Unit #7 - Thermochemistry
What is enthalpy?
'Heat content' - the amount of heat something has.
What is the relationship between the moles of a substance in a chemical reaction and the amount of heat absorbed/released by the system produced by the chemical reaction? What does the sign of ^H tell you?
+-Heat in kJ = # of moles of each substance in the balanced chemical equation. Where heat is + if an endothermic reaction occurs (system gains energy from the surroundings) and heat is - if an exothermic reaction occurs (system loses energy to the surroundings).
When writing a balanced chemical equation for an exothermic reaction which side of the equation does +heat go on? why?
+HEAT would go on the product side because since there was release of heat that occurred to the system during this chemical reaction there would be less heat on the product side of the equation then there would be on the reactant side so you must add heat to the product side to keep the equation balanced.
When writing a balanced chemical equation for an endothermic reaction which side of the equation does the +heat go on? why?
+HEAT would go on the reactant side because since an absorption of energy occurred to the system during this chemical reaction there would be more heat on the product side of the equation then the reactant side so heat must be added to the reactant side of the chemical equation to balance it.
How do you calculate the total amount of energy required to heat/cool an object that undergoes phase changes?
1.) Draw a graph of the situation to get a better understanding of the problem. 2.) Then break the problem down into each line segment on the graph. For example if you are calculating the amount of energy needed to change ice into water vapor you would break the problem down into: starting temp to melting point, phase change of solid to liquid, melting point temp to boiling point temp, phase change from liquid to gas, and boiling point temp to final temp. 3.) To calculate ^H where the temperature is increasing/decreasing use h=mc^t relationship where h is positive for increasing temp and negative for decreasing temp. 4.) To calculate ^H for the phase change part of the graph (flat portion) use the heat of fusion (+ for freezing, - for melting) or heat of vaporization relationship (+ for evaporating, - for condensing) in kj/mol. Then convert moles of the substance you're given to kj of energy required for the phase change. 5.) Once you have the values for the amount of energy required for each step of the process (steps 3+4) add them together (AFTER MAKING SURE ALL VALUES HAVE THE SAME THE SAME ENERGY UNIT) to find the total energy required for the complete phase change proceeds. 6.) To check your work if the substance is increasing in temperature ^Htot should be positive (endothermic reaction) and if the substance is decreasing in temperature ^Htot should be negative (exothermic reaction).
How do you find ^H for a reaction given the change in enthalpy for two equations for two similar reactions? Using Hess's Law
1.) Flip the reactants and products for either of the equations so that they match up as best as possible with the original equation we are trying to find ^H for. - Note that if you flip the reactants and products of an equation the sign of ^H for that equation. 2.) Multiply or divide the coefficients one of the equations by a small value so that when you add them together in the next step. Hopefully get as many reactants to cancel out with products that are the same and aren't part of the original equation. Note that if you multiply the coefficients of the equations you also need to multiply ^H for that equation by the same amount. 3.) Add the two equations together by writing out the reactants from both the equations yielding the products from both the equations. 4.) Cross out any reactant that is the exact same as a product (same substance and same amount of moles) and if they are the same substance but not the same amount of moles you can add or subtract one of them to both all of that substance on the same side of the equation. If you did this step right you should end up with one equation that matches the original equation you are solving for ^H in. If not you will need to try steps 1-3 again until it works. 5.) When adding the two equations together you also add the change in enthalpy for each equation together which results in the change in enthalpy for the original equation.
What is temperature?
A measure of the average kinetic energy in the particles of a sample.
What is Hess's Law?
A reaction can take place in more than one route but if the initial and final conditions are the same then the change in enthalpy is the same too.
What is a state function? Is heat of formation a state function?
A state function is a value that stays constant for a substance even when its surroundings change. Mass and heat of formation are both examples of state functions.
What is heat?
A transfer of energy from one object to another due to a difference in temperature.
How do you convert Celsius to Kelvin?
C + 273 = K
What is heat (q)?
Change in enthalpy
What is the difference between heat capacity and specific heat capacity?
Heat capacity is the amount of heat required to heat a substance 1 degree C while specific heat capacity is the amount of heat required to heat 1 gram of a substance by 1 degree C.
