Energy Changes

Learning Objectives…

• Describe the term exothermic as a process or chemical reaction which transfers energy, often in the form of heat, to the surroundings and may be detected by an increase in temperature, e.g. the reaction between sodium hydroxide and hydrochloric acid

• Describe the term endothermic as a process or chemical reaction which takes in energy, often in the form of heat, from the surroundings and may be detected by a decrease in temperature, e.g. the dissolving of ammonium nitrate in water

Introduction

• Endo = In• Exo = Out

• Thermic = Heat

What is an ENDOTHERMIC reaction?

• A reaction that takes in heat (thermal energy)• Causes a drop in temperature in the

surroundings.

What is an EXOTHERMIC reaction?• A reaction that gives out heat (thermal

energy)• Cause a temperature rise in the surroundings.

Heat of reaction

ΔHHeat of reaction

Unit: kilojoules (kJ)

The amount of heat energy given out or taken in (absorbed) during a

chemical reaction.

Exothermic reaction Endothermic reactionHeat energy lost Heat energy gained

- ΔH + ΔH

Example 1

Reaction: Magnesium and hydrochloric acidEquation: Mg + 2HCl MgCl2 + H2 [ΔH = -462 KJ]ΔH ΔH = -462 KJExothermic/Endothermic reaction:

Explanation: Negative value of ΔH shows that the reaction is exothermic. 462 kJ means that 462 kilojoules of energy is given out.

Example 1

Reaction: Magnesium and hydrochloric acidEquation: Mg + 2HCl MgCl2 + H2 [ΔH = -462 KJ]ΔH ΔH = -462 KJExothermic/Endothermic reaction:

exothermic reaction

Explanation: Negative value of ΔH shows that the reaction is exothermic. 462 kJ means that 462 kilojoules of energy is given out.

Example 2Reaction: Decomposition of calcium carbonateEquation: CaCO3 CaO + CO2 [ΔH = +222 KJ]ΔH ΔH = +222 KJExothermic/Endothermic reaction:

endothermic reaction

Explanation: Positive value of ΔH shows that the reaction is endothermic. 222 kJ means that 222 kilojoules of energy is given out.

CHECKPOINT!

1. DETERMINE IF THE REACTIONS STATED BELOW ARE ENDOTHERMIC OR EXOTHERMIC REACTIONS. (A) COMBUSTION OF METHANE: EXOTHERMIC/ENDOTHERMIC REACTION

CH4 + 2O2 CO2 + 2H2O [ΔH = -882 KJ]

(b) Decomposition of copper(II) carbonate: exothermic/endothermic reaction

CuCO3 CuO + CO2 [ΔH = +46 kJ]

(c) Formation of nitrogen oxide in a car engine: exothermic/endothermic reaction

N2 + O2 2NO [ΔH = +181 kJ]

(d) Reaction of sodium hydroxide and hydrochloric acid: exothermic/endothermic reaction

NaOH + HCl NaCl + H2O [ΔH = -57 kJ]

1. DETERMINE IF THE REACTIONS STATED BELOW ARE ENDOTHERMIC OR EXOTHERMIC REACTIONS. (A) COMBUSTION OF METHANE: EXOTHERMIC/ENDOTHERMIC REACTION

CH4 + 2O2 CO2 + 2H2O [ΔH = -882 KJ]

(b) Decomposition of copper(II) carbonate: exothermic/endothermic reaction

CuCO3 CuO + CO2 [ΔH = +46 kJ]

(c) Formation of nitrogen oxide in a car engine: exothermic/endothermic reaction

N2 + O2 2NO [ΔH = +181 kJ]

(d) Reaction of sodium hydroxide and hydrochloric acid: exothermic/endothermic reaction

NaOH + HCl NaCl + H2O [ΔH = -57 kJ]

Exothermicreaction Exothermic reaction

only occurs when the chemical energy present in the substance is converted to thermal energy (and sometimes light and sound energy).

1. Change of state

• Particles move closer together, at a lower energy level

• Excess energy is lost in the form of heat energy

freezingcondensation

2. Combustion reactions

• Combustion is the reaction between a substance and oxygen.

• Energy is released in the form of heat (sometimes light and sound energy).

• Respiration is also a type of combustion reaction.

3. Neutralisation

• Acid + Alkali Thermal energy given out– Combining of H+ and OH- ions to form H2O

– H+ (aq)+ OH- (aq) H2O (l)

Endothermic reaction

Endothermic reaction occurs when heat energy is absorbed.

1. Change of state

• Particles move further apart, breaking forces of attraction holding them together

• Energy is absorbed and used to break bonds• Surrounding temperature drops.

2. Decomposition of compounds

• Most compounds require heating to be decomposed. – Thermal energy absorbed– Compound is decomposed

bond-making vs. bond-breaking

Heat changes in a reaction

heat of reaction, ΔH

Overall heat change (heat of reaction, ΔH)= heat given out in making bonds + heat

absorbed in breaking bonds