Heat Energy Changes in Chemical Reactions Flashcards

All 24 cards in this deck

What is an exothermic reaction or change?

One in which heat energy is given out to the surroundings, so the temperature of the surroundings rises.

What is an endothermic reaction or change?

One in which heat energy is taken in from the surroundings, so the temperature of the surroundings falls.

Name four types of change that are accompanied by heat energy changes measurable as a temperature change in solution.

Salts dissolving in water, neutralisation reactions, displacement reactions and precipitation reactions.

What are the steps to measure the temperature change of a reaction in solution?

e.g. dilute hydrochloric acid neutralised by sodium hydroxide solution

1. Measure a fixed volume of the acid into a polystyrene cup and record its starting temperature.
2. Add the sodium hydroxide solution and stir.
3. Record the highest (or lowest) temperature reached; temperature change = final − starting temperature.

In an experiment measuring a temperature change in solution, how can heat loss be reduced?

Put a lid on the cup and insulate it (e.g. cotton wool), so less heat energy escapes to the surroundings.

True or false? If the temperature of the solution falls during a reaction, the reaction is exothermic.

False. A temperature fall means heat energy has been taken in, so the reaction is endothermic.

Is the breaking of bonds exothermic or endothermic?

Endothermic — energy must be taken in (absorbed) to break bonds.

Is the making of bonds exothermic or endothermic?

Exothermic — energy is released (given out) when bonds are formed.

In terms of bond breaking and bond making, when is a reaction overall exothermic?

When more heat energy is released in forming the bonds in the products than is required to break the bonds in the reactants.

In terms of bond breaking and bond making, when is a reaction overall endothermic?

When less heat energy is released in forming the bonds in the products than is required to break the bonds in the reactants.

True or false? Energy is released when a chemical bond is broken.

False. Breaking a bond absorbs energy; energy is only released when new bonds are made.

True or false? In an endothermic reaction the energy needed to break the reactant bonds is greater than the energy released when the product bonds form.

True. That is why heat energy is taken in overall.

What is the formula for the overall energy change of a reaction from bond energies?

Energy change = (total energy to break bonds in reactants) − (total energy released making bonds in products).

What are the steps to calculate the energy change of a reaction from bond energies?

e.g. $\mathrm{H_2 + Cl_2 \rightarrow 2HCl}$; H–H = 436, Cl–Cl = 244, H–Cl = 431 kJ mol$^{-1}$

1. Energy to break bonds: $436 + 244 = 680$ kJ mol$^{-1}$.
2. Energy released making bonds: $2 \times 431 = 862$ kJ mol$^{-1}$.
3. Energy change $= 680 - 862 = -182$ kJ mol$^{-1}$ (exothermic).

What does the sign of a calculated energy change tell you about a reaction?

Negative = exothermic (energy released); positive = endothermic (energy taken in).

What units are used for bond energies and for the energy change of a reaction?

kJ mol$^{-1}$.

What are the steps to find an unknown bond energy from the overall energy change?

e.g. $\mathrm{H_2 + Cl_2 \rightarrow 2HCl}$, energy change $-180$ kJ mol$^{-1}$; H–H = 436, Cl–Cl = 244 kJ mol$^{-1}$

1. Total energy to break bonds: $436 + 244 = 680$ kJ mol$^{-1}$.
2. Energy released making bonds = bonds broken − energy change $= 680 - (-180) = 860$ kJ mol$^{-1}$.
3. Divide by the number of those bonds: $860 \div 2 = 430$ kJ mol$^{-1}$ for H–Cl.

True or false? In the products $\mathrm{2H_2O}$ there are 4 O–H bonds to count in a bond energy calculation.

True. Multiply the bonds in one molecule by the balancing number in the equation.

What is meant by the term activation energy?

The minimum amount of energy that colliding particles must have for a reaction to occur.

On a reaction profile, how is the activation energy shown?

As the energy difference from the reactants level up to the peak of the curve.

How can you tell from a reaction profile that a reaction is endothermic?

The products are at a higher energy than the reactants.

True or false? On a reaction profile, the overall energy change is the difference between the reactants and the peak of the curve.

False. That is the activation energy; the overall energy change is the difference between the reactants and the products.

True or false? An exothermic reaction has no activation energy.

False. Every reaction has an activation energy, shown as a peak on its reaction profile.

When drawing the curve on a reaction profile for an endothermic reaction, how high must the peak be?

Higher than the energy of the products (so higher than both reactants and products).