Chapter 2:Unit 11. Energy

Energy

Energy is the capacity to do work. There are two types of energy

1)      Potential energy is stored energy

2)      Kinetic energy is the energy of motion

The Law of conservation of energy states that total energy of the universe is constant.  Energy can neither be destroyed nor created. Energy can be converted from one form to another. energy is a scalar quantity, it doesn’t have any direction.

Unit: There are two different units, calories(C) and Joules.(J) Calorie is defined as amount of energy required to raise the temp of 1 g of water by 1⁰C. metric prefixes can be applied to both calorie and Joule. The relationship between Cal and Joule can be written as 1 cal= 4.184 J

Two important forms of energy are chemical energy and  heat energy which are very closely related to chemical science and reactions. Most reactions either release heat or absorb heat. If the reaction releases heat, it is called exothermic reaction. If the reaction absorbs heat, it is an endothermic reaction. Similarly, most chemicals have stored energy called chemical energy. When the substance is burned or allowed to react, chemical energy is converted to heat or other energy.

The amount of stored energy in food is measured using nutritional Calories ( C) where 1 Cal= 1 kcal= 1000 cal.

Caloric value of three classes of compounds present in food

Example: A baked food contains 5.0 g of protein and 15 g of carbohydrates. Estimate the amount of energy in Cal.

5.0 g Protein ×    4.0 Cal                    =20. Cal
                            1 g protein

15.0 g Carbohydrate ×    4.0 Cal                     = 60. Cal                                        1 g Carbohydrate

Total energy= 80. Cal

Effect of energy on matter:

A matter is solid, liquid or gas can be determined by the amount of kinetic energy of  the particles and strength of the interactions between the particles. Attractive force between the molecules in solid, liquid or gas is called intermolecular forces. Higher the magnitude of intermolecular forces, higher the boiling point and melting point.

The effect of heat energy on matter can be seen when a substance changes state, i.e., solid to liquid or liquid to gas. During the phase change temperature always stays constant until the transformation is complete. Any process that absorbs heat is called endothermic process. The processes releasing heat are called exothermic process.  When a substance changes from solid to liquid, the process is called melting and heat is absorbed. The opposite of melting would be freezing and heat is released in this process. Solid to liquid close intermolecular forces are broken in this process. Similarly, when liquid changes to gas, the process is called vaporization  or evaporation, requires addition of heat. Gas to liquid is exothermic process.

Sometimes solids  can be directly converted into gas without going via liquid. This type of phase change is called sublimation, reverse process is deposition.

Phase change processes are shown graphically where amount of heat released or absorbed is plotted against X-axis and corresponding temperature change is plotted along Y-axis. THis is called heating or cooling curve.

Below is the heating curve of water that shows how phase phase occurs when temperature stays constant. Amount of heat required to melt or freeze a substance is same and it is called heat of fusion.(∆Hf)

Amount of heat required to boil or condense a substance is same and it is called heat of vaporization. (∆Hv)

Knowing the amount of sample and heat of vaporization or fusion, total amount of heat required or given off can be determined in  a phase change process.

watch this out!

Following activity has been taken from AACT.

Heating Curve of Water

Background

The three states of matter are solid, liquid, and gas. To change from one state to another, energy is either added to or removed from the system. In this investigation, you will look at heating up water. Does that involve adding or removing energy? Explain.

Below, draw how you think particles are arranged in the three states. Use spheres to represent the particles and label each box with the state it represents.

When changing from one state to another, what happens to particles in terms of energy?

Procedure

1. Visit teachchemistry.org/heating-curve. You should see this on your screen.

1. On the heating curve above, label the states of matter. Include the state changes.
2. Choose an initial point on the graph. This is your starting temperature/state. Choose a second point that is the same state. Draw the two particle diagrams in the squares. Explain why the diagrams look the way they do.

1. Calculate the energy required to heat up 65.0 mL of the sample. Show your work.

Verify your answer by clicking “Calculate.”

The following part is optional. It requires heat of fusion and and heat of vaposrization.

1. Choose an initial point on the graph. This is your starting temperature/state. Choose a second point that is a different state. Draw the two particle diagrams in the squares. Explain why the diagrams look the way they do.

1. Calculate the energy required for 30.0 mL of water to undergo this state change.

Verify your answer by clicking “Calculate.”

• Pick a point on the ice part of the heating curve. Click on a point about 75 ^oC warmer. What state is this? _________ Record the T₁ and T₂ How much energy is required to heat 15.0 g ice to T₂? Show your work.

Verify your answer by clicking “Calculate.”

• Pick a point on the liquid part of the heating curve. Click on a point about 75 ^oC warmer. What state is this? _________ Record the T₁ and T₂ How much energy is required to heat 15.0 mL of water to T₂? Show your work.

9. Why are your answers to questions 7 and 8 not the same? Explain.