Hess's Law

Kinetics and Hess's Law Review

Let's make sure we've got this :)

Multiple Choice

In this graph, ΔE is ___

• Activation Energy

• Enthalpy

• Exothermic

Multiple Choice

A chemical compound has an activation energy of 45 kJ, and absorbs 10 kJ. Which graph best represents this chemical compound?

• A

• B

• C

• D

Multiple Choice

Which letter shows the activation energy?

• A

• B

• C

• D

• E

Multiple Choice

For which reaction is the activation energy higher?

• With the catalyst

• Without the catalyst

Multiple Choice

Is this showing an endothermic or an exothermic reaction?

• Endothermic

• Exothermic

Multiple Choice

In an exothermic reaction...

• heat is released

• heat is absorbed

Multiple Choice

In an endothermic reaction...

• heat is released

• heat is absorbed

Multiple Choice

Does this show an endothermic or an exothermic reaction?

• Endothermic

• Exothermic

Multiple Choice

The ___________ the surface area of the reactants, the faster the reaction rate.

• larger

• smaller

Multiple Choice

Increasing the temperature will increase the kinetic energy of particles, therefore increasing the collisions between particles.

• false

• true

Multiple Choice

A solution is heated and bubbles form. Do you assume a chemical reaction took place?

• Yes

• No

Multiple Choice

You observe bubbles after adding metal to a solution. Do you assume a chemical reaction took place?

• Yes

• No

Multiple Choice

A catalyst increases the efficiency of collisions between reactants.

• true

• false

Multiple Choice

Exothermic reactions release heat to the surroundings.

• true

• false

Multiple Choice

Increasing the concentration of the reactants will slow down the reaction.

• false

• true

Multiple Choice

Usually lowering the temperature will slow down a reaction.

• false

• true

Multiple Choice

Increasing the temperature will increase the kinetic energy of particles, therefore increasing the collisions between particles.

• false

• true

Multiple Choice

The ___________ the surface area of the reactants, the faster the reaction rate.

• larger

• smaller

Multiple Choice

Smaller particle size allows for a _________ surface area to be exposed for the reaction.

• larger
• smaller

Multiple Choice

Catalysts permit reactions to proceed along a ___________energy path.

• lower

• higher

Multiple Choice

A ______________ is a substance that increases the rate of a reaction without being used up during the reaction.

• catalyst

• product

• reactant

• solute

Multiple Choice

Products will form faster if____________

• the particle size of the reactants are larger.

• temperature is decreased.

• concentration of the reactants are increased.

• the reaction is not stirred.

Multiple Choice

The rate of a chemical reaction is NOT affected by which of the following:

• temperature

• concentration

• particle size (surface area)

• All of these affect reaction rates

Multiple Choice

The rate of a chemical reaction is NOT affected by which of the following:

• temperature

• concentration

• particle size (surface area)

• All of these affect reaction rates

Multiple Choice

The following equations show the oxidation of carbon and carbon monoxide to carbon dioxide.

C_(s) +O_2(g) → CO_2(g) ΔH = –x kJ mol^–1

CO_(g) + O_2(g) → CO_2(g) ΔH = –y kJ mol^–1

What is the enthalpy change, in kJ mol^–1, for the oxidation of carbon to carbon monoxide?

C_(s) + O_2(g) → CO_(g)

• x + y

• -x - y

• y - x

• x - y

Multiple Choice

Consider the following equations.

Mg_(s) + O_2(g) → MgO_(s) ∆H = –602 kJ

H_2(g) + O_2(g) → H₂O_(g) ∆H = –242 kJ

What is the ∆H value (in kJ) for the following reaction?

MgO_(s) + H_2(g) → Mg_(s) + H₂O_(g)

• -844

• -360

• +360

• +844

Multiple Choice

Using the equations below

Cu_(s) + ¹/₂O_2(g) → CuO_(s) ∆H = –156 kJ

2Cu_(s) + O_2(g) → Cu₂O_(s) ∆H = –170 kJ

what is the value of ∆H (in kJ) for the following reaction?

2CuO_(s) → Cu₂O_(s) + ¹/₂O_2(g)

• 142

• 15

• -15

• -142

Multiple Choice

When you flip a chemical reaction using Hess's Law, what is done to the heat of reaction value?

