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Chemistry 101: General Chemistry14 chapters | 132 lessons | 11 flashcard sets

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Lesson Transcript

Instructor:
*Nicola McDougal*

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

This video lesson explains the technique of calorimetry used to measure heat transfer in chemical reactions. You will see how different materials have different specific heat capacities. You will learn how to carry out heat calculations using a simple equation.

Have you ever read the food label of your hamburger and seen how many calories your burger contains? Ever wondered how they know this? The technique used is **calorimetry**. This is the science of measuring heat transfer when a substance absorbs or gives out heat. A **calorimeter** is a device that we use to experimentally determine the heat associated with a chemical reaction. We will discuss two types of calorimeters later in the lesson.

Before we start to measure heat transfer, let me introduce you to two common units of heat or energy you will come across. Firstly, the **calorie**; this is defined as the amount of energy, or heat, required to raise the temperature of one gram of water by one Celsius degree. The calorie is an old unit, but is still commonly used today. The calorie used to describe the amount of energy in food is actually a kilocalorie (which is 1,000 calories). So, our 500-calorie burger is really a 500,000-calorie burger!

The SI unit for heat is called the **joule** and is defined in terms of the calorie: 1 cal = 4.184 J. So, now our burger is equal to 500 kilocalories, 500,000 calories, and over 2 million joules! It is important that you can easily convert between calories and joules, so let us do an example now.

Express 28.4 J of heat (or energy) in units of calories. Our conversion factor is 1 cal = 4.184 J. So, 28.4 J * 1 cal / 4.184 J = 6.79 cal. Notice we have set up the calculation so we can cancel the units to leave us with calories. And, don't forget to check for significant figures.

In your everyday lives, you probably know that different substances respond differently when heated. It is important to know how materials respond to heat! You also need to think about how much of the material you have. How does the amount of the substance affect the energy required to heat it? Well, I'm sure you can see that heating up a large hamburger takes longer than heating up a small one. The amount of heat energy needed to change the temperature of a substance depends on three things:

- The amount of the substance you are heating (which is the number of grams) - the larger hamburger takes longer to increase in temperature compared to the small one.
- The amount of temperature change (the number of degrees) - the higher you want the temperature to go the longer you need to heat it.
- The identity of the substance.

Now, we have already observed that different substances react differently when heated. It is also true that some substances require relatively large amounts of energy to change their temperatures, while others require relatively little. We call these differences 'heat capacity.' **Specific heat capacity** is the amount of energy required to change the temperature of one gram of a substance by one Celsius degree.

Here, we can see a table of specific heat capacities of common substances. The higher the number, the more energy is needed to change the temperature of the substance:

Substance | Specific Heat Capacity (J/g degrees C) |
---|---|

Water (l) | 4.184 |

Beeswax | 3.4 |

Wool | 1.26 |

Aluminum | 0.89 |

Iron | 0.45 |

Bone | 0.44 |

Carbon (diamond) | 0.52 |

Silver | 0.24 |

Gold | 0.13 |

Notice that water has a very high specific heat capacity compared to the other substances. This is hugely important and is the reason why lakes and oceans are relatively slow to respond to heating or cooling compared to the land around it. Now we have the three factors that affect how much heat is needed to change the temperature. We can represent this by the following equation:

*Q* = *s* * *m* * change in temperature, where *Q* = heat (or energy), *s* is specific heat capacity, *m* is mass in grams, and delta T is the change in temperature in degrees Celsius.

In the next section, we will use this equation to do some heat calculations.

Let us return to the way we measure heat transfer using calorimetry. There are many different types of calorimeters, but I am only going to talk about the two you are most likely to come across. The first one is called the 'Bomb Calorimeter.' Here, a chemical reaction is performed inside of a metal container that is immersed in water. All of the energy released by the reaction goes into heating the water.

Our newly learned heat equation is used to figure out the heat of reaction. We know the specific heat capacity of the water, we know how much water we have, and we can measure the change in temperature. This is exactly the way nutritionists work out the energy content of food. The food is burned in the chamber and the temperature of the water is measured. The higher the temperature rises, the more heat is given out, and the more calories the food contains.

The other type of calorimeter you may encounter in the school or college lab is the 'Coffee Cup Calorimeter.' The setup is much simpler, with just an insulated Styrofoam cup, stirrer, and a thermometer. The chemical reaction takes place in the coffee cup and the same calculations can be made. So, let us finish with a couple of simple heat calculations.

Question 1: 'How much energy does it take to heat 120 grams of water from 20 to 45°C?'

Here, we can simply use our equation and put in the numbers. So, *Q* = *s* * *m* * delta T, where *s* is 4.184 J per degrees Celsius per gram multiplied by 120 g multiplied by the change in temperature, which is 25°C. And, if you calculate this out, it comes out to 13,000 J or 13 kJ.

Question 2: 'You have a 1.6-gram metal nugget, and it requires 5.8 joules of heat to change its temperature from 23 to 41°C. Is it pure gold?'

This is a little trickier, but we know that materials have specific heat capacities. We can use this as an identification tool. We can simply rearrange our equation above to calculate the specific heat capacity and put in the information we are given. Once we have this, we can compare it to the known specific heat capacity and see if it really is gold.

So, rearranging the equation: *s* is equal to *Q* divided by mass multiplied by the change in temperature. And, if you put in the numbers that we have, the answer comes out to *s* is equal to 0.20 J per gram per degree Celsius. So, looking back at the table we see that the specific heat of gold is actually 0.13, and so this nugget is not pure gold!

In this lesson, you have learned that **calorimetry** is the science of measuring heat transfer when a substance absorbs or gives out heat. A **calorimeter** is a device used to experimentally determine the heat associated with a chemical reaction. We have learned two units of heat. The **calorie** is defined as the amount of heat required to raise the temperature of one gram of water by one Celsius degree. A **joule** is defined in terms of the calorie: 1 cal = 4.184 J. We learned that substances have a **specific heat capacity**, which is the amount of energy required to change the temperature of one gram of a substance by one Celsius degree. And, the simple equation ** Q = s * m * delta T** can be used to work out heat calculations.

Watch and review the video lesson, then ensure your preparedness to:

- Distinguish between calorimetry, calorimeter, calorie and joule
- Identify two types of calorimeters, discuss the two units of heat, and convert between joules and calories
- Express understanding of specific heat capacity
- Work out heat calculations using an equation and a table of specific heat capacity

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Chemistry 101: General Chemistry14 chapters | 132 lessons | 11 flashcard sets

- Go to Atom

- Go to Gases

- Go to Solutions

- Go to Kinetics

- State Functions in Thermochemistry 6:14
- Enthalpy: Energy Transfer in Physical and Chemical Processes 8:50
- Using Hess's Law to Calculate the Change in Enthalpy of a Reaction 8:44
- Calorimetry: Measuring Heat Transfer and Heat Capacity 8:49
- Free Energy: Predicting the Spontaneity of a Reaction 7:07
- The Relationship Between Enthalpy (H), Free Energy (G) and Entropy (S) 8:33
- Electrochemistry: Free Energy and Cell Potential Energy 8:24
- The Relationship Between Free Energy and the Equilibrium Constant 9:25
- Go to Thermodynamics

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