Lesson 1: Spontaneity and Energy
Part b: Spontaneous Processes
Part a:
What is Thermodynamics?
Part b: Spontaneous Processes
Part c:
The First Law of Thermodynamics
The Big Idea
Some changes occur naturally on their own — without sustained intervention. Spontaneous processes in chemistry are those changes that proceed forward under given conditions. This page will help a student learn to recognize such processes.
What Does Spontaneous Mean?
On the previous page of this lesson, we described Chemical Thermodynamics as the science of predicting and explaining whether or not a chemical or physical change will occur naturally, the direction in which it naturally occurs, and the extent to which it occurs. The general term given to a process that occurs naturally under a specific set of conditions is the term spontaneous. A spontaneous process is a process that occurs naturally on its own without any sustained or continued intervention.
The rolling of a ball down a hill is an example of a spontaneous process. The ball may require a gentle push or nudge to get it started. But once started, the ball will continue rolling down a hill of any given length. Additional pushes or nudges will not be necessary to sustain the motion. It occurs naturally on its own without any continued external intervention.
Chemistry students are familiar with the combustion of methane gas in the Chemistry lab. Methane, the main component of natural gas, is often ignited as it exits a Bunsen burner during a chemistry lab. The ignition may occur by use of a lit match or a spark from a striker. Once ignited, the combustion of methane continues forever as long as there is a supply of methane and oxygen gas. The reaction is spontaneous and occurs naturally without requiring a continued intervention.
Conditions of Spontaneity
Identifying a process as being spontaneous often assumes a set of temperature and pressure conditions. Combustion reactions are always spontaneous at 1 atm of pressure and room temperature. But there are conditions. First, they must be activated. Similar to the combustion of methane exiting the barrel of a Bunsen burner, a candle must first be lit to initiate or start the process. The activation is required to overcome the high activation energy and initiate the reaction. It is a one-time deal; it is not a continued intervention. Second, there must be a supply of reactants - the candle wax and oxygen. Provided the fuel and the oxygen are available and the reaction has been activated, the combustion of the fuel is considered spontaneous. Once activated, the process is naturally self-sustaining without the need for continued, external intervention.
The Direction of Spontaneity
A process that occurs spontaneously in one direction is not spontaneous in the opposite direction under the same conditions. An ice cube melts at room temperature. That is to say that the change of state described by the equation
H2O(s) → H2O(l) (25°C)
is spontaneous at ~25°C. However, at the same temperature, water in the liquid state does not change to ice at room temperature.
Similarly, liquid water will freeze at -10°C. However, it will not melt at -10°C. The freezing of water is spontaneous at temperatures below 0°C while the reverse process (melting) is not spontaneous under the same conditions.
Examples of Spontaneous Processes
There are endless examples of spontaneous and non-spontaneous reactions. We have already mentioned a ball rolling down a hill, the combution of methane from a Bunsen burner, and the burning of a candle as spontaneous processes. Here are a few more.
The dissolving of sodium chloride in water is an example of a spontaneous process. As long the NaCl crystals and the water are in contact with each other, the water molecules will solvate the ions and dissolving will occur. The process will continue without intervention until the solution is saturated with sodium chloride. There are interventions that can be taken to increase the rate at which the salt dissolves - stirring, increasing the temperature of the water - but the rate at which a process occurs has nothing to do with it being spontaneous or non-spontaneous. The process of dissolving would continue on its own without these interventions.
The diffusion of a gas is another example of a spontaneous process. Diffusion involves the spread of a gas from a location where it is concentrated to the surrounding space. Consider a two-bulb system separated by a valve like that shown below, with all the gas initially present in the Bulb A. If the valve is opened, some gas from Bulb A will diffuse to Bulb B. This occurs spontaneously and does not require a fan or the input of energy.
The rusting of iron is our final example of a spontaneous process. Rusting involves the reaction between an iron surface and oxygen gas in a moist environment to produce flakes of hydrated iron(III) oxide. The process takes time but occurs naturally. It does not require the continuous input of energy or intervention from external influences. Steel is an alloy of iron and despite efforts to protect it from rust by the addition of additives, steel still rusts. Steel tools are particularly susceptible to rust over the course of time, as shown in the photo at the right.
Examples of Non-Spontaneous Processes
A non-spontaneous process is one that does not occur naturally on its own. A constant input of energy is often required for a non-spontaneous process to occur. A non-spontaneous process is akin to riding a bicycle up a hill. The process can occur, but it requires the continuous input of energy since the uphill ascent is not sustainable on its own.
Photosynthesis is an example of a non-spontaneous process. The process involves the reaction of carbon dioxide with water to produce glucose and oxygen. The process does not take place without sunlight.
The charging of a battery is another example of a non-spontaneous process. Your phone battery won’t charge if you fail to plug it in to the charger. The continued supply of energy from the charger to the phone is required to charge the phone battery.
