Constant-Pressure Calorimetry - Online Tutor, Practice Problems & Exam Prep

Calorimetry is a method used to measure the amount of heat absorbed or released during a chemical or physical process. In constant pressure calorimetry, often utilized with a coffee cup calorimeter, the heat transfer occurs under atmospheric pressure, making it constant. The calorimeter's heat capacity (C), defined as the heat required to change its temperature, is crucial in calculating heat changes. The key equation for heat transfer is:

$Q=mc\Delta T$

where Q is the heat in joules, m is the mass, c is the specific heat capacity, and ΔT is the temperature change. For the calorimeter, since its mass is typically unknown, the equation simplifies to:

$Q=C\Delta T$

This allows for determining the heat lost or gained by the object and solution, ensuring units are consistent for accurate calculations.

Constant-Pressure Calorimetry

Now, a calorimeter is just a container, usually insulated against heat loss, that contains a liquid with a given heat capacity. We're going to say usually when a heated object is placed in a liquid, both the liquid and the container that it's in will absorb this released heat. And we're going to say here that our standard heat capacity, which is capital C for a calorimeter, it's just the amount of heat required to change its temperature.

Now the weight of the calorimeter is usually unknown, so we're going to exclude it from our mass in terms of the formula. Now standard heat capacity equals C=Q/ΔT, which is our heat divided by change in temperature. Our standard heat capacity is usually in units of Joules over degrees Celsius. Now this temperature could be given to us in Kelvin and if it is given to us in Kelvin, then the temperature must be converted to Kelvin as well.

The units here both have to match one another. Here Q, which is our heat, can be in Joules or in Kilojoules, so always be on the lookout for that too. Make sure your units all match so that they cancel out to isolate the missing variable.

Constant-Pressure Calorimetry Example 1

What is the standard heat capacity in kilojoules per degree Celsius of a calorimeter that absorbs 87 joules at its temperature as its temperature goes from 33�C to 38.1�C? All right, so your standard heat capacity is C capital C and remember it equals Q divided by change in temperature.

Here we need to convert our joules into kilojoules because they want the unit in kilojoules. So we have 87.0 joules. Remember that 1K is 10 to the three joules here, so that's 0.087 kilojoules. Take that and plug it in, and then delta T is our change in temperature.

Final temperature minus initial temperature O. When we do that, we're going to get our answer in kilojoules over degrees Celsius. This comes out to be 0.0171 kilojoules over degrees Celsius.

Here. Our answer is going to be given to us in three sig figs. Because I'm using 87.0 Joules as my reference here, our temperatures could also be used. This has two significant figures. This has three. But just to give more detail into our answer, we're going to go with three significant figures.

Constant-Pressure Calorimetry

Now, constant pressure calorimetry is associated with our coffee cup calorimeter. Now we're going to say here it uses the coffee cup calorimeter to determine heat transfers occurring in a liquid solution. Now, a coffee cup calorimeter is basically an insulated Styrofoam cup with a lid. And we're going to say we call it constant pressure because the calorimeter measuring heat is open to the atmosphere where pressure is going to be fixed.

So here we're going to take a look at the images provided to us. We're going to start out with our coffee cup calorimeter here on the left. And when we look at this coffee cup calorimeter, we're going to say it has some key portions to it, one of them being this thermometer given to us on top. So that's going to measure my temperature difference. Then we're going to have here our Styrofoam cover or Styrofoam top. Here, this portion just represents the Styrofoam cup itself. In here we have our water and then finally here we have our stir.

When we place the heated object inside of here, we're going to be able to measure the amount of heat that's being released. Now realize here when it comes to the constant cut on constant pressure calorimeter and this coffee cup calorimeter, we have formulas that are associated with it. We're going to say when both the liquid and calorimeter absorb heat from the hot object, we get the heat lost by the object. So minus Q_lost equals plus heat gained by the water plus the heat also gained by the calorimeter.

Now we can expand this a little bit further. We're going to say expanding them to their heat capacity formulas. Where Q=MCΔT, we're going to have Qlost=-MCΔT, Qgained by the water is +MCΔT. And remember for the calorimeter, the mass of it is usually unknown, so we ignore its mass within our formula. So it just becomes the heat capacity of the calorimeter, which is C times change in temperature. So that becomes our equation for constant pressure calorimetry. It's going to be -MCΔT=+MCΔT+CΔT.

