Unit 5: Thermochemistry
How chemists put a number on the energy a reaction releases or absorbs, and why that number doesn't care which path the reaction took to get there.
7 topics · 27 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
5.1 Energy, Heat, and Work
- How it worksHow do heat and work both move energy around without either one being a form of energy that an object stores?
- How it worksWalk me through tracking energy as it flows out of a hot cup of coffee into the cooler air around it.
- Common mistakeWhy isn't heat something a hot object contains and hands off to a colder one?
- Going furtherWhat happens to the internal energy of a gas that does work by expanding against a piston with no heat added?
Your turn1 of 2
A hot cup of coffee sits on a table and warms the surrounding air. Why isn't it accurate to say the coffee "contains heat" that it hands off to the air?5.1 Specific Heat and Calorimetry Basics
- How it worksHow does a substance's specific heat capacity decide how many degrees it rises for a given amount of heat?
- Worked exampleFind the heat needed to raise the temperature of 250 g of water from 22.0 degrees C to 80.0 degrees C.
- Common mistakeWhy doesn't every substance heat up by the same number of degrees when given the same amount of heat?
Your turn1 of 2
Find the heat needed to raise the temperature of 363 g of water (specific heat 4.184 J per gram per degree C) from 23 degrees C to 87 degrees C.5.2 Constant-Pressure Calorimetry with a Coffee Cup
- How it worksHow does measuring a temperature change inside an insulated cup let you calculate a reaction's released heat?
- Worked exampleFind the heat released when 50.0 mL of 1.0 M HCl neutralizes 50.0 mL of 1.0 M NaOH and the temperature rises 6.5 degrees C.
- Common mistakeWhy isn't the heat measured in a coffee cup calorimeter exactly the reaction's enthalpy change at constant volume?
- Going furtherWhat happens to the measured temperature change if heat leaks out through the calorimeter's lid during the reaction?
Your turn1 of 2
In a coffee-cup calorimeter, 154 mL of combined solution (density 1.00 g/mL, specific heat 4.184 J per gram per degree C) rises 10 degrees C during a neutralization reaction. Find the heat released by the reaction.5.2 Bomb Calorimetry at Constant Volume
- How it worksHow does a bomb calorimeter's fixed volume mean the heat it measures equals a change in internal energy, not enthalpy?
- Worked exampleFind the energy released when burning a 1.000 g food sample raises a calorimeter 3.20 degrees C, given a calorimeter constant of 8.20 kJ per degree C.
- Common mistakeWhy isn't the heat measured by a bomb calorimeter the same quantity as delta H for the reaction?
- Ties togetherHow does the difference between a bomb calorimeter and a coffee cup calorimeter connect to the difference between internal energy and enthalpy?
Your turn1 of 2
Burning a 1.54 g food sample inside a bomb calorimeter raises its temperature by 4 degrees C. Given a calorimeter constant of 12 kJ per degree C, find the energy released per gram of sample.5.2 Enthalpy and Thermochemical Equations
- Common mistakeWhy does reversing a thermochemical equation flip the sign of delta H instead of leaving it unchanged?
- How it worksHow does writing delta H alongside a balanced equation turn a chemical reaction into a statement about energy?
- Worked exampleFind the heat released when 2.50 mol of methane burns completely, given that burning 1 mol releases 890.4 kJ.
- Going furtherWhat happens to delta H for a reaction when every coefficient in its thermochemical equation is doubled?
Your turn1 of 2
Burning 1 mol of methane releases 890.4 kJ. Find the heat released when 7.12 mol of methane burns completely.5.3 Hess's Law
- How it worksHow does adding up the enthalpies of a series of steps give the same total as running the overall reaction directly?
- Worked exampleFind the enthalpy of formation of CO2 by combining two other reactions' enthalpies with Hess's law.
- Worked exampleFind the enthalpy change for converting graphite directly into diamond using two combustion reactions and Hess's law.
- Common mistakeWhy doesn't the path a reaction actually takes affect its total enthalpy change, only the starting and ending points?
- Ties togetherHow does Hess's law connect back to enthalpy being a state function?
Your turn1 of 2
Reaction 1 has delta H = -187 kJ and Reaction 2 has delta H = 89 kJ. Adding Reaction 1 and Reaction 2 together, by Hess's law, gives the overall reaction. Find the overall delta H.5.3 Standard Enthalpies of Formation
- How it worksHow do you calculate a reaction's enthalpy from a table of standard enthalpies of formation for products and reactants?
- Worked exampleFind the standard enthalpy change for the combustion of methane using standard enthalpies of formation.
- Common mistakeWhy isn't the standard enthalpy of formation of an element like O2 gas some nonzero number?
Your turn1 of 2
A reaction's products have standard enthalpies of formation summing to -313 kJ, and its reactants have standard enthalpies of formation summing to -176 kJ. Find the reaction's standard enthalpy change.