Some processes are spontaneous at one set of pressure-temperature conditions but non-spontaneous at other conditions. Liquid water freezes at temperatures below 0°C but does not freeze at temperatures above the freezing point of 0°C.
Can an Endothermic Process by Spontaneous?
The short answer to the question is YES! The long answer to the question is in discussion of Gibbs Free Energy found in Lesson 3. For now, we will elaborate on the short answer.
Spontaneous processes are those that occur naturally on their own without external intervention. Let’s consider two processes - the melting of an ice cube at room temperature and the photosynthesis in plants. Both processes are endothermic. The enthalpy of the products are greater than the enthalpy of the reactants. Both processes climb up the enthalpy hill. But the melting of an ice cube at room temperature occurs naturally on its own without intervention. Plant photosynthesis is not spontaneous; it does not occur naturally on its own. What’s the distinction?
An ice cube melts naturally on its own. The flow of energy from the surroundings to the ice cube occurs as warmer air particles collide with the colder ice cube particles. This is a natural process; we refer to it as heat transfer. There is no way to stop or start it. It is an inescapable, passive event that occurs naturally between two objects of different temperatures.
Photosynthesis is also endothermic. But the nature of the energy input is quite different than the passive flow of energy that occurs as heat transfer. Photosynthesis does not occur because of heat flow into the plan. Instead, the energy input must be actively supplied by an external source - the sun or a grow light - that illuminate the plant with photons (light particles). The plant is unable to naturally turn the sun or grow light on or off. The “on its own” feature is absent in photosynthesis. Intervention is necessary and the process is classified as non-spontaneous.
It’s easy to mix up the terms endothermic and non-spontaneous because both seem to involve energy going into something. But these two terms describe very different ideas. Endothermic describes the direction that energy flows relative to the system and surroundings. Spontaneous and non-spontaneous describe whether or not a process occurs naturally on its own without intervention. So, every non-spontaneous process has an energy input requirement; but not every process that absorbs energy is non-spontaneous. Ice melting and plant photosynthesis highlight this distinction.
Explanations of Spontaneity
The interesting questions (and answers) are the why questions. Exactly why does the melting of ice occur at 25°C but the opposite process - freezing - not occur under the same conditions? There are reasons and we will get to them. But we have some additional concepts to develop. By the end of Lesson 2, you will be able to provide conceptual explanations for spontaneity. And by the end of Lesson 3, you will be able to quantify those explanations. But before we start Lessons 2 and 3, we have to discuss the first law of thermodynamics. That’s our next page. Before plowing ahead, take some time to internalize the ideas presented here on spontaneity.
Before You Leave - Practice and Reinforcement
Now that you've done the reading, take some time to strengthen your understanding and to put the ideas into practice. Here's some suggestions.
- Try our Concept Builder titled Spontaneity and Driving Forces. The first of the three activities is a great follow-up to this lesson.
- The Check Your Understanding section below includes questions with answers and explanations. It provides a great chance to self-assess your understanding.
- Download our Study Card on Spontaneous Processes. Save it to a safe location and use it as a review tool.
Check Your Understanding of Spontaneous Processes
Use the following questions to assess your understanding the meaning of spontaneous and non-spontaneous. Tap the
Check Answer buttons when ready.
1. Anna Litical and Aaron Agin live in a northern climate that experiences relatively extreme winter weather. They’re talking about their spontaneity lesson during lunch. Aaron is struggling with the claim that the melting of ice is not spontaneous below 0°C. His claim is that he’s seen icicles on his home melting on a sunny day when the temperature is -5°C. Anna has an answer for Aaron. But before you read it, make an attempt at an explanation of why ice melting at -5°C is not spontaneous despite the accurate observation of Aaron’s.
Check Answer
Answer:
Hi Aaron,
I know what you're talking about Aaron because I've seen it happen as well. But the process of melting at -5°C only occurs because of the sustained intervention of the sun. The process does not take place at night when the sun has set or even during the day when the sun is behind the clouds. The ice cannot continue to sustain the melting on its own without the help of the sun. Because the melting of the ice at -5°C requires this sustained intervention of the sun, we regard it to be non-spontaneous.
-- Your friend, Anna
2. Identify the following statements as being
TRUE or
FALSE. For any
FALSE statement, correct the statement or prepare an explanation for why it is
FALSE.
- All spontaneous reactions are explosive reactions.
Check Answer
Answer: FALSE
Being spontaneous means to occur naturally on its own. The process may or may not be explosive.
- All spontaneous reactions, like combustion reactions, release a large amount of energy.
Check Answer
Answer: FALSE
Spontaneous reactions may or may not release energy or even lots of energy. They simply are reactions that occur naturally without sustained interventions. The release of energy is characteristic of an exothermic reactions and does not guarantee spontaneity.
- Spontaneous reactions occur very rapidly.
Check Answer
Answer: FALSE
Spontaneous reactions can occur very slowly. The transformation of a diamond to graphite is spontaneous and occurs very, very slowly.
