ENERGETICS [dchemlc]

A reaction that gives out heat to the surroundings is said to be _______________.

Answer explanation

Some chemical reactions give out energy in the form of heat. A reaction that gives out heat to the surroundings is said to be exothermic. If you are holding a test-tube in which an exothermic reaction is occurring, you will notice that the test-tube gets warmer.

Which of the following is not an exothermic reaction?

• Combustion reaction

• Displacement reaction

• Thermal decomposition reaction

• Neutralization reaction

Answer explanation

Any reaction that produces a flame is exothermic. apart from combustion reactions, other exothermic changes include:

• the reactions of metals with acids

• neutralization reactions

• displacement reactions.

Thermal decomposition is an endothermic reaction

The amount of heat energy taken in or released in a chemical reaction can be measured.

This energy is known as the _____________ of the reaction.

Answer explanation

It is the difference between the energy of the products and the energy of the reactants. enthalpy change is given a minus or a plus sign to show whether heat is being given out or absorbed by the reaction. You always look at it from the point of view of the reactants.

What is the unit used to measure enthalpy change?

• kJ/mol

• J/mol

• J/Kg

• J/g

Answer explanation

enthalpy change is measured in units of kJ/mol (kilojoules per mole).

If the combustion of ethanol was complete, carbon dioxide should be produced instead of carbon and the colour of the flame should be _______

• blue

• yellow

• white

• orange

Answer explanation

A reaction that absorbs heat from the surroundings is said to be __________

Answer explanation

the amount of heat needed to raise the temperature of 1 gram of a substance by 1⁰C is known as_____________.

Answer explanation

In the combustion reaction of alcohols, as the alcohol chain grows longer, the reaction gets___________

• More exothermic

• More endothermic

• Less exothermic

• Less endothermic

Answer explanation

we can see that the combustion reaction gets more exothermic as the alcohol chain becomes longer. In other words, longer alcohols give out more heat energy per mole when they burn than shorter ones.

During chemical reactions bonds in the reactants have to be broken and new ones formed to make the products. Breaking bonds are (a)__________ and making bonds are (b)____________.

choose the most suitable pair of answers for (a) and (b).

• (a)exothermic (b) exothermic

• (a)exothermic (b) endothermic

• (a)endothermic (b) endothermic

• (a)endothermic (b) exothermic

Answer explanation

Breaking bonds needs energy (endothermic) and making bonds releases energy (exothermic).

_________ is a measure of the number of ways for the energy in a system to be distributed.

Answer explanation

For example, dissolving ammonium nitrate into water causes the temperature of the solution to decrease, but it dissolves anyway. This is possible because of a factor called entropy.