# Thermodynamics

In this worksheet, students will study the laws of thermodynamics in chemical reactions. There is also an introduction to the concepts of entropy and enthalpy.

Key stage:  KS 4

Curriculum topic:  Chemistry: Structure, Bonding and the Properties of Matter

Curriculum subtopic:  Changes of State of Matter

Difficulty level:

### QUESTION 1 of 10

Ever looked at fire and really wondered what is going on there? We all know that it is hot, but where is that heat coming from? What is happening to make the fire give off so much energy? Well, it turns out it is all to do with some of the smallest stuff we study in science - particles.

When studying energy transfers in chemical reactions, it is important to remember that it all happens on a small scale, as we say at the atomic level. Atoms and molecules are responsible for the transmission of tiny amounts of energy, but when they come together we have energy flow on a bigger scale.

Energy moves between systems of different temperature (along a temperature gradient) in such a way that high energy systems give off energy to low energy systems, e.g. hot to cold. When the temperatures become equal there is no transfer. Remember, it's all about balance!

Energy moves by convection, conduction and radiation and when things get hotter, they also get bigger. This is because when atoms gain kinetic energy (energy associated with movement), they move around faster, so they expand. When they lose heat and consequently kinetic energy (i.e. get colder), they contract.

A thermodynamic system is one that exchanges energy with the area around it. If the system is 'in equilibrium', it is actually balanced and 'happy' on its own, so it becomes a closed system, i.e. it does not exchange energy.

Thermodynamics are 'guided' by a number of laws:

Law 0

If two systems are in equilibrium with a third system, then they are in equilibrium with each other.

This means that, for three systems (A, B and C), if A has the same energy as C and B has the same energy as C, then A also has the same energy as B.

Law 1

When energy is added to a system, some of it stays in and increases the overall energy and some of it leaves the system, but does work in the area around it. By 'work' we mean that there is an effect in the area around the system. For example, when we heat a pot of water, the energy increases first. Further, some energy is released and heats the air around it.

Law 2

No reaction is 100 per cent efficient. Some amount of energy is always lost to heat. Moreover, it is impossible for heat to flow from a cold object to a hot one without work, as this would go against what is natural; heat flowing from hot to cold.

Enthalpy is a measure of heat in a system. It depends on the amount of internal energy, the pressure and volume of a system. If you have more of a substance the enthalpy increases.

Entropy is a measure of random activity in a system at a specific time. Increase in temperature results in increase in entropy. Gases have more entropy than liquids and solids. A chemical reaction that produces gases increases the entropy of the system.

At what level does energy transfer occur in chemical reactions?

What brings energy flow on a big scale?

Tiny molecules moving rapidly

Tiny energy transmissions at an atomic level put together

Tiny bits of kinetic energy

What is the direction of energy flow between two systems?

There is no energy flow.

Energy flows from hot to cold.

Energy flows from cold to hot.

What happens to objects when they gain heat?

Contract

Expand

Stay the same

What is a thermodynamic system?

A system that does not interact with its surrounding area

A system that interacts with its surrounding area

A system that does not take part in energy transfers

What happens to a system 'in equilibrium' in terms of energy transfers?

It takes in energy from its surroundings.

It does not take energy from its surroundings nor does it give any energy off.

System X is 'in equilibrium' with system Z. System Y is also 'in equilibrium' with system Z.

State the relationship between systems X and Y according to Law 0 of thermodynamics.

X and Y are not 'in equilibrium'.

X and Y are 'in equilibrium'.

X and Y are independent of each other.

Is this statement true or false?

When energy is added to a system, it all stays in and increases the overall energy.

True

False

Complete this sentence.

For energy to flow from a cold object to a hot one, ______ needs to be done.

What state of matter has more entropy than the others: solid, liquid or gas?

• Question 1

At what level does energy transfer occur in chemical reactions?

atomic
atomic level
the atomic level
EDDIE SAYS
Energy transfer in chemical reactions occurs at an atomic level. It is changes in the chemicals that make up the reactants that release the energy.
• Question 2

What brings energy flow on a big scale?

Tiny energy transmissions at an atomic level put together
EDDIE SAYS
Tiny energy transmissions at an atomic level put together bring about energy flow on a big scale. If you have enough of these tiny energy transfers, they can all get added together to make something huge.
• Question 3

What is the direction of energy flow between two systems?

Energy flows from hot to cold.
EDDIE SAYS
Energy flows from a system with high energy levels to a system with low energy levels, i.e. hot to cold.
• Question 4

What happens to objects when they gain heat?

Expand
EDDIE SAYS
Objects expand when they gain heat because the atoms they are made of gain kinetic energy, which makes them move more and faster. This, in turn, increases the size of the gaps int eh particles and that increases the size of the object.
• Question 5

What is a thermodynamic system?

A system that interacts with its surrounding area
EDDIE SAYS
A thermodynamic system interacts with its surrounding area in a way that allows energy transfers to take place. All things in the world use thermodynamics all of the time as they are all trying to become the same temperature.
• Question 6

What happens to a system 'in equilibrium' in terms of energy transfers?

It does not take energy from its surroundings nor does it give any energy off.
EDDIE SAYS
A system 'in equilibrium' does not take energy from its surroundings nor does it give any energy off. This is because it is the same temperature as its surroundings.
• Question 7

System X is 'in equilibrium' with system Z. System Y is also 'in equilibrium' with system Z.

State the relationship between systems X and Y according to Law 0 of thermodynamics.

X and Y are 'in equilibrium'.
EDDIE SAYS
According to Law 0 of thermodynamics, X and Y are 'in equilibrium'. This is because the two systems mentioned above are in equilibrium, meaning that all three need to be in equilibrium as they are all connected.
• Question 8

Is this statement true or false?

When energy is added to a system, it all stays in and increases the overall energy.

False
EDDIE SAYS
The statement is false. When energy is added to a system, some stays in and increases the overall energy, but some of it leaves the system.
• Question 9

Complete this sentence.

For energy to flow from a cold object to a hot one, ______ needs to be done.

work
EDDIE SAYS
For energy to flow from a cold object to a hot one, work needs to be done. Work is needed any time there is a flow of energy - so work is needed here as well as when moving objects.
• Question 10

What state of matter has more entropy than the others: solid, liquid or gas?

gas
EDDIE SAYS
Gases have increased entropy in relation to liquids and solids. This is because all of the particles are spread out and moving in random directions.
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