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### The flow of energy

Heat and work

Objectives

- When you complete this presentation, you will be able to
- define energy, heat, and work
- describe the flow of energy in exothermic and endothermic processes
- use the proper units when calculating energy, heat, and work
- perform specific heat calculations

Energy transformations

- Energy is the capacity for doing workor supplying heat.
- Energy is detected only by its effects:
- the motionof a bounced basketball
- The heatgenerated by a chemical reaction

- Energy is detected only by its effects:
- Thermochemistry is the study of changesin chemical reactions and changes of state.
- Chemical potentialenergy is the energy stored in chemical bonds.

Energy transformations

- Chemical potential energy is seen in the combustion of methane in a laboratory burner.
- Methane, CH4, is added to the burner with oxygen gas, O2.
- A spark is added to the mixture and there is a flame.
- C-H (in CH4), and O-O (in O2) bonds are broken.
- C-O (in CO2) and O-H (in H2O) bonds are formed.
- Energy is released.

Energy transformations

- Looking at the reaction CH4 +2 O2 → CO2+ 2 H2O

Energy transformations

- Heat, symbolized by q,
- is the transfer of energyfrom one object to another
- due to the temperature difference between the two objects.

- Heat always flows from a warmobject to a coolerobject.
- If the two objects are in contact, and remain in contact, they will come to the same temperature.

Exothermic and endothermic Processes

- A chemical reaction is part of a system.
- Everything outside the system is called the surroundings.

Surroundings

System

Exothermic and endothermic Processes

- Energy is not created or destroyed.
- The total amount of energy in the system and the surroundings must remain the same.

Surroundings

System

Exothermic and endothermic Processes

- If the chemical reaction in the system uses energy
- then energy is transferred from the surroundingsinto the system.

- This is an endothermicprocess
- In an endothermic process, the system gains heat while the surroundings cool down.

Surroundings

System

energy

Exothermic and endothermic Processes

- If the chemical reaction in the system makes energy
- then energy is transferred from the system out to the surroundings.

- This is an exothermicprocess
- In an exothermic process, the system cools down while the surroundings gain heat.

Surroundings

System

energy

Units for measuring heat flow

- We measure heat flow in two common units:
- the calorie(cal)
- the joule(J)

- The calorie (with a lower case “c”) is ...
- a non-SI unit.
- the amount of heat to increase the temperature of water by 1°C.
- equal to 0.001 Calories (food calories or Cal).
- There are 1,000 cal in 1 Cal.

Units for measuring heat flow

- We measure heat flow in two common units:
- the calorie(cal)
- the joule(J)

- The joule is ...
- the SI unit for heat (and energy and work).
- the energy required to apply 1 newton of force (about 3.6 ounces) over a distance of 1 meter.
- equal to 0.2390 cal.
- There are 4.184 J in 1 cal.

Units for measuring heat flow

- We measure heat flow in two common units:
- the calorie(cal)
- the joule(J)

- We will be using the joule exclusively in this course.
- You will not be required to convert from J ➔ cal or from cal ➔ J.

Heat capacity and specific heat

- The amount of heat needed to increase the temperature of an object by exactly 1°C is called the heat capacity of that object.
- The heat capacity of an object depends on both its ...
- mass
- chemical composition

- It takes more heat to increase the temperature of 1 kg of water than it does to increase the temperature of 1 g of water.
- It takes more heat to increase the temperature of 1 kg of water than it does to increase the temperature of 1 kg of iron.

Heat capacity and specific heat

- The amount of heat needed to increase the temperature of an object with a mass of exactly 1 g by exactly 1°C is called the specific heat capacity or the specific heat, C, of that object.
- Different objects have different specific heats.

Heat capacity and specific heat

- Water has a very highspecific heat.
- C = 4.18 J/g•°C.

- This means that you must add a lot of heat to water to increase its temperature.
- It also means that we get a lot of heat out of water when we decrease its temperature.
- Farmers use this to protect crops in danger of freezing.

Heat capacity and specific heat

- Water has a very highspecific heat.
- C = 4.18 J/g•°C.

- This means that you must add a lot of heat to water to increase its temperature.
- It also means that we get a lot of heat out of water when we decrease its temperature.
- This is why the filling in a hot apple pie is more likely to burn your tongue than is the crust.

Heat capacity and specific heat

- To calculate the specific heat, C, of a material
you divide the amount of heat input, q,

by

the mass, m, times the temperature change, ∆T.

C =

- We can use this to calculate the specific heat of any material.

q

m∙∆T

Heat capacity and specific heat

Example 1:

The temperature of a 95.4 g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849 J of heat. What is the specific heat of copper?

m = 95.4 g

∆T = Tf – Ti = 48.0°C – 25.0°C = 23.0°C

q = 849 J

C =

q

m∙∆T

849 J

= 0.387 J/g∙°C

=

(95.4 g)(23.0°C)

Heat capacity and specific heat

Sample Problems: Find the specific heat of each of the following metals that are heated from an initial temperature of 25.0°C to the indicated final temperature using the indicated amount of heat.

- 37.0 g of iron heated to 50.0°C using 415 J of heat.
- 15.0 g of benzene heated to 52.0°C using 705 J of heat.
- 78.0 g of calcium heated to 30.0°C using 252 J of heat.

C = 0.449 J/g∙°C

C = 1.74 J/g∙°C

C = 0.646 J/g∙°C

summary

- Energy is the capacity for doing work or supplying heat.
- Thermochemistry is the study of energy changes in chemical reactions and changes of state.
- Chemical potential energy is the energy stored in chemical bonds.
- Heat, symbolized by q, is the transfer of energy from one object to another due to the temperature difference between the two objects.

summary

- A chemical reaction is part of a system.
- Everything outside the system is called the surroundings.
- Energy is not created or destroyed, which means that the total amount of energy in the system and the surroundings must remain the same.

summary

- If the chemical reaction in the system uses energy, then energy is transferred from the surroundings into the system (an endothermic reaction).
- If the chemical reaction in the system makes energy, then energy is transferred from the system into the surroundings (an exothermic reaction).

summary

- We measure heat flow in two common units: the calorie (cal), which we will not use and the joule (J), which we will use.
- The amount of heat needed to increase the temperature of an object by exactly 1°C is called the heat capacity of that object.
- The heat capacity of an object depends on both its mass and its chemical composition.

summary

- The amount of heat needed to increase the temperature of an object with a mass of exactly 1 g by exactly 1°C is called the specific heat capacity or the specific heat, C, of that object.
- To calculate the specific heat, C, of a material you divide the amount of heat input, q, by the mass, m, times the temperature change, ∆T.

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