Constant-Pressure Calorimetry Example 2

Here we're told when 60 grams of lead at 68.3�C is poured into 90 grams water at 30�C within a coffee cup calorimeter, the temperature increases to 48�C. Based on this information, what is the heat capacity of the calorimeter? We're told the specific heat capacity, specific heat of lead and water are .128 and 4.184 respectively.

All right, so we're going to plug. Well, first of all, we need to realize we have our container with water and we're placing it lacing into it. Our lead lead is at a higher temperature than the water is. Remember, the hotter object always releases its heat, so it's going to release its heat. So the water is releasing its heat. So negative Q of water. Well, no negative Q of lead. Sorry. The object that's releasing its heat equals positive Q of water, which is absorbing it, plus Q of the calorimeter.

The calorimeter is absorbing some of it as well because they tell us that we need to find AT capacity. We don't need to find AT capacity if it was involved within the calculation, All right. So if their queues are involved, that means their mcats for some of them are involved. So negative MCAT of our lead equals positive MCAT of the water. Remember calorimeters? Their masses tend to be unknown, so it's just the heat capacity that they have times the change in temperature.

All right, So here we're going to say -60 grams of our lead times its specific heat capacity times the change in temperature. We're told that the temperature increases to 48�C, that the final temperature for everyone O. Here the change in temperature for the lead is the final temperature minus its initial temperature of 68.3�C that equals positive. And cat of water and cat on. So water is 90 grams of water times its specific heat capacity times its final temperature minus the initial temperature that it had.

Plus, we don't know the heat capacity of the calimeter, and that's what we're looking for. And here's the thing, before we add the lead to the water, it was just the water in the calorimeter hanging out with each other, so they both must have been at the same initial temperature. So change in temperature would be 48�C minus the initial temperature of the calorimeter, which again it was with the water, and we assumed they both were at the same initial temperature of 30�C.

All we have to do now is solve for this one missing variable. So what we're going to do now is we're going to multiply everything here on this side. Grams will cancel out, degrees Celsius will cancel out. We'll have Joules left on this side, which comes out to 155.904 joules equals grams cancel out, degrees Celsius cancel out. So we're going to have everything here also in joules. So that is 6778.08 Joules. Plus, when you subtract these, that's going to give me 18�C times my specific capacity or my heat capacity of C.

We got to isolate our heat capacity of C So next we're going to subtract this from both sides here. So when we do that, we're going to have now negative 66.2217600. Actually negative 6622.176 equals 18�C times my heat capacity of C divide out 18.0�C. So remember jewels are still here, so one 8.0�C heat capacity is a number that is positive. So this is an absolute terms. When we do that, that's Joules divided by degrees Celsius. So here my heat capacity which is capital C = 367.9 Joules over degrees Celsius. So that would be the heat capacity of our calorimeter within this given question.

A 115.6 g piece of copper metal at 182.5 ºC is placed into 120.0 mL of methylene chloride at 31.0 ºC within a coffee-cup calorimeter. If the final temperature of the solution is 50.3 ºC, what is the specific heat of methylene chloride? Assume the calorimeter absorbs a negligible amount of heat. The specific heat of copper is 4.184 J/g ∙ ºC and the density of methylene chloride is 1.33 g/cm³.

• A. 5.02 J/g°C
• B. 8.18 J/g°C
• C. 12.9 J/g°C
• D. 17.3 J/g°C
• E. 20.8 J/g°C

You place 75.0 mL of 0.100 M NaOH in a calorimeter at 25.00 ºC and carefully add 55.0 mL of 0.200 M HNO₃, also at 25.00 ºC. After stirring, the final temperature is 53.35 ºC. Calculate the enthalpy ∆H_rxn (in J/mol) for the formation of water. (Specific heat capacity, Cs, and density of the solution:4.184 J/g∙K and 1.00 g/mL).

• A. -9.25x10^-3 J/mol
• B. -7.72x10^-5 J/mol
• C. -4.57x10^-5 J/mol
• D. -2.05x10⁶ J/mol
• E. -6.39x10^-8 J/mol

Here’s what students ask on this topic:

What is calorimetry?

Calorimetry is an experimental technique used to measure the amount of heat energy released or absorbed in a chemical or physical process. This is done using an instrument called a calorimeter. There are different types of calorimeters, but they all operate on the principle of heat transfer. During a reaction, if heat is released, the process is exothermic, and if heat is absorbed, it's endothermic.