- If a truly spontaneous reaction is occurring, it will sustain itself without continuous intervention as long as it does not run out of one or more reactants.
Check Answer
Answer: TRUE
Yep! This is a good definition of spontaneous.
- Sugar wont dissolve in room-temperature water unless it is stirred. Therefore, the dissolving of sugar in water is not spontaneous.
Check Answer
Answer: FALSE
Sugar does dissolve in water even when not stirred. It just dissolves very slowly without any intervention. But it does dissolve and that makes it spontaneous.
- Homework (the paper variety) won’t burn unless you light it. Since the burning of paper requires intervention, it is not a spontaneous process.
Check Answer
Answer: FALSE
The initiation of the reaction involves sparking or lighting the paper to get the combustion started. But once started, it continues on its own without sustained intervention. This makes the combustion of paper spontaneous.
- Homework does not get completed unless you put sustained energy into its completion. Therefore, homework completion is not a spontaneous process.
Check Answer
Answer: TRUE
But wait! Just because it isn't spontaneous does not mean that it cannot occur. It just doesn't occur naturally without sustained intervention. In other words: quit looking for excuses to not do your homework.
Sorry. Chemistry is not on your side on this one!
3. Can a non-spontaneous process ever occur?
Check Answer
Answer: Yes
Non-spontaneous processes do occur and occur quite often. They just don't occur on their own. They require continued interventions - usually the supply of energy in the form of electricity, light, mechanical work. The charging of a battery, photosynthesis, and riding a bicycle up a hill are all examples of non-spontaneous processes that only occur do to the continued input of energy.
4. Identify the following processes as being spontaneous (
S) or non-spontaneous (
NS) under the conditions that are provided.
- Mrs. Mandocino has acquired a vessel full of liquid nitrogen for some demonstrations. Nitrogen has a boiling point of -196°C. She pours some of the liquid nitrogen onto the floor and it vaporizes.
Check Answer
Answer: S
This is spontaneous. Boiling of the liquid nitrogen occurs spontaneously at room temperatures (since it is a temperature above the boiling point).
- After several prompts, a few commands, and a final threat, Randy Mizashun violates his moral principles and decides to clean his room. To the surprise of his parents, he puts everything in its designated place.
Check Answer
Answer: NS
This process of ordering the room isn't going to occur without Randy's sustained intervention and input of energy. The opposite process - the slow and inevitable randomization of items within the room - is indeed a spontaneous process and will occur without Randy's help.
- Cameron Per is (his friends call him Cam) is on a camping outing with his friends. After some hot dogs and several helpings of baked beans for dinner and some fireside chatting, they finally crowd into their tent like sardines in a can. Unfortunately for his friends, the beans are not settling well for Cam. He flatulates. The gas then fills the entire volume of the tent.
Check Answer
Answer: S
The spread of the gas throughout the tent is known as diffusion. It is a spontaneous process. (There's other words to describe it, but that's the one we've chosen.)
- In a classroom demonstration, Mr. Lebrum drops a couple of small chunks of calcium carbide in water. It immediately reacts to produce acetylene gas and calcium hydroxide.
Check Answer
Answer: S
The reaction occurs without any sustained intervention from Mr. Lebrum. Once the reactants are in contact with each other, the reaction occurs (immediately) until one of the reactants is depleted. (PS - Don't do this at home. It should only be done by a trained professional like Mr. Lebrum.)
- On their Physics field trip to Great America, a group of students are loaded into the roller coaster cars of the Batman ride. They are pulled from the loading station to the top of the tallest point on the track.
Check Answer
Answer: NS
The pulling of the cars up the hill to the highest point requires energy. It is a non-spontaneous process.
- After the ascent up the first hill is over, the coaster car begins its path around the turns, drops, hills, and loops for a 45-second thrill ride.
Check Answer
Answer: S
Once the roller coaster car has reached its highest point, it can traverse the entirety of the track without sustained intervention. That makes the second half of this roller coaster spontaneous.
- Mr. Tooker planted several sugar snap peas in his backyard garden. They grew to 5 feet tall within six weeks.
Check Answer
Answer: NS
The peas grow by photosynthesis. As discussed, this is a non-spontaneous process that requires the sustained inteverntion of the sun.
- A large pump at the bottom of the water slide at the water park is used to pump water from the ground to the top of the hill.
Check Answer
Answer: NS
Water does not spontaneously move up hill ... even in a magical water park. Energy must be continuously supplied to it to move the water up to the top of the water slide.
- Suzie Lavtaski glides effortlessly down the ski hill to the ski lodge at its base.
Check Answer
Answer: S
A downhill motion is a spontaneous motion. And Suzie loves it!
- The freezing point of mercury is -38°C. The mercury in a glass thermometer freezes at 0°C.
Check Answer
Answer: NS
Liquids do not freeze at temperatures above their freezing point. This is a non-spontaneous and a non-occuring event.