Which direction does heat travel?
Heat flows from the hot object to the cold object.
What is an endothermic reaction? What temperature does the system appear to us (the surroundings) as it undergoes an endothermic reaction?
Heat flows into the system - heat is moving from surroundings to the system (ENter). Essentially this also means that heat is exiting the surroundings so from the surroundings perspective (to us) the system "feels cold."
What is an exothermic reaction? What temperature does the system appear to us (the surroundings) as it undergoes an endothermic reaction?
Heat flows out of the system - heat is moving from system to sourroundings (EXit). Essentially this means that heat is entering the surroundings so from the surroundings perspective (to us) the system "feels hot."
What is heat of formation? What units is it measured in?
Heat of formation is the amount of energy needed to form a compound using the elements of that compound in their standard state. ^Hf0 is measured in kJ/mol of an element at 25 degrees C and 1 atm.
What is heat of fusion? ^Hf
Heat of fusion is the amount of energy needed to transform 1 mole of a solid substance into a liquid (+^Hf) or the amount of energy needed to transform 1 mole of a liquid substance into a solid (-^Hf).
What is heat of vaporization? ^Hv
Heat of vaporization is the amount of energy required to transform 1 mile of a liquid substance into a gas (+ ^Hv) or the amount of energy required to transform 1 mole of a gas into a liquid (-^Hv).
How do you write chemical equations using ^H? What are the units of ^H?
If the reaction is endothermic (HEAT reactant side) then ^H is positive and if the reaction is exothermic (HEAT on the product side) then ^H is negative. You always write ^H = x to the right of the equation and the units of x for a physical reactions are kJ/mol since there is only 1 substance that the mol could refer to and the units for a chemical reaction are kJ.
How do you convert Kelvin to Celcuis
K - 273 = C
What does the kelvin temperature scale measure?
Kelvin scale actually measures average kinetic energy of the particles in a substance. So 0K = 0J KE Also Kelvins can't be negative because you can't have negative joules of energy.
How do you calculate the heat of formation of a substance?
Multiply the number of moles of the element/compound (refer to balanced chemical equation) by the standard heat of formation for one mole of that element/compound (refer to reference sheet).
What is the difference between potential and kinetic energy?
Potential energy is the energy of motion and kinetic energy is the energy due to position.
If ^H is positive what does that tell you?
Reaction for the system is endothermic because heat is entering the system.
If ^H is negative what does that tell you?
Reaction for the system is exothermic because heat is exiting the system.
How do you calculate the change in enthalpy for a reaction when you aren't able to conduct an experiment in a lab?
Subtract the sum of the heat of formation of each of the products by the the sum of the heat of formation of each of the reactants.
What is energy?
The ability to do work or generate heat.
What is specific heat capacity (C)?
The amount of energy required to raise the temperature of 1 gram of a substance by 1 degree Celsius.
What is the standard heat of formation formula?
The change in enthalpy of a chemical reaction is the the sum of the heat of formations of the reactants subtracted by the sum of the heat of formations of the products. ^Hrxn = sum of ^Hf products - sum of ^Hf reactants Note to calculate the ^Hf for a reactant or product you have to multiply the number of moles of that element/compound by it's standard heat of formation in kJ/mol.
What is thermochemistry?
The study of energy associated with chemical processes.
How do you determine the system and surroundings?
The system is just the object you are observing or the substances in a physical/chemical reaction and the surroundings are literally everything else in the universe that is not the system.
How do you solve problems asking the amount of heat released for every n moles of a chemical in a reaction?
Use kJ/mol relationship to convert n moles of a substance to kJ of energy required. But make sure to take the absolute value of the energy required because you can't have negative energy.
Which substance has a higher specific heat capacity (water or sand)?
Water has a higher specific heat capacity because it takes longer to heat up than sand (think of the beach).
What is the relationship between heat, mass, specific heat capacity, and change of temperature?
^H = mC^T where ^H = change in heat to the system, m = mass of the system, C = specific heat capacity of the system, and ^T = final temp of the system - initial temp of the system