In a typical calorimetry experiment, the substance under investigation reacts inside the calorimeter. The calorimeter is insulated to prevent heat exchange with the surroundings. Any change in temperature, caused by the release or absorption of heat during the reaction, is measured with a thermometer or temperature sensor. By knowing the mass of the substance and the specific heat capacity (the amount of heat per unit mass required to raise the temperature by one degree Celsius), you can calculate the total heat energy involved in the process using the formula:

$Q=mc\Delta T$

where Q is the heat energy, m is the mass, c is the specific heat capacity, and ΔT is the change in temperature.

How does calorimetry work?

Calorimetry is a technique used to measure the amount of heat involved in a chemical or physical process. At its core, the process involves using a calorimeter, which is a device that contains the sample and substance under study. When a chemical reaction or physical change occurs, it either absorbs or releases heat, which changes the temperature of the surroundings.

In a typical experiment, the sample is placed in the calorimeter, and any temperature change is monitored. This change is used to calculate the amount of heat absorbed or released. The calorimeter is usually insulated to prevent heat exchange with the environment, ensuring that all heat transfer is between the sample and the substance inside the calorimeter (often water).

The amount of heat ($q$) involved in the process is calculated using the formula:

$q=mc\Delta T$

where $m$ is the mass of the substance that absorbs or releases heat, $c$ is the specific heat capacity of the substance, and $\Delta T$ is the change in temperature. By measuring these variables, you can determine the heat involved in the process.

What does calorimetry measure?

Calorimetry is a technique used to measure the amount of heat absorbed or released during a chemical or physical process. When a substance undergoes a chemical reaction or a physical change, energy is either released to the environment (exothermic process) or absorbed from the environment (endothermic process). Calorimetry quantifies this energy change, which is usually expressed in joules or calories.

The device used for this measurement is called a calorimeter. There are different types of calorimeters, but they all work on the principle of heat transfer. For example, in a simple coffee cup calorimeter, the temperature change of the water is measured before and after a reaction takes place in it. By knowing the mass of the water and its specific heat capacity (the amount of heat required to raise the temperature of one gram of water by one degree Celsius), the heat change of the reaction can be calculated. This information is vital in fields such as chemistry, biology, and material science, where understanding energy changes is crucial.

What is constant pressure calorimetry?

Constant pressure calorimetry is a technique used to measure the heat of a chemical reaction or a physical change under conditions of constant atmospheric pressure. This method is relevant when you want to understand the enthalpy change (ΔH) of a process, which is the total heat content of a system.

In a typical constant pressure calorimetry experiment, a calorimeter is used, which might simply be an insulated container like a coffee cup calorimeter or a more sophisticated device. The sample is placed inside the calorimeter, and any changes in temperature are measured over the course of the reaction. Since the pressure remains constant (usually at atmospheric pressure), any heat exchange between the calorimeter and its surroundings is due to the process happening inside it.

By knowing the mass of the substances involved and the specific heat capacities, you can calculate the heat change ($q=m*c*\Delta T$), which is equivalent to the enthalpy change at constant pressure. This technique is widely used in chemistry and biochemistry to understand energetic changes during reactions and processes.

How to do coffee cup calorimetry?

Coffee cup calorimetry is a simple method to measure the heat changes during chemical reactions at constant pressure. Here's how you can perform it:

1. Gather Materials: You'll need a styrofoam cup, a thermometer or temperature probe, a lid to cover the cup, and the reactants for your experiment.
2. Set Up: Place one of the reactants in the coffee cup and measure its initial temperature. If you're studying a reaction between two solutions, one reactant will be in the cup first.
3. Reaction: Quickly add the second reactant to the cup and immediately place the lid on top to minimize heat loss. Stir the mixture gently if necessary.
4. Measure Temperature Change: Insert the thermometer through the lid and record the temperature at regular intervals. You'll notice a temperature change as the reaction proceeds – it will either increase (exothermic reaction) or decrease (endothermic reaction).
5. Calculate Heat Transfer: Use the formula $q=mc\Delta T$, where 'q' is the heat transferred, 'm' is the mass of the solution (assuming it's similar to water, you can use the volume of the solution since the density of water is 1 g/mL), 'c' is the specific heat capacity (for water, it's 4.184 J/g°C).