard states and under standard conditions
Define standard enthalpy of combustion
Enthalpy involved (change) when 1 mole of a substance is completely burnt in excess of oxygen under standard temperatures and pressure
What is the standard enthalpy of combustion and why
0kJ mol because oxygen cannot combust with itself
Why aren’t values you get after calculation the same as in the question
• Incomplete combustion
• Heat loss
How can enthalpy of combustion be measured?
Using a calorimeter
What is formula for heat energy (Q)
Q= mass x specific heat capacity x change in Temperature
What is the formula for enthalpy change
Heat capacity/ mol
Draw the graphs of endothermic and exothermic reactions
Check physical flashcards to check if you’re right
Define mean bond enthalpy
The average bond enthalpy term is the average amount of energy per mole needed to break a specific type of bond, measured over a wide variety of different molecules
What is the simple equation of calculating enthalpy
Enthalpy change = sum of bonds broken - sum of bonds made
What direction does the arrow go for enthalpy of formation
Towards the products and reactants
What direction do the arrows go when calculations enthalpy is of combustion
Away from the the reactants and products
Describe the required practical
• Measure 50cm3 of water using a measuring cylinder
• Place into an insulated container (polystyrene cup)
• Record the initial temperature for at least 4 minutes
• Add a known mass
• Measure and record the temperature every minute for another 7mins
• Plot temperature vs time on a graph
• Extrapolate the line of best fit for the initial temperature and after the solid was added, use this to find the temperature rise
• Use Q= mct to find Q in J (/1000)
• divide my no of moles to work change in H
what is the general formula for change in enthalpy
total energy absorbed-total energy released
what is the formula for enthalpy of formation
change in f= products - reactants
What is the formula for enthalpy of combustion
reactants-products
Give the equation for the enthalpy of combustion of propanol
C3H8(g) + 5O2(g) = 3O2(g) + 4H2O(l)
Give the equation for the enthalpy of formation for methylbenzene, C7H8
7C(s) + 4H2(g) = C7H8(l)
Suggest why the student records the times to the nearest second and not to the nearest 0.01s
Because it’s harder to judge
Suggest why the student chose not to carry out experiments at temperatures in the the range 1-10C
Reaction would be too slow
Suggest by considering the products of this reaction why small amounts of reactants are used in this reaction
• Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + SO2(g) + S(s) + H2O(l)
SO2 is a toxic gas/ limit the amount of SO2 formed
Explain why the value given for the O=O bond enthalpy is not a mean value?
Oxygen is the only substance in the reaction that has O=O bond
Suggest one reason other than the use of mean bond enthalpies, why a value for the enthalpy of combustion of a liquid alkane is different from the value obtained using the expression in question 4.2
• Alkane is not gaseous
• Equation relates to gaseous alkanes
• It takes energy to convert it to gas
• That water/alkane/substances are gaseous in calculations using bond enthalpies
Give examples of exothermic and endothermic reactions
Exothermic- oxidation reactions
Endothermic-thermal decomposition of calcium carbonate
What are the characteristics of breaking and making bonds
- reactant bonds are broken and product bonds are formed
- You need energy to break bonds, so breaking is endothermic, stronger bonds need more energy to break
- Stronger bonds release more energy when they form
- The enthalpy change for a reaction is the overall effect of these two changes. If you need more energy to break bonds than is released when bonds are made, the enthalpy is positive
Why aren’t mean bond enthalpies exact
- Bond enthalpy is the energy required to break bonds
- Every time you broke the same bond, it would not require the same energy
- The energy needed to break a specific type of bond depends on the environment
- Mean bond enthalpies are the average energy needed to break a certain type of bond
- Breaking bonds is always endothermic so mean bond enthalpies are always positive
- So they are slightly less accurate than enthalpy change values calculated using Hess’s law
Define Hess’s law and give its purpose
- The enthalpy change at constant pressure in a reaction is independent of the route/path taken
- Handy for working out enthalpy changes you can’t find directly by doing an experiment
What is the enthalpy of formation of elements
zero
What value is used to calculate enthalpy of formation
Enthalpy of formation of products and reactants that are compounds
Suggest why the students value for enthalpy of combustion of methanol is different from the data book
• Experiment not done under standard conditions
• Evaporation of methanol
• Heat up calorimeter
How many significant figures must you convert your answers to?
3 significant figures
How many significant figures is the enthalpy change given
3 significant figures
What are the advantages of magnesium being a smaller ion than calcium ion
• Attracts chloride ion more strongly/ more ionic
When calculating in energetic what must your answer be
negative for combustion and enthalpy change reactions
In an experiment, the water was heated for 5 minutes. The student thought that the experiment could be improved by heating the water for 10 minutes.
Explain whether the accuracy in the student’s calculated value for 🔼cH may or may not be improved by heating for longer
- Less accurate due to greater heat loss
- More accurate due to smaller % uncertainty in temperature change
What should you always calculate in terms of temperature readings?
How many readings are taken before you calculate the uncertainty
Explain the meaning of the term mean bond enthalpy
• Enthalpy to break a bond
• Averaged over different molecules
Explain why the value in part b for the mean bond enthalpies is different from that obtained in part cii
Mean bond enthalpies are not exact
When do we use Hess’s law to calculate enthalpy in contrast with products-reactants/reactants-products
- We use Hess’s law when the product cannot be measured directly i.e. one product is given in the question
- If all the products are given in the question, you can use products/reactants/reactants-products to calculate the enthalpy change