• nothing

• the sign is flipped

• 1/delta H

Multiple Choice

4. The enthalpies of combustion of C(s), H2(g) and C4H9OH(l) (in kJmol^-1) are as follows

C(s) + O₂(g) -> CO₂(g) ∆H=-393kJ/mol

H₂(g) + ½O₂(g) -> H₂O(l) ∆H=-286kJ/mol

C₄H₉OH(l) + 6O₂(g) -> 4CO₂(g) + 5H₂O(l) ∆H=-1371kJ/mol

What is the enthalpy change for the reaction shown below?

4C(s) + 5H₂(l) + ½O₂(g) -> C₄H₉OH(l)

• +845kJ

• -2275 kJ

• -1631kJ

• -845

Multiple Choice

When you multiply a chemical equation by 2, what is done to the heat of reaction?

• multiplied by 1/2

• multiplied by 2

• nothing

Multiple Choice

What is the relationship between enthalpies p, q, r and s?

S(s) + H₂(g) -> H₂S(g) ∆H=p

H₂(g) + ½O₂(g) -> H₂O(l) ∆H=q

S(s) + O₂(g) -> SO₂(g) ∆H=r

H₂S(g) + 1½O₂(g) -> H₂O(l) + SO₂(g) ∆H=s

• p = q + r – s

• p = s – q – r

• p = q – r – s

• p = s + r – q

Multiple Choice

C₂H₄(g) + H₂(g) -> C₂H₆(g) ∆H°=-137 kJ mol^-1

Which statement about this information is correct?

• The total energy of the bonds broken in the reactants is greater

  than the total energy of the bonds

  formed in the product

• The bonds broken and the bonds made are of the same strength

• The total energy of the bonds broken in the reactants is less than the total energy of the bonds formed in the product

• No conclusion can be made about the sums of the bond enthalpies in the product compared with the reactants

Multiple Choice

The standard enthalpy changes of combustion of carbon, hydrogen and methane are shown in the table.

Which one of the following expressions gives the correct value for the standard enthalpy change of formation of methane in kJ mol^–1?

C(s) + 2H₂(g) → CH₄(g)

• 394 + (2 × 286) – 891

• –394 – (2 × 286) + 891

• 394 + 286 – 891

• –394 – 286 + 891

Multiple Choice

The enthalpy change for the reaction

C(s, graphite) + 1⁄2O₂(g) --> CO(g) cannot be measured directly since some carbon dioxide is always formed in the reaction. It can be calculated using Hess’s Law and the enthalpy changes of combustion of graphite and of carbon monoxide.

C(s, graphite) + O₂(g) --> CO₂ ΔH=-394 kJmol^–1

CO(g) + 1⁄2O₂(g) --> CO₂ ΔH=-283 kJmol^–1

The enthalpy change for the reaction of graphite with oxygen to give carbon monoxide is ... ?

• -677 kJ mol^–1

• +111 kJ mol^–1

• -111 kJ mol^–1

• +677 kJ mol^–1

Multiple Choice

The enthalpy change for the reaction, ∆H_r , is equal to 

• ∆H₁ + ∆H₂
• ∆H₁ - ∆H₂
• -∆H₁ - ∆H₂
• -∆H₁ + ∆H₂

Multiple Choice

Hess's Law states that...

• all reactions are reversible

• the enthalpy change of a reaction is independent of the route taken

• the direct route results in a greater enthalpy change

• the indirect route results in a lower enthalpy change as energy is lost in the second step

Multiple Choice

(Hess's Law)Calculate the ∆H for the following reaction: 2H₂O₂ → 2H₂O + 1 O₂

You are given these two equations:

2H₂ + O₂ → 2H₂O ∆H = -572 kJ

H₂ + O₂ → H₂O₂ ∆H = -188 kJ

• ∆H = -948 kJ

• ∆H = -196 kJ

• ∆H = -384 kJ

• ∆H = -188 kJ

Multiple Choice

Enthalpy (H)

• The heat content of a system at constant pressure.

• A chemical equation that includes the amount of heat released or absorbed during the reaction.

• The heat released or absorbed during a chemical reaction.

• The heat released after burning 1 mole of a substance.